Expand functionality of function! macro 🛰

20 files changed, 1039 insertions, 767 deletions

diff --git a/src/export/pdf.rs b/src/export/pdf.rs
index 76fc59b8..8370f454 100644
--- a/src/export/pdf.rs
+++ b/src/export/pdf.rs
@@ -80,7 +80,7 @@ impl<'d, W: Write> ExportProcess<'d, W> {
     ) -> PdfResult<ExportProcess<'d, W>>
     {
         let (fonts, font_remap) = Self::subset_fonts(layouts, font_loader)?;
-        let offsets = Self::calculate_offsets(layouts.count(), fonts.len());
+        let offsets = Self::calculate_offsets(layouts.len(), fonts.len());
 
         Ok(ExportProcess {
             writer: PdfWriter::new(target),
diff --git a/src/func/args.rs b/src/func/args.rs
new file mode 100644
index 00000000..2e9e80cc
--- /dev/null
+++ b/src/func/args.rs
@@ -0,0 +1,215 @@
+//! Parsing, storing and deduplication of function arguments.
+
+use super::prelude::*;
+use Expression::*;
+
+/// Provides a convenient interface to parse the arguments to a function.
+pub struct ArgParser<'a> {
+    args: &'a FuncArgs,
+    positional_index: usize,
+}
+
+impl<'a> ArgParser<'a> {
+    pub fn new(args: &'a FuncArgs) -> ArgParser<'a> {
+        ArgParser {
+            args,
+            positional_index: 0,
+        }
+    }
+
+    /// Get the next positional argument of the given type.
+    ///
+    /// If there are no more arguments or the type is wrong,
+    /// this will return an error.
+    pub fn get_pos<T>(&mut self) -> ParseResult<Spanned<T::Output>>
+    where T: Argument<'a> {
+        Self::expected(self.get_pos_opt::<T>()?)
+    }
+
+    /// Get the next positional argument if there is any.
+    ///
+    /// If the argument is of the wrong type, this will return an error.
+    pub fn get_pos_opt<T>(&mut self) -> ParseResult<Option<Spanned<T::Output>>>
+    where T: Argument<'a> {
+        let arg = self.args.positional
+            .get(self.positional_index)
+            .map(T::from_expr)
+            .transpose();
+
+        if let Ok(Some(_)) = arg {
+            self.positional_index += 1;
+        }
+
+        arg
+    }
+
+    /// Get a keyword argument with the given key and type.
+    pub fn get_key<T>(&mut self, key: &str) -> ParseResult<Spanned<T::Output>>
+    where T: Argument<'a> {
+        Self::expected(self.get_key_opt::<T>(key)?)
+    }
+
+    /// Get a keyword argument with the given key and type if it is present.
+    pub fn get_key_opt<T>(&mut self, key: &str) -> ParseResult<Option<Spanned<T::Output>>>
+    where T: Argument<'a> {
+        self.args.keyword.iter()
+            .find(|entry| entry.val.0.val == key)
+            .map(|entry| T::from_expr(&entry.val.1))
+            .transpose()
+    }
+
+    /// Assert that there are no positional arguments left. Returns an error
+    /// otherwise.
+    pub fn done(&self) -> ParseResult<()> {
+        if self.positional_index == self.args.positional.len() {
+            Ok(())
+        } else {
+            pr!("unexpected argument");
+        }
+    }
+
+    /// Covert an option to a result with an error on `None`.
+    fn expected<T>(val: Option<Spanned<T::Output>>) -> ParseResult<Spanned<T::Output>>
+    where T: Argument<'a> {
+        val.ok_or_else(|| pr!(@"expected {}", T::ERROR_MESSAGE))
+    }
+}
+
+/// A kind of argument.
+pub trait Argument<'a> {
+    type Output;
+    const ERROR_MESSAGE: &'static str;
+
+    fn from_expr(expr: &'a Spanned<Expression>) -> ParseResult<Spanned<Self::Output>>;
+}
+
+macro_rules! arg {
+    ($type:ident, $err:expr, $doc:expr, $output:ty, $wanted:pat => $converted:expr) => (
+        #[doc = $doc]
+        #[doc = " argument for use with the [`ArgParser`]."]
+        pub struct $type;
+        impl<'a> Argument<'a> for $type {
+            type Output = $output;
+            const ERROR_MESSAGE: &'static str = $err;
+
+            fn from_expr(expr: &'a Spanned<Expression>) -> ParseResult<Spanned<Self::Output>> {
+                #[allow(unreachable_patterns)]
+                match &expr.val {
+                    $wanted => Ok(Spanned::new($converted, expr.span)),
+                    _ => pr!("expected {}", $err),
+                }
+            }
+        }
+    );
+}
+
+arg!(ArgExpr,  "expression", "A generic expression", &'a Expression, expr => &expr);
+arg!(ArgIdent, "identifier", "An identifier (e.g. `horizontal`)", &'a str, Ident(s) => s.as_str());
+arg!(ArgStr,   "string", "A string (e.g. `\"Hello\"`)", &'a str, Str(s) => s.as_str());
+arg!(ArgNum,   "number", "A number (e.g. `5.4`)", f64, Num(n) => *n);
+arg!(ArgSize,  "size", "A size (e.g. `12pt`)", crate::size::Size, Size(s) => *s);
+arg!(ArgBool,  "bool", "A boolean (`true` or `false`)", bool, Bool(b) => *b);
+
+/// An argument key which identifies a layouting axis.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum AxisKey {
+    Primary,
+    Secondary,
+    Vertical,
+    Horizontal,
+}
+
+impl AxisKey {
+    /// The generic version of this axis key in the given system of axes.
+    pub fn generic(&self, axes: LayoutAxes) -> GenericAxisKind {
+        match self {
+            Primary => GenericAxisKind::Primary,
+            Secondary => GenericAxisKind::Secondary,
+            Vertical => axes.vertical(),
+            Horizontal => axes.horizontal(),
+        }
+    }
+
+    /// The specific version of this axis key in the given system of axes.
+    pub fn specific(&self, axes: LayoutAxes) -> SpecificAxisKind {
+        match self {
+            Primary => axes.primary(),
+            Secondary => axes.secondary(),
+            Vertical => SpecificAxisKind::Vertical,
+            Horizontal => SpecificAxisKind::Horizontal,
+        }
+    }
+}
+
+/// An argument key which identifies a target alignment.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum AlignmentKey {
+    Left,
+    Top,
+    Right,
+    Bottom,
+    Origin,
+    Center,
+    End,
+}
+
+impl AlignmentKey {
+    /// The generic axis this alignment key corresopnds to in the given system
+    /// of layouting axes. Falls back to `default` if the alignment is generic.
+    pub fn axis(&self, axes: LayoutAxes, default: GenericAxisKind) -> GenericAxisKind {
+        use AlignmentKey::*;
+        match self {
+            Origin | Center | End => default,
+            Left | Right => axes.horizontal(),
+            Top | Bottom => axes.vertical(),
+        }
+    }
+
+    /// The generic version of this alignment in the given system of layouting
+    /// axes. Returns an error if the alignment is invalid for the given axis.
+    pub fn generic(&self, axes: LayoutAxes, axis: GenericAxisKind) -> LayoutResult<Alignment> {
+        use AlignmentKey::*;
+
+        let horizontal = axis == axes.horizontal();
+        Ok(match self {
+            Origin => Alignment::Origin,
+            Center => Alignment::Center,
+            End => Alignment::End,
+            Left if horizontal => axes.left(),
+            Right if horizontal => axes.right(),
+            Top if !horizontal => axes.top(),
+            Bottom if !horizontal => axes.bottom(),
+            _ => lr!(
+                "invalid alignment `{}` for {} axis",
+                format!("{:?}", self).to_lowercase(),
+                format!("{:?}", axis).to_lowercase()
+            )
+        })
+    }
+
+    /// The specific version of this alignment in the given system of layouting
+    /// axes.
+    pub fn specific(&self, axes: LayoutAxes, axis: SpecificAxisKind) -> AlignmentKey {
+        use AlignmentKey::*;
+        match (self, axis) {
+            (Origin, SpecificAxisKind::Horizontal) => Left,
+            (End, SpecificAxisKind::Horizontal) => Right,
+            (Origin, SpecificAxisKind::Vertical) => Top,
+            (End, SpecificAxisKind::Vertical) => Bottom,
+            _ => *self,
+        }
+    }
+}
+
+/// An argument key which identifies a margin or padding target.
+///
+/// A is the axis type used.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum PaddingKey<A> {
+    /// All four sides should have the specified padding.
+    All,
+    /// Both sides of the given axis should have the specified padding.
+    Axis(A),
+    /// Only the given side of the given axis should have the specified padding.
+    AxisAligned(A, AlignmentKey),
+}
diff --git a/src/func/helpers.rs b/src/func/helpers.rs
deleted file mode 100644
index c82a90e1..00000000
--- a/src/func/helpers.rs
+++ /dev/null
@@ -1,203 +0,0 @@
-//! Helper types and macros for creating custom functions.
-
-use super::prelude::*;
-use Expression::*;
-
-/// Lets you implement the function trait more concisely.
-#[macro_export]
-macro_rules! function {
-    (data: $ident:ident, $($tts:tt)*) => (
-        #[allow(unused_imports)]
-        use $crate::func::prelude::*;
-
-        impl Function for $ident {
-            function!(@parse $ident, $($tts)*);
-        }
-    );
-
-    (@parse $ident:ident, parse: plain, $($tts:tt)*) => (
-        fn parse(header: &FuncHeader, body: Option<&str>, _: ParseContext) -> ParseResult<Self>
-        where Self: Sized {
-            ArgParser::new(&header.args).done()?;
-            if body.is_some() {
-                perr!("expected no body");
-            }
-            Ok($ident)
-        }
-
-        function!(@layout $($tts)*);
-    );
-
-    (
-        @parse $ident:ident,
-        parse($args:ident, $body:ident, $ctx:ident)
-        $block:block
-        $($tts:tt)*
-    ) => (
-        fn parse(header: &FuncHeader, body: Option<&str>, ctx: ParseContext) -> ParseResult<Self>
-        where Self: Sized {
-            #[allow(unused_mut)]
-            let mut $args = ArgParser::new(&header.args);
-            let $body = body;
-            let $ctx = ctx;
-            $block
-        }
-
-        function!(@layout $($tts)*);
-    );
-
-    (@layout layout($this:pat, $ctx:pat) $block:block) => (
-        fn layout(&self, ctx: LayoutContext) -> LayoutResult<CommandList> {
-            let $ctx = ctx;
-            let $this = self;
-            $block
-        }
-    );
-}
-
-/// Parse the body of a function.
-/// - If the function does not expect a body, use `forbidden`.
-/// - If the function can have a body, use `optional`.
-/// - If the function must have a body, use `required`.
-#[macro_export]
-macro_rules! parse {
-    (forbidden: $body:expr) => {
