Non-fatal argument parsing 🌋

4 files changed, 702 insertions, 0 deletions

diff --git a/src/syntax/func/keys.rs b/src/syntax/func/keys.rs
new file mode 100644
index 00000000..dff97bde
--- /dev/null
+++ b/src/syntax/func/keys.rs
@@ -0,0 +1,121 @@
+use crate::layout::prelude::*;
+use super::*;
+
+use AxisKey::*;
+use PaddingKey::*;
+use AlignmentValue::*;
+
+
+pub trait Key {
+    type Output: Eq;
+
+    fn parse(key: Spanned<&str>) -> Option<Self::Output>;
+}
+
+impl<K: Key> Key for Spanned<K> {
+    type Output = Spanned<K::Output>;
+
+    fn parse(key: Spanned<&str>) -> Option<Self::Output> {
+        K::parse(key).map(|v| Spanned { v, span: key.span })
+    }
+}
+
+macro_rules! key {
+    ($type:ty, $output:ty, $($($p:pat)|* => $r:expr),* $(,)?) => {
+        impl Key for $type {
+            type Output = $output;
+
+            fn parse(key: Spanned<&str>) -> Option<Self::Output> {
+                match key.v {
+                    $($($p)|* => Some($r)),*,
+                    other => None,
+                }
+            }
+        }
+    };
+}
+
+/// An argument key which identifies a layouting axis.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum AxisKey {
+    Generic(GenericAxis),
+    Specific(SpecificAxis),
+}
+
+impl AxisKey {
+    /// The generic version of this axis key in the given system of axes.
+    pub fn to_generic(self, axes: LayoutAxes) -> GenericAxis {
+        match self {
+            Generic(axis) => axis,
+            Specific(axis) => axis.to_generic(axes),
+        }
+    }
+
+    /// The specific version of this axis key in the given system of axes.
+    pub fn to_specific(self, axes: LayoutAxes) -> SpecificAxis {
+        match self {
+            Generic(axis) => axis.to_specific(axes),
+            Specific(axis) => axis,
+        }
+    }
+}
+
+key!(AxisKey, Self,
+    "horizontal" | "h" => Specific(Horizontal),
+    "vertical"   | "v" => Specific(Vertical),
+    "primary"    | "p" => Generic(Primary),
+    "secondary"  | "s" => Generic(Secondary),
+);
+
+pub struct ExtentKey;
+
+key!(ExtentKey, AxisKey,
+    "width"          | "w"  => Specific(Horizontal),
+    "height"         | "h"  => Specific(Vertical),
+    "primary-size"   | "ps" => Generic(Primary),
+    "secondary-size" | "ss" => Generic(Secondary),
+);
+
+/// An argument key which identifies an axis, but allows for positional
+/// arguments with unspecified axes.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum PosAxisKey {
+    /// The first positional argument.
+    First,
+    /// The second positional argument.
+    Second,
+    /// An axis keyword argument.
+    Keyword(AxisKey),
+}
+
+/// An argument key which identifies a margin or padding target.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum PaddingKey<Axis> {
+    /// All four sides should have the specified padding.
+    All,
+    /// Both sides of the given axis should have the specified padding.
+    Both(Axis),
+    /// Only the given side of the given axis should have the specified padding.
+    Side(Axis, AlignmentValue),
+}
+
+key!(PaddingKey<AxisKey>, Self,
+    "horizontal" | "h" => Both(Specific(Horizontal)),
+    "vertical"   | "v" => Both(Specific(Vertical)),
+    "primary"    | "p" => Both(Generic(Primary)),
+    "secondary"  | "s" => Both(Generic(Secondary)),
+
+    "left"   => Side(Specific(Horizontal), Left),
+    "right"  => Side(Specific(Horizontal), Right),
+    "top"    => Side(Specific(Vertical),   Top),
+    "bottom" => Side(Specific(Vertical),   Bottom),
+
+    "primary-origin"    => Side(Generic(Primary),     Align(Origin)),
+    "primary-end"       => Side(Generic(Primary),     Align(End)),
+    "secondary-origin"  => Side(Generic(Secondary),   Align(Origin)),
+    "secondary-end"     => Side(Generic(Secondary),   Align(End)),
+    "horizontal-origin" => Side(Specific(Horizontal), Align(Origin)),
+    "horizontal-end"    => Side(Specific(Horizontal), Align(End)),
+    "vertical-origin"   => Side(Specific(Vertical),   Align(Origin)),
+    "vertical-end"      => Side(Specific(Vertical),   Align(End)),
+);
diff --git a/src/syntax/func/maps.rs b/src/syntax/func/maps.rs
new file mode 100644
index 00000000..452c8ab1
--- /dev/null
+++ b/src/syntax/func/maps.rs
@@ -0,0 +1,211 @@
+//! Deduplicating maps and keys for argument parsing.