-        if $body.is_some() {
-            perr!("unexpected body");
-        }
-    };
-
-    (optional: $body:expr, $ctx:expr) => (
-        if let Some(body) = $body {
-            Some($crate::syntax::parse(body, $ctx)?)
-        } else {
-            None
-        }
-    );
-
-    (required: $body:expr, $ctx:expr) => (
-        if let Some(body) = $body {
-            $crate::syntax::parse(body, $ctx)?
-        } else {
-            perr!("expected body");
-        }
-    )
-}
-
-/// Early-return with a formatted parsing error or yield
-/// an error expression without returning when prefixed with `@`.
-#[macro_export]
-macro_rules! perr {
-    (@$($tts:tt)*) => ($crate::syntax::ParseError::new(format!($($tts)*)));
-    ($($tts:tt)*) => (return Err(perr!(@$($tts)*)););
-}
-
-/// Early-return with a formatted layouting error or yield
-/// an error expression without returning when prefixed with `@`.
-#[macro_export]
-macro_rules! lerr {
-    (@$($tts:tt)*) => ($crate::layout::LayoutError::new(format!($($tts)*)));
-    ($($tts:tt)*) => (return Err(lerr!(@$($tts)*)););
-}
-
-
-/// Easy parsing of function arguments.
-pub struct ArgParser<'a> {
-    args: &'a FuncArgs,
-    positional_index: usize,
-}
-
-impl<'a> ArgParser<'a> {
-    pub fn new(args: &'a FuncArgs) -> ArgParser<'a> {
-        ArgParser {
-            args,
-            positional_index: 0,
-        }
-    }
-
-    /// Get the next positional argument of the given type.
-    ///
-    /// If there are no more arguments or the type is wrong,
-    /// this will return an error.
-    pub fn get_pos<T>(&mut self) -> ParseResult<Spanned<T::Output>> where T: Argument<'a> {
-        self.get_pos_opt::<T>()?
-            .ok_or_else(|| perr!(@"expected {}", T::ERROR_MESSAGE))
-    }
-
-    /// Get the next positional argument if there is any.
-    ///
-    /// If the argument is of the wrong type, this will return an error.
-    pub fn get_pos_opt<T>(&mut self) -> ParseResult<Option<Spanned<T::Output>>>
-    where T: Argument<'a> {
-        let arg = self.args.positional
-            .get(self.positional_index)
-            .map(T::from_expr)
-            .transpose();
-
-        if let Ok(Some(_)) = arg {
-            self.positional_index += 1;
-        }
-
-        arg
-    }
-
-    /// Get a keyword argument with the given key and type.
-    pub fn get_key<T>(&mut self, key: &str) -> ParseResult<Spanned<T::Output>>
-    where T: Argument<'a> {
-        self.get_key_opt::<T>(key)?
-            .ok_or_else(|| perr!(@"expected {}", T::ERROR_MESSAGE))
-    }
-
-    /// Get a keyword argument with the given key and type if it is present.
-    pub fn get_key_opt<T>(&mut self, key: &str) -> ParseResult<Option<Spanned<T::Output>>>
-    where T: Argument<'a> {
-        self.args.keyword.iter()
-            .find(|entry| entry.val.0.val == key)
-            .map(|entry| T::from_expr(&entry.val.1))
-            .transpose()
-    }
-
-    /// Assert that there are no positional arguments left. Returns an error, otherwise.
-    pub fn done(&self) -> ParseResult<()> {
-        if self.positional_index == self.args.positional.len() {
-            Ok(())
-        } else {
-            perr!("unexpected argument");
-        }
-    }
-}
-
-/// A kind of argument.
-pub trait Argument<'a> {
-    type Output;
-    const ERROR_MESSAGE: &'static str;
-
-    fn from_expr(expr: &'a Spanned<Expression>) -> ParseResult<Spanned<Self::Output>>;
-}
-
-macro_rules! arg {
-    ($type:ident, $err:expr, $doc:expr, $output:ty, $wanted:pat => $converted:expr) => (
-        #[doc = $doc]
-        #[doc = " argument for use with the [`ArgParser`]."]
-        pub struct $type;
-        impl<'a> Argument<'a> for $type {
-            type Output = $output;
-            const ERROR_MESSAGE: &'static str = $err;
-
-            fn from_expr(expr: &'a Spanned<Expression>) -> ParseResult<Spanned<Self::Output>> {
-                #[allow(unreachable_patterns)]
-                match &expr.val {
-                    $wanted => Ok(Spanned::new($converted, expr.span)),
-                    _ => perr!("expected {}", $err),
-                }
-            }
-        }
-    );
-}
-
-arg!(ArgExpr,  "expression", "A generic expression", &'a Expression, expr => &expr);
-arg!(ArgIdent, "identifier", "An identifier (e.g. `horizontal`)", &'a str, Ident(s) => s.as_str());
-arg!(ArgStr,   "string", "A string (e.g. `\"Hello\"`)", &'a str, Str(s) => s.as_str());
-arg!(ArgNum,   "number", "A number (e.g. `5.4`)", f64, Num(n) => *n);
-arg!(ArgSize,  "size", "A size (e.g. `12pt`)", crate::size::Size, Size(s) => *s);
-arg!(ArgBool,  "bool", "A boolean (`true` or `false`)", bool, Bool(b) => *b);
diff --git a/src/func/macros.rs b/src/func/macros.rs
new file mode 100644
index 00000000..78cf1f56
--- /dev/null
+++ b/src/func/macros.rs
@@ -0,0 +1,149 @@
+//! Helper types and macros for creating custom functions.
+
+/// Defines function types concisely.
+#[macro_export]
+macro_rules! function {
+    // Parse a unit struct.
+    ($(#[$outer:meta])* pub struct $type:ident; $($rest:tt)*) => {
+        $(#[$outer])*
+        pub struct $type;
+        function!(@meta $type | $($rest)*);
+    };
+
+    // Parse a struct with fields.
+    ($(#[$outer:meta])* pub struct $type:ident { $($fields:tt)* } $($rest:tt)*) => {
+        $(#[$outer])*
+        pub struct $type { $($fields)* }
+        function!(@meta $type | $($rest)*);
+    };
+
+    // Parse a metadata type definition.
+    (@meta $type:ident | type Meta = $meta:ty; $($rest:tt)*) => {
+        function!(@parse $type $meta | $($rest)*);
+    };
+
+    // Set the metadata to `()` if there is not type definition.
+    (@meta $type:ident | $($rest:tt)*) => {
+        function!(@parse $type () | $($rest)*);
+    };
+
+    // Parse a `parse(default)`.
+    (@parse $type:ident $meta:ty | parse(default) $($rest:tt)*) => {
+        function!(@parse $type $meta |
+            parse(_args, _body, _ctx, _meta) { Default::default() }
+            $($rest)*
+        );
+    };
+
+    // (0-arg) Parse a parse-definition without arguments.
+    (@parse $type:ident $meta:ty | parse() $code:block $($rest:tt)*) => {
+        function!(@parse $type $meta | parse(_args, _body, _ctx, _meta) $code $($rest)*);
+    };
+
+    // (1-arg) Parse a parse-definition with only the first argument.
+    (@parse $type:ident $meta:ty | parse($args:ident) $code:block $($rest:tt)*) => {
+        function!(@parse $type $meta | parse($args, _body, _ctx, _meta) $code $($rest)*);
+    };
+
+    // (2-arg) Parse a parse-definition with only the first two arguments.
+    (@parse $type:ident $meta:ty |
+        parse($args:ident, $body:pat) $code:block $($rest:tt)*
+    ) => {
+        function!(@parse $type $meta | parse($args, $body, _ctx, _meta) $code $($rest)*);
+    };
+
+    // (3-arg) Parse a parse-definition with only the first three arguments.
+    (@parse $type:ident $meta:ty |
+        parse($args:ident, $body:pat, $ctx:pat) $code:block $($rest:tt)*
+    ) => {
+        function!(@parse $type $meta | parse($args, $body, $ctx, _meta) $code $($rest)*);
+    };
+
+    // (4-arg) Parse a parse-definition with all four arguments.
+    (@parse $type:ident $meta:ty |
+        parse($args:ident, $body:pat, $ctx:pat, $metadata:pat) $code:block
+        $($rest:tt)*
+    ) => {
+        impl $crate::func::ParseFunc for $type {
+            type Meta = $meta;
+
+            fn parse(
+                header: &FuncHeader,
+                $body: Option<&str>,
+                $ctx: ParseContext,
+                $metadata: Self::Meta,
+            ) -> ParseResult<Self> where Self: Sized {
+                let mut $args = $crate::func::args::ArgParser::new(&header.args);
+                let val = $code;
+                $args.done()?;
+                Ok(val)
+            }
+        }
+
+        function!(@layout $type | $($rest)*);
+    };
+
+    // (0-arg) Parse a layout-definition without arguments.
+    (@layout $type:ident | layout() $code:block) => {
+        function!(@layout $type | layout(self, _ctx) $code);
+    };
+
+    // (1-arg) Parse a layout-definition with only the first argument.
+    (@layout $type:ident | layout($this:ident) $code:block) => {
+        function!(@layout $type | layout($this, _ctx) $code);
+    };
+
+    // (2-arg) Parse a layout-definition with all arguments.
+    (@layout $type:ident | layout($this:ident, $ctx:pat) $code:block) => {
+        impl $crate::func::LayoutFunc for $type {
+            fn layout(&$this, $ctx: LayoutContext) -> LayoutResult<Commands> {
+                Ok($code)
+            }
+        }
+    };
+}
+
+/// Parse the body of a function.
+/// - If the function does not expect a body, use `parse!(forbidden: body)`.
+/// - If the function can have a body, use `parse!(optional: body, ctx)`.
+/// - If the function must have a body, use `parse!(expected: body, ctx)`.
+#[macro_export]
+macro_rules! parse {
+    (forbidden: $body:expr) => {
+        if $body.is_some() {
+            pr!("unexpected body");
+        }
+    };
+
+    (optional: $body:expr, $ctx:expr) => (
+        if let Some(body) = $body {
+            Some($crate::syntax::parse(body, $ctx)?)
+        } else {
+            None
+        }
+    );
+
+    (expected: $body:expr, $ctx:expr) => (
+        if let Some(body) = $body {
+            $crate::syntax::parse(body, $ctx)?
+        } else {
+            pr!("expected body");
+        }
+    )
+}
+
+/// Early-return with a formatted parsing error or yield
+/// an error expression without returning when prefixed with `@`.
+#[macro_export]
+macro_rules! pr {
+    (@$($tts:tt)*) => ($crate::syntax::ParseError::new(format!($($tts)*)));
+    ($($tts:tt)*) => (return Err(pr!(@$($tts)*)););
+}
+
+/// Early-return with a formatted layouting error or yield
+/// an error expression without returning when prefixed with `@`.
+#[macro_export]
+macro_rules! lr {
+    (@$($tts:tt)*) => ($crate::layout::LayoutError::new(format!($($tts)*)));
+    ($($tts:tt)*) => (return Err(lr!(@$($tts)*)););
+}
diff --git a/src/func/mod.rs b/src/func/mod.rs
index 33a6c756..b16eecb8 100644
--- a/src/func/mod.rs
+++ b/src/func/mod.rs
@@ -7,53 +7,64 @@ use std::fmt::{self, Debug, Formatter};
 use self::prelude::*;
 
 #[macro_use]
-pub mod helpers;
+pub mod macros;
+pub mod args;
 
 /// Useful imports for creating your own functions.
 pub mod prelude {
-    pub use crate::func::{Command, CommandList, Function};
-    pub use crate::layout::{layout_tree, Layout, MultiLayout, LayoutContext};
-    pub use crate::layout::{LayoutSpace, LayoutSpaces, SpacingKind};
-    pub use crate::layout::{LayoutAxes, Axis, AxisKind, LayoutAlignment, Alignment};
-    pub use crate::layout::{LayoutError, LayoutResult};
+    pub use Command::*;
+    pub use super::args::*;
+    pub use super::{Scope, ParseFunc, LayoutFunc, Command, Commands};
     pub use crate::syntax::{SyntaxTree, FuncHeader, FuncArgs, Expression, Spanned, Span};
     pub use crate::syntax::{parse, ParseContext, ParseError, ParseResult};
     pub use crate::size::{Size, Size2D, SizeBox};
     pub use crate::style::{PageStyle, TextStyle};
-    pub use super::helpers::*;
-    pub use Command::*;
+    pub use crate::layout::{
+        layout_tree, Layout, MultiLayout,
+        LayoutContext, LayoutSpace, LayoutSpaces,
+        LayoutAxes, Axis, GenericAxisKind, SpecificAxisKind,
+        LayoutAlignment, Alignment,
+        SpacingKind,
+        LayoutError, LayoutResult,
+    };
 }
 
-/// Typesetting function types.
-///
-/// These types have to be able to parse themselves from a string and build
-/// a list of layouting commands corresponding to the parsed source.
-///
-/// This trait is a supertrait of `FunctionBounds` for technical reasons.  The
-/// trait `FunctionBounds` is automatically implemented for types which can
-/// be used as functions, that is, all types which fulfill the bounds `Debug + PartialEq +
-/// 'static`.
-pub trait Function: FunctionBounds {
+/// Types representing functions that are parsed from source code.
+pub trait ParseFunc {
+    type Meta;
+
     /// Parse the header and body into this function given a context.
-    fn parse(header: &FuncHeader, body: Option<&str>, ctx: ParseContext) -> ParseResult<Self>
-    where Self: Sized;
+    fn parse(
+        header: &FuncHeader,
+        body: Option<&str>,
+        ctx: ParseContext,
+        metadata: Self::Meta,
+    ) -> ParseResult<Self> where Self: Sized;
+}
 
-    /// Layout this function given a context.
+/// Types representing functions which can be laid out in a layout context.
+///
+/// This trait is a supertrait of `[LayoutFuncBounds]` for technical reasons.
+/// The trait `[LayoutFuncBounds]` is automatically implemented for types which
+/// can be used as functions, that is, all types which fulfill the bounds `Debug
+/// + PartialEq + 'static`.
+pub trait LayoutFunc: LayoutFuncBounds {
+    /// Layout this function in a given context.
     ///
-    /// Returns optionally the resulting layout and a new context if changes to
-    /// the context should be made.
-    fn layout(&self, ctx: LayoutContext) -> LayoutResult<CommandList>;
+    /// Returns a sequence of layouting commands which describe what the
+    /// function is doing.
+    fn layout(&self, ctx: LayoutContext) -> LayoutResult<Commands>;
 }
 
-impl dyn Function {
-    /// Downcast a dynamic function to a concrete function type.
-    pub fn downcast<F>(&self) -> Option<&F> where F: Function + 'static {
+impl dyn LayoutFunc {
+    /// Downcast a function trait object to a concrete function type.
+    pub fn downcast<F>(&self) -> Option<&F> where F: LayoutFunc + 'static {
         self.help_cast_as_any().downcast_ref::<F>()
     }
 }
 
-impl PartialEq for dyn Function {
-    fn eq(&self, other: &dyn Function) -> bool {
+impl PartialEq for dyn LayoutFunc {
+    fn eq(&self, other: &dyn LayoutFunc) -> bool {
         self.help_eq(other)
     }
 }
@@ -63,22 +74,20 @@ impl PartialEq for dyn Function {
 ///
 /// Automatically implemented for all types which fulfill to the bounds `Debug +
 /// PartialEq + 'static`. There should be no need to implement this manually.
-pub trait FunctionBounds: Debug {
+pub trait LayoutFuncBounds: Debug {
     /// Cast self into `Any`.
     fn help_cast_as_any(&self) -> &dyn Any;
 
-    /// Compare self with another function.
-    fn help_eq(&self, other: &dyn Function) -> bool;
+    /// Compare self with another function trait object.
+    fn help_eq(&self, other: &dyn LayoutFunc) -> bool;
 }
 
-impl<T> FunctionBounds for T
-where T: Debug + PartialEq + 'static
-{
+impl<T> LayoutFuncBounds for T where T: Debug + PartialEq + 'static {
     fn help_cast_as_any(&self) -> &dyn Any {
         self
     }
 
-    fn help_eq(&self, other: &dyn Function) -> bool {
+    fn help_eq(&self, other: &dyn LayoutFunc) -> bool {
         if let Some(other) = other.help_cast_as_any().downcast_ref::<Self>() {
             self == other
         } else {
@@ -87,17 +96,17 @@ where T: Debug + PartialEq + 'static
     }
 }
 
-/// A sequence of commands requested for execution by a function.
-pub type CommandList<'a> = Vec<Command<'a>>;
+/// A sequence of layouting commands.
+pub type Commands<'a> = Vec<Command<'a>>;
 
-/// Commands requested for execution by functions.
+/// Layouting commands from functions to the typesetting engine.
 #[derive(Debug)]
 pub enum Command<'a> {
     LayoutTree(&'a SyntaxTree),
 
     Add(Layout),
     AddMultiple(MultiLayout),
-    AddSpacing(Size, SpacingKind, AxisKind),
+    AddSpacing(Size, SpacingKind, GenericAxisKind),
 
     FinishLine,
     FinishRun,
@@ -110,13 +119,18 @@ pub enum Command<'a> {
     SetAxes(LayoutAxes),
 }
 
-/// A map from identifiers to functions.
+/// A map from identifiers to function parsers.
 pub struct Scope {
-    parsers: HashMap<String, Box<ParseFunc>>,
+    parsers: HashMap<String, Box<Parser>>,
 }
 
-/// A function which parses a function invocation into a function type.
-type ParseFunc = dyn Fn(&FuncHeader, Option<&str>, ParseContext) -> ParseResult<Box<dyn Function>>;
+/// A function which parses the source of a function into a function type which
+/// implements [`LayoutFunc`].
+type Parser = dyn Fn(
+    &FuncHeader,
+    Option<&str>,
+    ParseContext
+) -> ParseResult<Box<dyn LayoutFunc>>;
 
 impl Scope {
     /// Create a new empty scope.
@@ -131,16 +145,27 @@ impl Scope {
         crate::library::std()
     }
 
-    /// Add a function type to the scope giving it a name.
-    pub fn add<F: Function + 'static>(&mut self, name: &str) {
+    /// Associate the given name with a type that is parseable into a function.
+    pub fn add<F>(&mut self, name: &str)
+    where F: ParseFunc<Meta=()> + LayoutFunc + 'static {
+        self.add_with_metadata::<F, ()>(name, ());
+    }
+
+    /// Add a parseable type with additional metadata  that is given to the
+    /// parser (other than the default of `()`).
+    pub fn add_with_metadata<F, T>(&mut self, name: &str, metadata: T)
+    where F: ParseFunc<Meta=T> + LayoutFunc + 'static, T: 'static {
         self.parsers.insert(
             name.to_owned(),
-            Box::new(|h, b, c| F::parse(h, b, c).map(|func| Box::new(func) as Box<dyn Function>)),
+            Box::new(|h, b, c| {
+                F::parse(h, b, c, metadata)
+                    .map(|f| Box::new(f) as Box<dyn LayoutFunc>)
+            })
         );
     }
 