+
+use std::collections::HashMap;
+use std::hash::Hash;
+use crate::layout::{LayoutAxes, SpecificAxis, GenericAxis};
+use crate::size::{PSize, ValueBox};
+use super::*;
+
+
+/// A deduplicating map type useful for storing possibly redundant arguments.
+#[derive(Debug, Clone, PartialEq)]
+pub struct DedupMap<K, V> where K: Eq {
+    map: Vec<Spanned<(K, V)>>,
+}
+
+impl<K, V> DedupMap<K, V> where K: Eq {
+    pub fn new() -> DedupMap<K, V> {
+        DedupMap { map: vec![] }
+    }
+
+    pub fn from_iter<I>(errors: &mut Errors, iter: I) -> DedupMap<K, V>
+    where I: IntoIterator<Item=Spanned<(K, V)>> {
+        let mut map = DedupMap::new();
+        for Spanned { v: (key, value), span } in iter.into_iter() {
+            map.insert(errors, key, value, span);
+        }
+        map
+    }
+
+    /// Add a key-value pair.
+    pub fn insert(&mut self, errors: &mut Errors, key: K, value: V, span: Span) {
+        if self.map.iter().any(|e| e.v.0 == key) {
+            errors.push(err!(span; "duplicate argument"));
+        } else {
+            self.map.push(Spanned { v: (key, value), span });
+        }
+    }
+
+    /// Add multiple key-value pairs.
+    pub fn extend<I>(&mut self, errors: &mut Errors, items: I)
+    where I: IntoIterator<Item=Spanned<(K, V)>> {
+        for Spanned { v: (k, v), span } in items.into_iter() {
+            self.insert(errors, k, v, span);
+        }
+    }
+
+    /// Get the value corresponding to a key if it is present.
+    pub fn get(&self, key: K) -> Option<&V> {
+        self.map.iter().find(|e| e.v.0 == key).map(|e| &e.v.1)
+    }
+
+    /// Get the value and its span corresponding to a key if it is present.
+    pub fn get_spanned(&self, key: K) -> Option<Spanned<&V>> {
+        self.map.iter().find(|e| e.v.0 == key)
+            .map(|e| Spanned { v: &e.v.1, span: e.span })
+    }
+
+    /// Call a function with the value if the key is present.
+    pub fn with<F>(&self, key: K, callback: F) where F: FnOnce(&V) {
+        if let Some(value) = self.get(key) {
+            callback(value);
+        }
+    }
+
+    /// Create a new map where keys and values are mapped to new keys and
+    /// values.
+    ///
+    /// Returns an error if a new key is duplicate.
+    pub fn dedup<F, K2, V2>(&self, errors: &mut Errors, mut f: F) -> DedupMap<K2, V2>
+    where F: FnMut(&K, &V) -> (K2, V2), K2: Eq {
+        let mut map = DedupMap::new();
+
+        for Spanned { v: (key, value), span } in self.map.iter() {
+            let (key, value) = f(key, value);
+            map.insert(errors, key, value, *span);
+        }
+
+        map
+    }
+
+    /// Iterate over the (key, value) pairs.
+    pub fn iter(&self) -> impl Iterator<Item=&(K, V)> {
+        self.map.iter().map(|e| &e.v)
+    }
+}
+
+/// A map for storing a value for two axes given by keyword arguments.
+#[derive(Debug, Clone, PartialEq)]
+pub struct AxisMap<V>(DedupMap<AxisKey, V>);
+
+impl<V: Clone> AxisMap<V> {
+    pub fn parse<KT: Key<Output=AxisKey>, VT: Value<Output=V>>(
+        errors: &mut Errors,
+        object: &mut Object,
+    ) -> AxisMap<V> {
+        let values: Vec<_> = object.get_all_spanned::<KT, VT>(errors).collect();
+        AxisMap(DedupMap::from_iter(errors, values))
+    }
+
+    /// Deduplicate from specific or generic to just specific axes.