     /// Return the parser with the given name if there is one.
-    pub(crate) fn get_parser(&self, name: &str) -> Option<&ParseFunc> {
+    pub(crate) fn get_parser(&self, name: &str) -> Option<&Parser> {
         self.parsers.get(name).map(|x| &**x)
     }
 }
diff --git a/src/layout/flex.rs b/src/layout/flex.rs
index afca23ec..3e8a64e1 100644
--- a/src/layout/flex.rs
+++ b/src/layout/flex.rs
@@ -14,7 +14,7 @@ pub struct FlexLayouter {
 #[derive(Debug, Clone)]
 enum FlexUnit {
     Boxed(Layout),
-    Space(Size, SpaceKind),
+    Space(Size, SpacingKind),
     SetAxes(LayoutAxes),
     Break,
 }
@@ -102,11 +102,11 @@ impl FlexLayouter {
         self.units.push(FlexUnit::Break);
     }
 
-    pub fn add_primary_space(&mut self, space: Size, kind: SpaceKind) {
+    pub fn add_primary_space(&mut self, space: Size, kind: SpacingKind) {
         self.units.push(FlexUnit::Space(space, kind))
     }
 
-    pub fn add_secondary_space(&mut self, space: Size, kind: SpaceKind) -> LayoutResult<()> {
+    pub fn add_secondary_space(&mut self, space: Size, kind: SpacingKind) -> LayoutResult<()> {
         if !self.run_is_empty() {
             self.finish_run()?;
         }
@@ -179,7 +179,7 @@ impl FlexLayouter {
             debug_render: false,
         })?;
 
-        self.stack.add_spacing(self.flex_spacing, SpaceKind::Independent);
+        self.stack.add_spacing(self.flex_spacing, SpacingKind::Independent);
 
         let remaining = self.axes.specialize(Size2D {
             x: self.part.usable
@@ -230,7 +230,7 @@ impl FlexLayouter {
 
             while size.x > self.line.usable {
                 if self.stack.space_is_last() {
-                    lerr!("box does not fit into line");
+                    lr!("box does not fit into line");
                 }
 
                 self.stack.finish_space(true);
@@ -238,7 +238,7 @@ impl FlexLayouter {
         }
 
         if let LastSpacing::Soft(space) = self.part.space {
-            self.layout_space(space, SpaceKind::Hard);
+            self.layout_space(space, SpacingKind::Hard);
         }
 
         let offset = self.part.dimensions.x;
@@ -251,8 +251,8 @@ impl FlexLayouter {
         Ok(())
     }
 
-    fn layout_space(&mut self, space: Size, kind: SpaceKind) {
-        if kind == SpaceKind::Soft {
+    fn layout_space(&mut self, space: Size, kind: SpacingKind) {
+        if kind == SpacingKind::Soft {
             if self.part.space != LastSpacing::Forbidden {
                 self.part.space = LastSpacing::Soft(space);
             }
@@ -263,7 +263,7 @@ impl FlexLayouter {
                 self.part.dimensions.x += space;
             }
 
-            if kind == SpaceKind::Hard {
+            if kind == SpacingKind::Hard {
                 self.part.space = LastSpacing::Forbidden;
             }
         }
diff --git a/src/layout/mod.rs b/src/layout/mod.rs
index f45ed10a..4304e46e 100644
--- a/src/layout/mod.rs
+++ b/src/layout/mod.rs
@@ -102,7 +102,7 @@ impl LayoutSpace {
 }
 
 /// The axes along which the content is laid out.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
 pub struct LayoutAxes {
     pub primary: Axis,
     pub secondary: Axis,
@@ -136,17 +136,66 @@ impl LayoutAxes {
         // at the call site, we still have this second function.
         self.generalize(size)
     }
-}
 
-/// The two kinds of axes.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub enum AxisKind {
-    Primary,
-    Secondary,
+    /// Returns the generic axis kind which is the horizontal axis.
+    pub fn horizontal(&self) -> GenericAxisKind {
+        match self.primary.is_horizontal() {
+            true => GenericAxisKind::Primary,
+            false => GenericAxisKind::Secondary,
+        }
+    }
+
+    /// Returns the generic axis kind which is the vertical axis.
+    pub fn vertical(&self) -> GenericAxisKind {
+        self.horizontal().inv()
+    }
+
+    /// Returns the specific axis kind which is the primary axis.
+    pub fn primary(&self) -> SpecificAxisKind {
+        match self.primary.is_horizontal() {
+            true => SpecificAxisKind::Horizontal,
+            false => SpecificAxisKind::Vertical,
+        }
+    }
+
+    /// Returns the specific axis kind which is the secondary axis.
+    pub fn secondary(&self) -> SpecificAxisKind {
+        self.primary().inv()
+    }
+
+    /// Returns the generic alignment corresponding to left-alignment.
+    pub fn left(&self) -> Alignment {
+        let positive = match self.primary.is_horizontal() {
+            true => self.primary.is_positive(),
+            false => self.secondary.is_positive(),
+        };
+
+        if positive { Alignment::Origin } else { Alignment::End }
+    }
+
+    /// Returns the generic alignment corresponding to right-alignment.
+    pub fn right(&self) -> Alignment {
+        self.left().inv()
+    }
+
+    /// Returns the generic alignment corresponding to top-alignment.
+    pub fn top(&self) -> Alignment {
+        let positive = match self.primary.is_horizontal() {
+            true => self.secondary.is_positive(),
+            false => self.primary.is_positive(),
+        };
+
+        if positive { Alignment::Origin } else { Alignment::End }
+    }
+
+    /// Returns the generic alignment corresponding to bottom-alignment.
+    pub fn bottom(&self) -> Alignment {
+        self.top().inv()
+    }
 }
 
 /// Directions along which content is laid out.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
 pub enum Axis {
     LeftToRight,
     RightToLeft,
@@ -180,8 +229,42 @@ impl Axis {
     }
 }
 
+/// The two generic kinds of layouting axes.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum GenericAxisKind {
+    Primary,
+    Secondary,
+}
+
+impl GenericAxisKind {
+    /// The other axis.
+    pub fn inv(&self) -> GenericAxisKind {
+        match self {
+            GenericAxisKind::Primary => GenericAxisKind::Secondary,
+            GenericAxisKind::Secondary => GenericAxisKind::Primary,
+        }
+    }
+}
+
+/// The two specific kinds of layouting axes.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum SpecificAxisKind {
+    Horizontal,
+    Vertical,
+}
+
+impl SpecificAxisKind {
+    /// The other axis.
+    pub fn inv(&self) -> SpecificAxisKind {
+        match self {
+            SpecificAxisKind::Horizontal => SpecificAxisKind::Vertical,
+            SpecificAxisKind::Vertical => SpecificAxisKind::Horizontal,
+        }
+    }
+}
+
 /// The place to put a layout in a container.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
 pub struct LayoutAlignment {
     pub primary: Alignment,
     pub secondary: Alignment,
@@ -194,13 +277,24 @@ impl LayoutAlignment {
 }
 
 /// Where to align content.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
 pub enum Alignment {
     Origin,
     Center,
     End,
 }
 
+impl Alignment {
+    /// The inverse alignment.
+    pub fn inv(&self) -> Alignment {
+        match self {
+            Alignment::Origin => Alignment::End,
+            Alignment::Center => Alignment::Center,
+            Alignment::End => Alignment::Origin,
+        }
+    }
+}
+
 /// The specialized anchor position for an item with the given alignment in a
 /// container with a given size along the given axis.
 pub fn anchor(axis: Axis, size: Size, alignment: Alignment) -> Size {
@@ -213,7 +307,7 @@ pub fn anchor(axis: Axis, size: Size, alignment: Alignment) -> Size {
 }
 
 /// Whitespace between boxes with different interaction properties.
-#[derive(Debug, Copy, Clone, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum SpacingKind {
     /// A hard space consumes surrounding soft spaces and is always layouted.
     Hard,
diff --git a/src/layout/stack.rs b/src/layout/stack.rs
index 793c2044..64823b67 100644
--- a/src/layout/stack.rs
+++ b/src/layout/stack.rs
@@ -108,7 +108,7 @@ impl StackLayouter {
         // Find the first (sub-)space that fits the layout.
         while !self.sub.usable.fits(new_size) {
             if self.space_is_last() && self.space_is_empty() {
-                lerr!("box does not fit into stack");
+                lr!("box does not fit into stack");
             }
 
             self.finish_space(true);
diff --git a/src/layout/text.rs b/src/layout/text.rs
index 343127e3..514bfe92 100644
--- a/src/layout/text.rs
+++ b/src/layout/text.rs
@@ -116,6 +116,6 @@ impl<'a, 'p> TextLayouter<'a, 'p> {
             self.classes.pop();
         }
 
-        lerr!("no suitable font for character `{}`", c);
+        lr!("no suitable font for character `{}`", c);
     }
 }
diff --git a/src/layout/tree.rs b/src/layout/tree.rs
index efa0c7b7..5fe3b6fd 100644
--- a/src/layout/tree.rs
+++ b/src/layout/tree.rs
@@ -99,8 +99,8 @@ impl<'a, 'p> TreeLayouter<'a, 'p> {
             Add(layout) => self.flex.add(layout),
             AddMultiple(layouts) => self.flex.add_multiple(layouts),
             AddSpacing(space, kind, axis) => match axis {
-                AxisKind::Primary => self.flex.add_primary_space(space, kind),
-                AxisKind::Secondary => self.flex.add_secondary_space(space, kind)?,
+                GenericAxisKind::Primary => self.flex.add_primary_space(space, kind),
+                GenericAxisKind::Secondary => self.flex.add_secondary_space(space, kind)?,
             }
 