+    pub fn dedup(&self, errors: &mut Errors, axes: LayoutAxes) -> DedupMap<SpecificAxis, V> {
+        self.0.dedup(errors, |key, val| (key.to_specific(axes), val.clone()))
+    }
+}
+
+/// A map for extracting values for two axes that are given through two
+/// positional or keyword arguments.
+#[derive(Debug, Clone, PartialEq)]
+pub struct PosAxisMap<V>(DedupMap<PosAxisKey, V>);
+
+impl<V: Clone> PosAxisMap<V> {
+    pub fn parse<KT: Key<Output=AxisKey>, VT: Value<Output=V>>(
+        errors: &mut Errors,
+        args: &mut FuncArgs,
+    ) -> PosAxisMap<V> {
+        let mut map = DedupMap::new();
+
+        for &key in &[PosAxisKey::First, PosAxisKey::Second] {
+            if let Some(value) = args.pos.get::<Spanned<VT>>(errors) {
+                map.insert(errors, key, value.v, value.span);
+            }
+        }
+
+        let keywords: Vec<_> = args.key
+            .get_all_spanned::<KT, VT>(errors)
+            .map(|s| s.map(|(k, v)| (PosAxisKey::Keyword(k), v)))
+            .collect();
+
+        map.extend(errors, keywords);
+
+        PosAxisMap(map)
+    }
+
+    /// Deduplicate from positional or specific to generic axes.
+    pub fn dedup<F>(
+        &self,
+        errors: &mut Errors,
+        axes: LayoutAxes,
+        mut f: F,
+    ) -> DedupMap<GenericAxis, V> where F: FnMut(&V) -> Option<GenericAxis> {
+        self.0.dedup(errors, |key, val| {
+            (match key {
+                PosAxisKey::First => f(val).unwrap_or(GenericAxis::Primary),
+                PosAxisKey::Second => f(val).unwrap_or(GenericAxis::Secondary),
+                PosAxisKey::Keyword(AxisKey::Specific(axis)) => axis.to_generic(axes),
+                PosAxisKey::Keyword(AxisKey::Generic(axis)) => *axis,
+            }, val.clone())
+        })
+    }
+}
+
+/// A map for extracting padding for a set of specifications given for all
+/// sides, opposing sides or single sides.
+#[derive(Debug, Clone, PartialEq)]
+pub struct PaddingMap(DedupMap<PaddingKey<AxisKey>, Option<PSize>>);
+
+impl PaddingMap {
+    pub fn parse(errors: &mut Errors, args: &mut FuncArgs) -> PaddingMap {
+        let mut map = DedupMap::new();
+
+        if let Some(psize) = args.pos.get::<Spanned<Defaultable<PSize>>>(errors) {
+            map.insert(errors, PaddingKey::All, psize.v, psize.span);
+        }
+
+        let paddings: Vec<_> = args.key
+            .get_all_spanned::<PaddingKey<AxisKey>, Defaultable<PSize>>(errors)
+            .collect();
+
+        map.extend(errors, paddings);
+
+        PaddingMap(map)
+    }
+
+    /// Apply the specified padding on a value box of optional, scalable sizes.