             FinishLine => self.flex.add_break(),
@@ -111,7 +111,7 @@ impl<'a, 'p> TreeLayouter<'a, 'p> {
             SetTextStyle(style) => self.style.text = style,
             SetPageStyle(style) => {
                 if !self.ctx.top_level {
-                    lerr!("page style cannot only be altered in the top-level context");
+                    lr!("page style cannot only be altered in the top-level context");
                 }
 
                 self.style.page = style;
diff --git a/src/library/align.rs b/src/library/align.rs
index 530120e7..14e329e3 100644
--- a/src/library/align.rs
+++ b/src/library/align.rs
@@ -1,155 +1,68 @@
 use crate::func::prelude::*;
 
-/// 📐 `align`: Aligns content in different ways.
-#[derive(Debug, PartialEq)]
-pub struct Align {
-    body: Option<SyntaxTree>,
-    positional_1: Option<AlignSpecifier>,
-    positional_2: Option<AlignSpecifier>,
-    primary: Option<AlignSpecifier>,
-    secondary: Option<AlignSpecifier>,
-    horizontal: Option<AlignSpecifier>,
-    vertical: Option<AlignSpecifier>,
-}
-
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-enum AlignSpecifier {
-    Origin,
-    Center,
-    End,
-    Left,
-    Right,
-    Top,
-    Bottom,
-}
-
 function! {
-    data: Align,
+    /// `align`: Aligns content along the layouting axes.
+    #[derive(Debug, PartialEq)]
+    pub struct Align {
+        body: Option<SyntaxTree>,
+        map: ArgMap<Key, AlignmentKey>,
+    }
 
     parse(args, body, ctx) {
-        let body = parse!(optional: body, ctx);
-
-        let mut align = Align {
-            body,
-            positional_1: None,
-            positional_2: None,
-            primary: None,
-            secondary: None,
-            horizontal: None,
-            vertical: None,
-        };
-
-        if let Some(arg) = args.get_pos_opt::<ArgIdent>()? {
-            align.positional_1 = Some(parse_align_specifier(arg)?);
+        let mut map = ArgMap::new();
+        map.put(Key::First, args.get_pos_opt::<ArgIdent>()?)?;
+        map.put(Key::Second, args.get_pos_opt::<ArgIdent>()?)?;
+
+        for arg in args.keys() {
+            let key = match arg.val.0.val {
+                "horizontal" => Key::Axis(AxisKey::Horizontal),
+                "vertical" => Key::Axis(AxisKey::Vertical),
+                "primary" => Key::Axis(AxisKey::Primary),
+                "secondary" => Key::Axis(AxisKey::Secondary),
+                _ => pr!("unexpected argument"),
+            };
+
+            let value = AlignmentKey::parse(arg.val.1.val)?;
+            map.add(key, value);
         }
 
-        if let Some(arg) = args.get_pos_opt::<ArgIdent>()? {
-            align.positional_2 = Some(parse_align_specifier(arg)?);
+        Align {
+            body: parse!(optional: body, ctx),
+            map,
         }
-
-        let mut parse_arg = |axis, target: &mut Option<AlignSpecifier>| {
-            Ok(if let Some(arg) = args.get_key_opt::<ArgIdent>(axis)? {
-                if target.is_none() {
-                    *target = Some(parse_align_specifier(arg)?);
-                } else {
-                    perr!("duplicate alignment specification for {} axis", axis);
-                }
-            })
-        };
-
-        parse_arg("primary", &mut align.primary)?;
-        parse_arg("secondary", &mut align.secondary)?;
-        parse_arg("horizontal", &mut align.horizontal)?;
-        parse_arg("vertical", &mut align.vertical)?;
-
-        args.done()?;
-
-        Ok(align)
     }
 
-    layout(this, ctx) {
-        let mut axes = ctx.axes;
-        let primary_horizontal = axes.primary.is_horizontal();
-
-        let mut primary = false;
-        let mut secondary = false;
-
-        let mut set_axis = |is_primary: bool, spec: Option<AlignSpecifier>| -> LayoutResult<()> {
-            if let Some(spec) = spec {
-                let (axis, was_set, name) = match is_primary {
-                    true => (&mut axes.primary, &mut primary, "primary"),
-                    false => (&mut axes.secondary, &mut secondary, "secondary"),
-                };
-
-                if *was_set {
-                    panic!("duplicate alignment for {} axis", name);
-                }
-
-                *was_set = true;
-
-                let horizontal = axis.is_horizontal();
-                let alignment = generic_alignment(spec, horizontal)?;
-
-                if axis.alignment == Alignment::End && alignment == Alignment::Origin {
-                    axis.expand = true;
-                }
-
-                axis.alignment = alignment;
-            }
-
-            Ok(())
-        };
-
-        if let Some(spec) = this.positional_1 {
-            let positional = generic_alignment(spec, primary_horizontal).is_ok();
-            set_axis(positional, this.positional_1)?;
+    layout(self, mut ctx) {
+        let axes = ctx.axes;
+        let basic = axes.primary.is_horizontal();
+
+        let map = self.map.dedup(|key, val| {
+            let axis = match key {
+                Key::First => val.axis(axes, GenericAxisKind::Primary),
+                Key::Second => val.axis(axes, GenericAxisKind::Secondary),
+                Key::Axis(AxisKey::Primary) => GenericAxisKind::Primary,
+                Key::Axis(AxisKey::Secondary) => GenericAxisKind::Secondary,
+                Key::Axis(AxisKey::Horizontal) => axes.horizontal(),
+                Key::Axis(AxisKey::Vertical) => axes.vertical(),
+            };
+
+            let alignment = val.generic(axes, axis)?;
+            Ok((key, alignment))
+        })?;
+
+        map.with(GenericAxisKind::Primary, |val| ctx.alignment.primary = val);
+        map.with(GenericAxisKind::Secondary, |val| ctx.alignment.secondary = val);
+
+        match &self.body {
+            Some(body) => vec![AddMultiple(layout_tree(&body, ctx)?)],
+            None => vec![Command::SetAlignment(ctx.alignment)],
         }
-
-        if let Some(spec) = this.positional_2 {
-            let positional = generic_alignment(spec, primary_horizontal).is_ok();
-            set_axis(positional, this.positional_2)?;
-        }
-
-        set_axis(true, this.primary)?;
-        set_axis(false, this.secondary)?;
-        set_axis(primary_horizontal, this.horizontal)?;
-        set_axis(!primary_horizontal, this.vertical)?;
-
-        Ok(match &this.body {
-            Some(body) => vec![AddMultiple(
-                layout_tree(body, LayoutContext {
-                    axes,
-                    .. ctx.clone()
-                })?
-            )],
-            None => vec![Command::SetAxes(axes)]
-        })
     }
 }
 
-fn parse_align_specifier(arg: Spanned<&str>) -> ParseResult<AlignSpecifier> {
-    Ok(match arg.val {
-        "origin" => AlignSpecifier::Origin,
-        "center" => AlignSpecifier::Center,
-        "end" => AlignSpecifier::End,
-        "left" => AlignSpecifier::Left,
-        "right" => AlignSpecifier::Right,
-        "top" => AlignSpecifier::Top,
-        "bottom" => AlignSpecifier::Bottom,
-        s => perr!("invalid alignment specifier: {}", s),
-    })
-}
-
-fn generic_alignment(spec: AlignSpecifier, horizontal: bool) -> LayoutResult<Alignment> {
-    use AlignSpecifier::*;
-    Ok(match (spec, horizontal) {
-        (Origin, _) | (Left, true) | (Top, false) => Alignment::Origin,
-        (Center, _) => Alignment::Center,
-        (End, _) | (Right, true) | (Bottom, false) => Alignment::End,
-        _ => lerr!(
-            "invalid alignment specifier `{}` for {} axis",
-            format!("{:?}", spec).to_lowercase(),
-            if horizontal { "horizontal" } else { "vertical" },
-        ),
-    })
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+enum Key {
+    First,
+    Second,
+    Axis(AxisKey),
 }
diff --git a/src/library/boxed.rs b/src/library/boxed.rs
index 8984417c..d4e39450 100644
--- a/src/library/boxed.rs
+++ b/src/library/boxed.rs
@@ -1,37 +1,44 @@
 use crate::func::prelude::*;
 
-/// `box`: Layouts content into a box.
-#[derive(Debug, PartialEq)]
-pub struct Boxed {
-    body: SyntaxTree,
-    width: Option<Size>,
-    height: Option<Size>,
-}
-
 function! {
-    data: Boxed,
+    /// `box`: Layouts content into a box.
+    #[derive(Debug, PartialEq)]
+    pub struct Boxed {
+        body: SyntaxTree,
+        map: ArgMap<AxisKey, Size>,
+    }
 
     parse(args, body, ctx) {
-        let width = args.get_key_opt::<ArgSize>("width")?.map(|a| a.val);
-        let height = args.get_key_opt::<ArgSize>("height")?.map(|a| a.val);
-        args.done()?;
-
-        let body = parse!(required: body, ctx);
-        Ok(Boxed {
-            body,
-            width,
-            height,
-        })
-    }
+        let mut map = ArgMap::new();
+
+        for arg in args.keys() {
+            let key = match arg.val.0.val {
+                "width" | "w" => AxisKey::Horizontal,
+                "height" | "h" => AxisKey::Vertical,
+                "primary-size" => AxisKey::Primary,
+                "secondary-size" => AxisKey::Secondary,
+                _ => pr!("unexpected argument"),
+            };
 
-    layout(this, mut ctx) {
-        if let Some(width) = this.width {
-            ctx.spaces[0].dimensions.x = width;
+            let size = ArgParser::convert::<ArgSize>(arg.val.1.val)?;
+            map.add(key, size);
         }
-        if let Some(height) = this.height {
-            ctx.spaces[0].dimensions.y = height;
+
+        Boxed {
+            body: parse!(expected: body, ctx),
+            map,
         }
+    }
+
+    layout(self, mut ctx) {
+        let map = self.map.dedup(|key, val| {
+            Ok((key.specific(ctx.axes), val))
+        });
+
+        let mut dimensions = &mut ctx.spaces[0].dimensions;
+        map.with(AxisKey::Horizontal, |val| dimensions.x = val);
+        map.with(AxisKey::Vertical, |val| dimensions.y = val);
 
-        Ok(vec![AddMultiple(layout_tree(&this.body, ctx)?)])
+        vec![AddMultiple(layout_tree(&self.body, ctx)?)]
     }
 }
diff --git a/src/library/mod.rs b/src/library/mod.rs
index adcb974b..b09303bb 100644
--- a/src/library/mod.rs
+++ b/src/library/mod.rs
@@ -1,34 +1,274 @@
 //! The standard library for the _Typst_ language.
 