+    pub fn apply(
+        &self,
+        errors: &mut Errors,
+        axes: LayoutAxes,
+        padding: &mut ValueBox<Option<PSize>>
+    ) {
+        use PaddingKey::*;
+        use SpecificAxis::*;
+
+        let map = self.0.dedup(errors, |key, &val| {
+            (match key {
+                All => All,
+                Both(axis) => Both(axis.to_specific(axes)),
+                Side(axis, alignment) => {
+                    let axis = axis.to_specific(axes);
+                    Side(axis, alignment.to_specific(axes, axis))
+                }
+            }, val)
+        });
+
+        map.with(All, |&val| padding.set_all(val));
+        map.with(Both(Horizontal), |&val| padding.set_horizontal(val));
+        map.with(Both(Vertical), |&val| padding.set_vertical(val));
+
+        for &(key, val) in map.iter() {
+            if let Side(_, alignment) = key {
+                match alignment {
+                    AlignmentValue::Left => padding.left = val,
+                    AlignmentValue::Right => padding.right = val,
+                    AlignmentValue::Top => padding.top = val,
+                    AlignmentValue::Bottom => padding.bottom = val,
+                    _ => {},
+                }
+            }
+        }
+    }
+}
diff --git a/src/syntax/func/mod.rs b/src/syntax/func/mod.rs
new file mode 100644
index 00000000..b6691ab5
--- /dev/null
+++ b/src/syntax/func/mod.rs
@@ -0,0 +1,147 @@
+use super::*;
+
+pub_use_mod!(maps);
+pub_use_mod!(keys);
+pub_use_mod!(values);
+
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct FuncHeader {
+    pub name: Spanned<Ident>,
+    pub args: FuncArgs,
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct FuncArgs {
+    pub pos: Tuple,
+    pub key: Object,
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum Arg {
+    Pos(Spanned<Expr>),
+    Key(Pair),
+}
+
+impl Arg {
+    /// The span or the value or combined span of key and value.
+    pub fn span(&self) -> Span {
+        match self {
+            Arg::Pos(item) => item.span,
+            Arg::Key(Pair { key, value }) => Span::merge(key.span, value.span),
+        }
+    }
+}
+
+impl FuncArgs {
+    pub fn new() -> FuncArgs {
+        FuncArgs {
+            pos: Tuple::new(),
+            key: Object::new(),
+        }
+    }
+
+    /// Add an argument.
+    pub fn add(&mut self, arg: Arg) {
+        match arg {
+            Arg::Pos(item) => self.add_pos(item),
+            Arg::Key(pair) => self.add_key_pair(pair),
+        }
+    }
+
+    /// Add a positional argument.
+    pub fn add_pos(&mut self, item: Spanned<Expr>) {
+        self.pos.add(item);
+    }
+
+    /// Add a keyword argument.
+    pub fn add_key(&mut self, key: Spanned<Ident>, value: Spanned<Expr>) {
+        self.key.add(key, value);
+    }
+
+    /// Add a keyword argument from an existing pair.
+    pub fn add_key_pair(&mut self, pair: Pair) {
+        self.key.add_pair(pair);
+    }
+
+    pub fn into_iter(self) -> impl Iterator<Item=Arg> {
+        self.pos.items.into_iter().map(|item| Arg::Pos(item))
+            .chain(self.key.pairs.into_iter().map(|pair| Arg::Key(pair)))
+    }
+
+    // /// Force-extract the first positional argument.
+    // pub fn get_pos<E: ExpressionKind>(&mut self) -> ParseResult<E> {
+    //     expect(self.get_pos_opt())
+    // }
+
+    // /// Extract the first positional argument.
+    // pub fn get_pos_opt<E: ExpressionKind>(&mut self) -> ParseResult<Option<E>> {
+    //     Ok(if !self.positional.items.is_empty() {
+    //         let spanned = self.positional.items.remove(0);
+    //         Some(E::from_expr(spanned)?)
+    //     } else {
+    //         None
+    //     })
+    // }
+
+    // /// Force-extract a keyword argument.
+    // pub fn get_key<E: ExpressionKind>(&mut self, name: &str) -> ParseResult<E> {
+    //     expect(self.get_key_opt(name))
+    // }
+
+    // /// Extract a keyword argument.
+    // pub fn get_key_opt<E: ExpressionKind>(&mut self, name: &str) -> ParseResult<Option<E>> {
+    //     self.keyword.pairs.iter()
+    //         .position(|p| p.key.v.0 == name)
+    //         .map(|index| {
+    //             let value = self.keyword.pairs.swap_remove(index).value;
+    //             E::from_expr(value)
+    //         })
+    //         .transpose()
+    // }
+
+    // /// Iterator over positional arguments.
+    // pub fn iter_pos(&mut self) -> std::vec::IntoIter<Spanned<Expr>> {
+    //     let tuple = std::mem::replace(&mut self.positional, Tuple::new());
+    //     tuple.items.into_iter()
+    // }
+
+    // /// Iterator over all keyword arguments.
+    // pub fn iter_keys(&mut self) -> std::vec::IntoIter<Pair> {
+    //     let object = std::mem::replace(&mut self.keyword, Object::new());
+    //     object.pairs.into_iter()
+    // }
+
+    // /// Clear the argument lists.
+    // pub fn clear(&mut self) {
+    //     self.positional.items.clear();
+    //     self.keyword.pairs.clear();
+    // }
+
+    // /// Whether both the positional and keyword argument lists are empty.
+    // pub fn is_empty(&self) -> bool {
+    //     self.positional.items.is_empty() && self.keyword.pairs.is_empty()
+    // }
+}
+
+// /// Extract the option expression kind from the option or return an error.
+// fn expect<E: ExpressionKind>(opt: ParseResult<Option<E>>) -> ParseResult<E> {
+//     match opt {
+//         Ok(Some(spanned)) => Ok(spanned),
+//         Ok(None) => error!("expected {}", E::NAME),
+//         Err(e) => Err(e),
+//     }
+// }
+
+pub trait OptionExt: Sized {
+    fn or_missing(self, errors: &mut Errors, span: Span, what: &str) -> Self;
+}
+
+impl<T> OptionExt for Option<T> {
+    fn or_missing(self, errors: &mut Errors, span: Span, what: &str) -> Self {
+        if self.is_none() {
+            errors.push(err!(span; "missing argument: {}", what));
+        }
+        self
+    }
+}
diff --git a/src/syntax/func/values.rs b/src/syntax/func/values.rs
new file mode 100644
index 00000000..b29b9726
--- /dev/null
+++ b/src/syntax/func/values.rs
@@ -0,0 +1,223 @@
+use std::marker::PhantomData;
+use toddle::query::{FontStyle, FontWeight};
+
+use crate::layout::prelude::*;
+use crate::size::ScaleSize;
+use crate::style::Paper;
+use super::*;
+
+use AlignmentValue::*;
+
+
+pub trait Value {
+    type Output;
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error>;
+}
+
+impl<V: Value> Value for Spanned<V> {
+    type Output = Spanned<V::Output>;
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+        let span = expr.span;
+        V::parse(expr).map(|v| Spanned { v, span })
+    }
+}
+
+macro_rules! value {
+    ($type:ty, $output:ty, $name:expr, $($p:pat => $r:expr),* $(,)?) => {
+        impl Value for $type {
+            type Output = $output;
+
+            fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+                #[allow(unreachable_patterns)]
+                match expr.v {
+                    $($p => Ok($r)),*,
+                    other => Err(err!("expected {}, found {}",
+                                      $name, other.name())),
+                }
+            }
+        }
+    };
+}
+
+value!(Expr,   Self, "expression", e => e);
+
+value!(Ident,  Self, "identifier", Expr::Ident(i)  => i);
+value!(String, Self, "string",     Expr::Str(s)    => s);
+value!(f64,    Self, "number",     Expr::Number(n) => n);
+value!(bool,   Self, "bool",       Expr::Bool(b)   => b);
+value!(Size,   Self, "size",       Expr::Size(s)   => s);
+value!(Tuple,  Self, "tuple",      Expr::Tuple(t)  => t);
+value!(Object, Self, "object",     Expr::Object(o) => o);
+
+value!(ScaleSize, Self, "number or size",
+    Expr::Size(size)    => ScaleSize::Absolute(size),
+    Expr::Number(scale) => ScaleSize::Scaled(scale as f32),
+);
+
+pub struct StringLike;
+
+value!(StringLike, String, "identifier or string",
+    Expr::Ident(Ident(s)) => s,
+    Expr::Str(s) => s,
+);
+
+pub struct Defaultable<T>(PhantomData<T>);
+
+impl<T: Value> Value for Defaultable<T> {
+    type Output = Option<T::Output>;
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+        match expr.v {
+            Expr::Ident(ident) if ident.as_str() == "default" => Ok(None),
+            _ => T::parse(expr).map(Some)
+        }
+    }
+}
+
+impl Value for Direction {
+    type Output = Self;
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+        Ok(match Ident::parse(expr)?.as_str() {
+            "left-to-right" | "ltr" | "LTR" => Direction::LeftToRight,
+            "right-to-left" | "rtl" | "RTL" => Direction::RightToLeft,
+            "top-to-bottom" | "ttb" | "TTB" => Direction::TopToBottom,
+            "bottom-to-top" | "btt" | "BTT" => Direction::BottomToTop,
+            other => return Err(err!("invalid direction"))
+        })
+    }
+}
+
+impl Value for FontStyle {
+    type Output = Self;