-use crate::func::Scope;
+use crate::func::prelude::*;
+use toddle::query::FontClass;
 
-pub_use_mod!(boxed);
 pub_use_mod!(align);
-pub_use_mod!(spacing);
-pub_use_mod!(style);
-pub_use_mod!(page);
+pub_use_mod!(boxed);
 
 /// Create a scope with all standard functions.
 pub fn std() -> Scope {
     let mut std = Scope::new();
 
-    std.add::<Boxed>("box");
-
     std.add::<Align>("align");
+    std.add::<Boxed>("box");
+    std.add::<PageSize>("page.size");
+    std.add::<PageMargins>("page.margins");
 
     std.add::<LineBreak>("n");
     std.add::<LineBreak>("line.break");
-    std.add::<ParagraphBreak>("paragraph.break");
+    std.add::<ParBreak>("par.break");
     std.add::<PageBreak>("page.break");
-    std.add::<HorizontalSpace>("h");
-    std.add::<VerticalSpace>("v");
 
-    std.add::<Bold>("bold");
-    std.add::<Italic>("italic");
-    std.add::<Monospace>("mono");
+    std.add_with_metadata::<Spacing, Option<AxisKey>>("spacing", None);
+    for (name, key) in &[
+        ("h", AxisKey::Horizontal),
+        ("v", AxisKey::Vertical),
+    ] {
+        std.add_with_metadata::<Spacing, Option<AxisKey>>(name, Some(*key));
+    }
 
-    std.add::<PageSize>("page.size");
-    std.add::<PageMargins>("page.margins");
+    for (name, class) in &[
+        ("bold", FontClass::Bold),
+        ("italic", FontClass::Italic),
+        ("mono", FontClass::Monospace),
+    ] {
+        std.add_with_metadata::<StyleChange, FontClass>(name, *class);
+    }
 
     std
 }
+
+function! {
+    /// `line.break`, `n`: Ends the current line.
+    #[derive(Debug, Default, PartialEq)]
+    pub struct LineBreak;
+
+    parse(default)
+    layout() { vec![FinishLine] }
+}
+
+function! {
+    /// `par.break`: Ends the current paragraph.
+    ///
+    /// self has the same effect as two subsequent newlines.
+    #[derive(Debug, Default, PartialEq)]
+    pub struct ParBreak;
+
+    parse(default)
+    layout() { vec![BreakParagraph] }
+}
+
+
+function! {
+    /// `page.break`: Ends the current page.
+    #[derive(Debug, Default, PartialEq)]
+    pub struct PageBreak;
+
+    parse(default)
+    layout() { vec![FinishSpace] }
+}
+
+function! {
+    /// `page.size`: Set the size of pages.
+    #[derive(Debug, PartialEq)]
+    pub struct PageSize {
+        width: Option<Size>,
+        height: Option<Size>,
+    }
+
+    parse(args, body) {
+        parse!(forbidden: body);
+        PageSize {
+            width: args.get_key_opt::<ArgSize>("width")?.map(|a| a.val),
+            height: args.get_key_opt::<ArgSize>("height")?.map(|a| a.val),
+        }
+    }
+
+    layout(self, ctx) {
+        let mut style = ctx.style.page;
+        if let Some(width) = self.width { style.dimensions.x = width; }
+        if let Some(height) = self.height { style.dimensions.y = height; }
+        vec![SetPageStyle(style)]
+    }
+}
+
+function! {
+    /// `page.margins`: Set the margins of pages.
+    #[derive(Debug, PartialEq)]
+    pub struct PageMargins {
+        map: ArgMap<PaddingKey, Size>,
+    }
+
+    parse(args, body) {
+        use PaddingKey::*;
+        use AlignmentKey::*;
+
+        let mut map = ArgMap::new();
+        map.add_opt(All, args.get_pos_opt::<ArgSize>()?);
+
+        for arg in args.keys() {
+            let key = match arg.val.0.val {
+                "horizontal" => Axis(AxisKey::Horizontal),
+                "vertical" => Axis(AxisKey::Vertical),
+                "primary" => Axis(AxisKey::Primary),
+                "secondary" => Axis(AxisKey::Secondary),
+
+                "left" => AxisAligned(AxisKey::Horizontal, Left),
+                "right" => AxisAligned(AxisKey::Horizontal, Right),
+                "top" => AxisAligned(AxisKey::Vertical, Top),
+                "bottom" => AxisAligned(AxisKey::Vertical, Bottom),
+
+                "primary-origin" => AxisAligned(AxisKey::Primary, Origin),
+                "primary-end" => AxisAligned(AxisKey::Primary, End),
+                "secondary-origin" => AxisAligned(AxisKey::Secondary, Origin),
+                "secondary-end" => AxisAligned(AxisKey::Secondary, End),
+                "horizontal-origin" => AxisAligned(AxisKey::Horizontal, Origin),
+                "horizontal-end" => AxisAligned(AxisKey::Horizontal, End),
+                "vertical-origin" => AxisAligned(AxisKey::Vertical, Origin),
+                "vertical-end" => AxisAligned(AxisKey::Vertical, End),
+
+                _ => pr!("unexpected argument"),
+            };
+
+            let size = ArgParser::convert::<ArgSize>(arg.val.1.val)?;
+            map.add(key, size);
+        }
+
+        parse!(forbidden: body);
+        PageMargins { map }
+    }
+
+    layout(self, ctx) {
+        use PaddingKey::*;
+
+        let axes = ctx.axes;
+        let map = self.map.dedup(|key, val| {
+            match key {
+                All => All,
+                Axis(axis) => Axis(axis.specific(axes)),
+                AxisAligned(axis, alignment) => {
+                    let axis = axis.specific(axes);
+                    AxisAligned(axis, alignment.specific(axes, axis))
+                }
+            }
+        });
+
+        let style = ctx.style.page;
+        let padding = &mut style.margins;
+
+        map.with(All, |val| padding.set_all(val));
+        map.with(Axis(AxisKey::Horizontal), |val| padding.set_horizontal(val));
+        map.with(Axis(AxisKey::Vertical), |val| padding.set_vertical(val));
+
+        for (key, val) in map.iter() {
+            if let AxisAligned(axis, alignment) = key {
+                match alignment {
+                    AlignmentKey::Left => padding.left = val,
+                    AlignmentKey::Right => padding.right = val,
+                    AlignmentKey::Top => padding.top = val,
+                    AlignmentKey::Bottom => padding.bottom = val,
+                    _ => {},
+                }
+            }
+        }
+
+        vec![SetPageStyle(style)]
+    }
+}
+
+function! {
+    /// `spacing`, `h`, `v`: Add spacing along an axis.
+    #[derive(Debug, PartialEq)]
+    pub struct Spacing {
+        axis: AxisKey,
+        spacing: SpacingValue,
+    }
+
+    type Meta = Option<AxisKey>;
+
+    parse(args, body, _, meta) {
+        let spacing = if let Some(axis) = meta {
+            Spacing {
+                axis,
+                spacing: SpacingValue::from_expr(args.get_pos::<ArgExpr>()?)?,
+            }
+        } else {
+            if let Some(arg) = args.get_key_next() {
+                let axis = match arg.val.0.val {
+                    "horizontal" => AxisKey::Horizontal,
+                    "vertical" => AxisKey::Vertical,
+                    "primary" => AxisKey::Primary,
+                    "secondary" => AxisKey::Secondary,
+                    _ => pr!("unexpected argument"),
+                };
+
+                let spacing = SpacingValue::from_expr(arg.val.1.val)?;
+                Spacing { axis, spacing }
+            } else {
+                pr!("expected axis and expression")
+            }
+        };
+
+        parse!(forbidden: body);
+        spacing
+    }
+
+    layout(self, ctx) {
+        let axis = self.axis.generic(ctx.axes);
+        let spacing = match self.spacing {
+            SpacingValue::Absolute(s) => s,
+            SpacingValue::Relative(f) => f * ctx.style.text.font_size,
+        };
+
+        vec![AddSpacing(spacing, SpacingKind::Hard, axis)]
+    }
+}
+
+#[derive(Debug, PartialEq)]
+enum SpacingValue {
+    Absolute(Size),
+    Relative(f32),
+}
+
+impl SpacingValue {
+    fn from_expr(expr: Spanned<&Expression>) -> ParseResult<SpacingValue> {
+        Ok(match expr.val {
+            Expression::Size(s) => SpacingValue::Absolute(*s),
+            Expression::Num(f) => SpacingValue::Relative(*f as f32),
+            _ => pr!("invalid spacing: expected size or number"),
+        })
+    }
+}
+
+function! {
+    /// Sets text with a different style.
+    #[derive(Debug, PartialEq)]
+    pub struct StyleChange {
+        body: Option<SyntaxTree>,
+        class: FontClass,
+    }
+
+    type Meta = FontClass;
+
+    parse(args, body, ctx, meta) {
+        StyleChange {
+            body: parse!(optional: body, ctx),
+            class: meta,
+        }
+    }
+
+    layout(self, ctx) {
+        let mut style = ctx.style.text.clone();
+        style.toggle_class(self.class);
+
+        match &self.body {
+            Some(body) => vec![
+                SetTextStyle(style),
+                LayoutTree(body),
+                SetTextStyle(ctx.style.text.clone()),
+            ],
+            None => vec![SetTextStyle(style)]
+        }
+    }
+}
diff --git a/src/library/page.rs b/src/library/page.rs
deleted file mode 100644
index e8a80870..00000000
--- a/src/library/page.rs
+++ /dev/null
@@ -1,79 +0,0 @@
-use crate::func::prelude::*;
-
-/// `page.break`: Ends the current page.
-#[derive(Debug, PartialEq)]
-pub struct PageBreak;
-
-function! {
-    data: PageBreak,
-    parse: plain,
-    layout(_, _) { Ok(vec![FinishSpace]) }
-}
-
-/// `page.size`: Set the size of pages.
-#[derive(Debug, PartialEq)]
-pub struct PageSize {
-    width: Option<Size>,
-    height: Option<Size>,
-}
-
-function! {
-    data: PageSize,
-
-    parse(args, body, _ctx) {
-        parse!(forbidden: body);
-        Ok(PageSize {
-            width: args.get_key_opt::<ArgSize>("width")?.map(|a| a.val),
-            height: args.get_key_opt::<ArgSize>("height")?.map(|a| a.val),
-        })
-    }
-
-    layout(this, ctx) {
-        let mut style = ctx.style.page;
-
-        if let Some(width) = this.width { style.dimensions.x = width; }
-        if let Some(height) = this.height { style.dimensions.y = height; }
-
-        Ok(vec![SetPageStyle(style)])
-    }
-}
-
-/// `page.margins`: Set the margins of pages.
-#[derive(Debug, PartialEq)]
-pub struct PageMargins {
-    left: Option<Size>,
-    top: Option<Size>,
-    right: Option<Size>,
-    bottom: Option<Size>,
-}
-
-function! {
-    data: PageMargins,
-
-    parse(args, body, _ctx) {
-        parse!(forbidden: body);
-        let default = args.get_pos_opt::<ArgSize>()?;
-        let mut get = |which| {
-            args.get_key_opt::<ArgSize>(which)
-                .map(|size| size.or(default).map(|a| a.val))
-        };
-
-        Ok(PageMargins {
-            left: get("left")?,
-            top: get("top")?,
-            right: get("right")?,
-            bottom: get("bottom")?,
-        })
-    }
-
-    layout(this, ctx) {
-        let mut style = ctx.style.page;
-
-        if let Some(left) = this.left { style.margins.left = left; }
-        if let Some(top) = this.top { style.margins.top = top; }
-        if let Some(right) = this.right { style.margins.right = right; }
-        if let Some(bottom) = this.bottom { style.margins.bottom = bottom; }
-
-        Ok(vec![SetPageStyle(style)])
-    }
-}
diff --git a/src/library/spacing.rs b/src/library/spacing.rs
deleted file mode 100644
index 47fe9fff..00000000
--- a/src/library/spacing.rs
+++ /dev/null
@@ -1,71 +0,0 @@
-use crate::func::prelude::*;
-
-/// `line.break`, `n`: Ends the current line.
-#[derive(Debug, PartialEq)]
-pub struct LineBreak;
-
-function! {
-    data: LineBreak,
-    parse: plain,
-    layout(_, _) { Ok(vec![FinishLine]) }
-}
-
-/// `paragraph.break`: Ends the current paragraph.
-///
-/// This has the same effect as two subsequent newlines.
-#[derive(Debug, PartialEq)]
-pub struct ParagraphBreak;
-
-function! {
-    data: ParagraphBreak,
-    parse: plain,
-    layout(_, _) { Ok(vec![BreakParagraph]) }
-}
-
-macro_rules! space_func {
-    ($ident:ident, $doc:expr, $var:ident => $command:expr) => (
-        #[doc = $doc]
-        #[derive(Debug, PartialEq)]
-        pub struct $ident(Spacing);
-
-        function! {
-            data: $ident,
-
-            parse(args, body, _ctx) {
-                parse!(forbidden: body);
-
-                let arg = args.get_pos::<ArgExpr>()?;
-                let spacing = match arg.val {
-                    Expression::Size(s) => Spacing::Absolute(*s),
-                    Expression::Num(f) => Spacing::Relative(*f as f32),
-                    _ => perr!("invalid spacing, expected size or number"),
-                };
-
-                Ok($ident(spacing))
-            }
-
-            layout(this, ctx) {
-                let $var = match this.0 {
-                    Spacing::Absolute(s) => s,
-                    Spacing::Relative(f) => f * ctx.style.text.font_size,
-                };
-
-                Ok(vec![$command])
-            }
-        }
-    );
-}
-
-/// Absolute or font-relative spacing.
-#[derive(Debug, PartialEq)]
-enum Spacing {
-    Absolute(Size),
-    Relative(f32),
-}
-
-// FIXME: h != primary and v != secondary.
-space_func!(HorizontalSpace, "📖 `h`: Adds horizontal whitespace.",
-    space => AddSpacing(space, SpacingKind::Hard, AxisKind::Primary));
-
-space_func!(VerticalSpace, "📑 `v`: Adds vertical whitespace.",
-    space => AddSpacing(space, SpacingKind::Hard, AxisKind::Secondary));
diff --git a/src/library/style.rs b/src/library/style.rs
deleted file mode 100644
index b2de19bb..00000000
--- a/src/library/style.rs
+++ /dev/null
@@ -1,39 +0,0 @@
-use crate::func::prelude::*;
-use toddle::query::FontClass;
-
-macro_rules! stylefunc {
-    ($ident:ident, $doc:expr) => (
-        #[doc = $doc]
-        #[derive(Debug, PartialEq)]
-        pub struct $ident {
-            body: Option<SyntaxTree>
-        }
-
-        function! {
-            data: $ident,
-
-            parse(args, body, ctx) {
-                args.done()?;
-                Ok($ident { body: parse!(optional: body, ctx) })
-            }
-
-            layout(this, ctx) {
-                let mut style = ctx.style.text.clone();
-                style.toggle_class(FontClass::$ident);
-
-                Ok(match &this.body {
-                    Some(body) => vec![
-                        SetTextStyle(style),
-                        LayoutTree(body),
-                        SetTextStyle(ctx.style.text.clone()),
-                    ],
-                    None => vec![SetTextStyle(style)]
-                })
-            }
-        }
-    );
-}
-
-stylefunc!(Italic, "`italic`: Sets text in _italics_.");
-stylefunc!(Bold, "`bold`: Sets text in **bold**.");
-stylefunc!(Monospace, "`mono`: Sets text in `monospace`.");
diff --git a/src/size.rs b/src/size.rs
index 50b3ace5..a9c3198c 100644
--- a/src/size.rs
+++ b/src/size.rs
@@ -51,25 +51,19 @@ impl Size {
     /// Create a size from an amount of millimeters.
     #[inline]
     pub fn mm(mm: f32) -> Size {
-        Size {
-            points: 2.83465 * mm,
-        }
+        Size { points: 2.83465 * mm }
     }
 