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+        FontStyle::from_str(Ident::parse(expr)?.as_str())
+            .ok_or_else(|| err!("invalid font style"))
+    }
+}
+
+impl Value for FontWeight {
+    type Output = (Self, bool);
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+        match expr.v {
+            Expr::Number(weight) => {
+                let weight = weight.round();
+
+                if weight >= 100.0 && weight <= 900.0 {
+                    Ok((FontWeight(weight as i16), false))
+                } else {
+                    let clamped = weight.min(900.0).max(100.0) as i16;
+                    Ok((FontWeight(clamped), true))
+                }
+            }
+            Expr::Ident(id) => {
+                FontWeight::from_str(id.as_str())
+                    .ok_or_else(|| err!("invalid font weight"))
+                    .map(|weight| (weight, false))
+            }
+            other => Err(err!("expected identifier or number, \
+                               found {}", other.name())),
+        }
+    }
+}
+
+impl Value for Paper {
+    type Output = Self;
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+        Paper::from_str(Ident::parse(expr)?.as_str())
+            .ok_or_else(|| err!("invalid paper type"))
+    }
+}
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+pub enum AlignmentValue {
+    Align(Alignment),
+    Left,
+    Top,
+    Right,
+    Bottom,
+}
+
+impl AlignmentValue {
+    /// The generic axis this alignment corresponds to in the given system of
+    /// layouting axes. `None` if the alignment is generic.
+    pub fn axis(self, axes: LayoutAxes) -> Option<GenericAxis> {
+        match self {
+            Left | Right => Some(Horizontal.to_generic(axes)),
+            Top | Bottom => Some(Vertical.to_generic(axes)),
+            Align(_) => None,
+        }
+    }
+
+    /// The generic version of this alignment in the given system of layouting
+    /// axes.
+    ///
+    /// Returns `None` if the alignment is invalid for the given axis.
+    pub fn to_generic(self, axes: LayoutAxes, axis: GenericAxis) -> Option<Alignment> {
+        let specific = axis.to_specific(axes);
+        let start = match axes.get(axis).is_positive() {
+            true => Origin,
+            false => End,
+        };
+
+        match (self, specific) {
+            (Align(alignment), _) => Some(alignment),
+            (Left, Horizontal) | (Top, Vertical) => Some(start),
+            (Right, Horizontal) | (Bottom, Vertical) => Some(start.inv()),
+            _ => None
+        }
+    }
+
+    /// The specific version of this alignment in the given system of layouting
+    /// axes.
+    pub fn to_specific(self, axes: LayoutAxes, axis: SpecificAxis) -> AlignmentValue {
+        let direction = axes.get_specific(axis);
+        if let Align(alignment) = self {
+            match (direction, alignment) {
+                (LeftToRight, Origin) | (RightToLeft, End) => Left,
+                (LeftToRight, End) | (RightToLeft, Origin) => Right,
+                (TopToBottom, Origin) | (BottomToTop, End) => Top,
+                (TopToBottom, End) | (BottomToTop, Origin) => Bottom,
+                (_, Center) => self,
+            }
+        } else {
+            self
+        }
+    }
+}
+
+impl Value for AlignmentValue {
+    type Output = Self;
+
+    fn parse(expr: Spanned<Expr>) -> Result<Self::Output, Error> {
+        Ok(match Ident::parse(expr)?.as_str() {
+            "origin" => Align(Origin),
+            "center" => Align(Center),
+            "end"    => Align(End),
+            "left"   => Left,
+            "top"    => Top,
+            "right"  => Right,
+            "bottom" => Bottom,
+            other => return Err(err!("invalid alignment"))
+        })
+    }
+}
+
+impl Display for AlignmentValue {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            Align(Origin) => write!(f, "origin"),
+            Align(Center) => write!(f, "center"),
+            Align(End) => write!(f, "end"),
+            Left => write!(f, "left"),
+            Top => write!(f, "top"),
+            Right => write!(f, "right"),
+            Bottom => write!(f, "bottom"),
+        }
+    }
+}