     /// Create a size from an amount of centimeters.
     #[inline]
     pub fn cm(cm: f32) -> Size {
-        Size {
-            points: 28.3465 * cm,
-        }
+        Size { points: 28.3465 * cm }
     }
 
     /// Create a size from an amount of inches.
     #[inline]
     pub fn inches(inches: f32) -> Size {
-        Size {
-            points: 72.0 * inches,
-        }
+        Size { points: 72.0 * inches }
     }
 
     /// Convert this size into points.
@@ -188,12 +182,33 @@ impl SizeBox {
 
     /// Create a box with all four fields set to the same value `s`.
     #[inline]
-    pub fn with_all(s: Size) -> SizeBox {
-        SizeBox { left: s, top: s, right: s, bottom: s }
+    pub fn with_all(value: Size) -> SizeBox {
+        SizeBox { left: value, top: value, right: value, bottom: value }
+    }
+
+    /// Set the `left` and `right` values.
+    #[inline]
+    pub fn set_all(&mut self, value: Size) {
+        *self = SizeBox::with_all(value);
+    }
+
+    /// Set the `left` and `right` values.
+    #[inline]
+    pub fn set_horizontal(&mut self, value: Size) {
+        self.left = value;
+        self.right = value;
+    }
+
+    /// Set the `top` and `bottom` values.
+    #[inline]
+    pub fn set_vertical(&mut self, value: Size) {
+        self.top = value;
+        self.bottom = value;
     }
 }
 
 /// The maximum of two sizes.
+#[inline]
 pub fn max(a: Size, b: Size) -> Size {
     if a >= b {
         a
@@ -203,6 +218,7 @@ pub fn max(a: Size, b: Size) -> Size {
 }
 
 /// The minimum of two sizes.
+#[inline]
 pub fn min(a: Size, b: Size) -> Size {
     if a <= b {
         a
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index 9406fdf4..33413317 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -2,13 +2,15 @@
 
 use std::fmt::{self, Display, Formatter};
 
-use crate::func::Function;
+use crate::func::LayoutFunc;
 use crate::size::Size;
 
 mod tokens;
 #[macro_use]
 mod parsing;
+mod span;
 
+pub use span::{Span, Spanned};
 pub use tokens::{tokenize, Tokens};
 pub use parsing::{parse, ParseContext, ParseError, ParseResult};
 
@@ -90,7 +92,13 @@ pub enum Node {
 #[derive(Debug)]
 pub struct FuncCall {
     pub header: Spanned<FuncHeader>,
-    pub body: Spanned<Box<dyn Function>>,
+    pub body: Spanned<Box<dyn LayoutFunc>>,
+}
+
+impl PartialEq for FuncCall {
+    fn eq(&self, other: &FuncCall) -> bool {
+        (self.header == other.header) && (&self.body == &other.body)
+    }
 }
 
 /// Contains header information of a function invocation.
@@ -134,12 +142,6 @@ pub enum Expression {
     Bool(bool),
 }
 
-impl PartialEq for FuncCall {
-    fn eq(&self, other: &FuncCall) -> bool {
-        (self.header == other.header) && (&self.body == &other.body)
-    }
-}
-
 impl Display for Expression {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         use Expression::*;
@@ -154,72 +156,3 @@ impl Display for Expression {
 }
 
 debug_display!(Expression);
-
-/// Annotates a value with the part of the source code it corresponds to.
-#[derive(Copy, Clone, Eq, PartialEq)]
-pub struct Spanned<T> {
-    pub val: T,
-    pub span: Span,
-}
-
-impl<T> Spanned<T> {
-    pub fn new(val: T, span: Span) -> Spanned<T> {
-        Spanned { val, span }
-    }
-
-    pub fn value(self) -> T {
-        self.val
-    }
-
-    pub fn span_map<F, U>(self, f: F) -> Spanned<U> where F: FnOnce(T) -> U {
-        Spanned::new(f(self.val), self.span)
-    }
-}
-
-impl<T> Display for Spanned<T> where T: std::fmt::Debug {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "({:?}:{})", self.val, self.span)
-    }
-}
-
-debug_display!(Spanned; T where T: std::fmt::Debug);
-
-/// Describes a slice of source code.
-#[derive(Copy, Clone, Eq, PartialEq)]
-pub struct Span {
-    pub start: usize,
-    pub end: usize,
-}
-
-impl Span {
-    pub fn new(start: usize, end: usize) -> Span {
-        Span { start, end }
-    }
-
-    pub fn merge(a: Span, b: Span) -> Span {
-        Span {
-            start: a.start.min(b.start),
-            end: a.end.max(b.end),
-        }
-    }
-
-    pub fn at(index: usize) -> Span {
-        Span { start: index, end: index + 1 }
-    }
-
-    pub fn pair(&self) -> (usize, usize) {
-        (self.start, self.end)
-    }
-
-    pub fn expand(&mut self, other: Span) {
-        *self = Span::merge(*self, other)
-    }
-}
-
-impl Display for Span {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "[{}, {}]", self.start, self.end)
-    }
-}
-
-debug_display!(Span);
diff --git a/src/syntax/parsing.rs b/src/syntax/parsing.rs
index 77697161..b56094e1 100644
--- a/src/syntax/parsing.rs
+++ b/src/syntax/parsing.rs
@@ -2,7 +2,7 @@
 
 use unicode_xid::UnicodeXID;
 
-use crate::func::{Function, Scope};
+use crate::func::{LayoutFunc, Scope};
 use crate::size::Size;
 use super::*;
 
@@ -120,10 +120,10 @@ impl<'s> Parser<'s> {
                 if is_identifier(word) {
                     Ok(Spanned::new(word.to_owned(), span))
                 } else {
-                    perr!("invalid identifier: '{}'", word);
+                    pr!("invalid identifier: '{}'", word);
                 }
             }
-            _ => perr!("expected identifier"),
+            _ => pr!("expected identifier"),
         }?;
 
         self.skip_white();
@@ -132,7 +132,7 @@ impl<'s> Parser<'s> {
         let args = match self.tokens.next().map(Spanned::value) {
             Some(Token::RightBracket) => FuncArgs::new(),
             Some(Token::Colon) => self.parse_func_args()?,
-            _ => perr!("expected arguments or closing bracket"),
+            _ => pr!("expected arguments or closing bracket"),
         };
 
         let end = self.tokens.string_index();
@@ -158,7 +158,7 @@ impl<'s> Parser<'s> {
             match self.tokens.next().map(Spanned::value) {
                 Some(Token::Comma) => {},
                 Some(Token::RightBracket) => break,
-                _ => perr!("expected comma or closing bracket"),
+                _ => pr!("expected comma or closing bracket"),
             }
         }
 
@@ -183,7 +183,7 @@ impl<'s> Parser<'s> {
                         self.skip_white();
 
                         let name = token.span_map(|_| name.to_string());
-                        let next = self.tokens.next().ok_or_else(|| perr!(@"expected value"))?;
+                        let next = self.tokens.next().ok_or_else(|| pr!(@"expected value"))?;
                         let val = Self::parse_expression(next)?;
                         let span = Span::merge(name.span, val.span);
 
@@ -219,18 +219,19 @@ impl<'s> Parser<'s> {
                 }
             }
 
-            _ => perr!("expected expression"),
+            _ => pr!("expected expression"),
         }, token.span))
     }
 
     /// Parse the body of a function.
-    fn parse_func_body(&mut self, header: &FuncHeader) -> ParseResult<Spanned<Box<dyn Function>>> {
+    fn parse_func_body(&mut self, header: &FuncHeader)
+    -> ParseResult<Spanned<Box<dyn LayoutFunc>>> {
         // Now we want to parse this function dynamically.
         let parser = self
             .ctx
             .scope
             .get_parser(&header.name.val)
-            .ok_or_else(|| perr!(@"unknown function: '{}'", &header.name.val))?;
+            .ok_or_else(|| pr!(@"unknown function: '{}'", &header.name.val))?;
 
         let has_body = self.tokens.peek().map(Spanned::value) == Some(Token::LeftBracket);
 
@@ -298,7 +299,7 @@ impl<'s> Parser<'s> {
                     state = NewlineState::Zero;
                     match token.val {
                         Token::LineComment(_) | Token::BlockComment(_) => self.advance(),
-                        Token::StarSlash => perr!("unexpected end of block comment"),
+                        Token::StarSlash => pr!("unexpected end of block comment"),
                         _ => break,
                     }
                 }
@@ -454,7 +455,7 @@ mod tests {
     #![allow(non_snake_case)]
 
     use super::*;
-    use crate::func::{CommandList, Function, Scope};
+    use crate::func::{Commands, Scope};
     use crate::layout::{LayoutContext, LayoutResult};
     use funcs::*;
     use Node::{Func as F, Newline as N, Space as S};
@@ -464,37 +465,36 @@ mod tests {
     mod funcs {
         use super::*;
 
-        /// A testing function which just parses it's body into a syntax tree.
-        #[derive(Debug)]
-        pub struct TreeFn(pub SyntaxTree);
-
         function! {
-            data: TreeFn,
+            /// A testing function which just parses it's body into a syntax
+            /// tree.
+            #[derive(Debug)]
+            pub struct TreeFn { pub tree: SyntaxTree }
+
+            parse(args, body, ctx) {
+                args.clear();
+                TreeFn {
+                    tree: parse!(expected: body, ctx)
+                }
+            }
 
-            parse(_args, body, ctx) { Ok(TreeFn(parse!(required: body, ctx))) }
-            layout(_, _) { Ok(vec![]) }
+            layout() { vec![] }
         }
 
         impl PartialEq for TreeFn {
             fn eq(&self, other: &TreeFn) -> bool {
-                assert_tree_equal(&self.0, &other.0);
+                assert_tree_equal(&self.tree, &other.tree);
                 true
             }
         }
 
-        /// A testing function without a body.
-        #[derive(Debug)]
-        pub struct BodylessFn;
-
         function! {
-            data: BodylessFn,
-
-            parse(_args, body, _ctx) { parse!(forbidden: body); Ok(BodylessFn) }
-            layout(_, _) { Ok(vec![]) }
-        }
+            /// A testing function without a body.
+            #[derive(Debug, Default, PartialEq)]
+            pub struct BodylessFn;
 
-        impl PartialEq for BodylessFn {
-            fn eq(&self, _: &BodylessFn) -> bool { true }
+            parse(default)
+            layout() { vec![] }
         }
     }
 
@@ -583,7 +583,7 @@ mod tests {
             func!(@$name, Box::new(BodylessFn), FuncArgs::new())
         );
         (name => $name:expr, body => $tree:expr $(,)*) => (
-            func!(@$name, Box::new(TreeFn($tree)), FuncArgs::new())
+            func!(@$name, Box::new(TreeFn { tree: $tree }), FuncArgs::new())
         );
         (@$name:expr, $body:expr, $args:expr) => (
             FuncCall {
@@ -789,7 +789,7 @@ mod tests {
         assert_eq!(func.header.val.args.positional[0].span.pair(), (13, 16));
         assert_eq!(func.header.val.args.positional[1].span.pair(), (18, 23));
 
-        let body = &func.body.val.downcast::<TreeFn>().unwrap().0.nodes;
+        let body = &func.body.val.downcast::<TreeFn>().unwrap().tree.nodes;
         assert_eq!(func.body.span.pair(), (24, 37));
         assert_eq!(body[0].span.pair(), (0, 4));
         assert_eq!(body[1].span.pair(), (4, 5));
diff --git a/src/syntax/span.rs b/src/syntax/span.rs
new file mode 100644
index 00000000..aa494224
--- /dev/null
+++ b/src/syntax/span.rs
@@ -0,0 +1,72 @@
+//! Spans map elements to the part of source code they originate from.
+
+use std::fmt::{self, Display, Formatter};
+
+/// Annotates a value with the part of the source code it corresponds to.
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub struct Spanned<T> {
+    pub val: T,
+    pub span: Span,
+}
+
+impl<T> Spanned<T> {
+    pub fn new(val: T, span: Span) -> Spanned<T> {
+        Spanned { val, span }
+    }
+
+    pub fn value(self) -> T {
+        self.val
+    }
+
+    pub fn span_map<F, U>(self, f: F) -> Spanned<U> where F: FnOnce(T) -> U {
+        Spanned::new(f(self.val), self.span)
+    }
+}
+
+impl<T> Display for Spanned<T> where T: std::fmt::Debug {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "({:?}:{})", self.val, self.span)
+    }
+}
+
+debug_display!(Spanned; T where T: std::fmt::Debug);
+
+/// Describes a slice of source code.
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub struct Span {
+    pub start: usize,
+    pub end: usize,
+}
+
+impl Span {
+    pub fn new(start: usize, end: usize) -> Span {
+        Span { start, end }
+    }
+
+    pub fn merge(a: Span, b: Span) -> Span {
+        Span {
+            start: a.start.min(b.start),
+            end: a.end.max(b.end),
+        }
+    }
+
+    pub fn at(index: usize) -> Span {
+        Span { start: index, end: index + 1 }
+    }
+
+    pub fn pair(&self) -> (usize, usize) {
+        (self.start, self.end)
+    }
+
+    pub fn expand(&mut self, other: Span) {
+        *self = Span::merge(*self, other)
+    }
+}
+
+impl Display for Span {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "[{}, {}]", self.start, self.end)
+    }
+}
+
+debug_display!(Span);