Add Value type and replace dyn-nodes with call-exprs 🏗

- In addition to syntax trees there are now `Value`s, which syntax trees can be evaluated into (e.g. the tree is `5+5` and the value is `10`)
- Parsing is completely pure, function calls are not parsed into nodes, but into simple call expressions, which are resolved later
- Functions aren't dynamic nodes anymore, but simply functions which receive their arguments as a table and the layouting context
- Functions may return any `Value`
- Layouting is powered by functions which return the new `Commands` value, which informs the layouting engine what to do
- When a function returns a non-`Commands` value, the layouter simply dumps the value into the document in monospace

1 files changed, 474 insertions, 0 deletions

diff --git a/src/compute/value.rs b/src/compute/value.rs
new file mode 100644
index 00000000..daa3b17b
--- /dev/null
+++ b/src/compute/value.rs
@@ -0,0 +1,474 @@
+//! Computational values: Syntactical expressions can be evaluated into these.
+
+use std::fmt::{self, Debug, Formatter};
+use std::rc::Rc;
+
+use fontdock::{FontStyle, FontWeight, FontWidth};
+
+use crate::color::RgbaColor;
+use crate::layout::{Commands, Dir, LayoutContext, SpecAlign};
+use crate::length::{Length, ScaleLength};
+use crate::paper::Paper;
+use crate::syntax::span::{Span, Spanned};
+use crate::syntax::tree::SyntaxTree;
+use crate::syntax::Ident;
+use crate::{DynFuture, Feedback, Pass};
+use super::table::{BorrowedKey, SpannedEntry, Table};
+
+/// A computational value.
+#[derive(Clone)]
+pub enum Value {
+    /// An identifier: `ident`.
+    Ident(Ident),
+    /// A string: `"string"`.
+    Str(String),
+    /// A boolean: `true, false`.
+    Bool(bool),
+    /// A number: `1.2, 200%`.
+    Number(f64),
+    /// A length: `2cm, 5.2in`.
+    Length(Length),
+    /// A color value with alpha channel: `#f79143ff`.
+    Color(RgbaColor),
+    /// A table value: `(false, 12cm, greeting="hi")`.
+    Table(TableValue),
+    /// A syntax tree containing typesetting content.
+    Tree(SyntaxTree),
+    /// An executable function.
+    Func(FuncValue),
+    /// Layouting commands.
+    Commands(Commands),
+}
+
+impl Value {
+    /// A natural-language name of the type of this expression, e.g.
+    /// "identifier".
+    pub fn name(&self) -> &'static str {
+        use Value::*;
+        match self {
+            Ident(_) => "identifier",
+            Str(_) => "string",
+            Bool(_) => "bool",
+            Number(_) => "number",
+            Length(_) => "length",
+            Color(_) => "color",
+            Table(_) => "table",
+            Tree(_) => "syntax tree",
+            Func(_) => "function",
+            Commands(_) => "commands",
+        }
+    }
+}
+
+impl Debug for Value {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        use Value::*;
+        match self {
+            Ident(i) => i.fmt(f),
+            Str(s) => s.fmt(f),
+            Bool(b) => b.fmt(f),
+            Number(n) => n.fmt(f),
+            Length(s) => s.fmt(f),
+            Color(c) => c.fmt(f),
+            Table(t) => t.fmt(f),
+            Tree(t) => t.fmt(f),
+            Func(_) => f.pad("<function>"),
+            Commands(c) => c.fmt(f),
+        }
+    }
+}
+
+impl PartialEq for Value {
+    fn eq(&self, other: &Self) -> bool {
+        use Value::*;
+        match (self, other) {
+            (Ident(a), Ident(b)) => a == b,
+            (Str(a), Str(b)) => a == b,
+            (Bool(a), Bool(b)) => a == b,
+            (Number(a), Number(b)) => a == b,
+            (Length(a), Length(b)) => a == b,
+            (Color(a), Color(b)) => a == b,
+            (Table(a), Table(b)) => a == b,
+            (Tree(a), Tree(b)) => a == b,
+            (Func(a), Func(b)) => {
+                a.as_ref() as *const _ == b.as_ref() as *const _
+            }
+            (Commands(a), Commands(b)) => a == b,
+            _ => false,
+        }
+    }
+}
+
+/// An executable function value.
+///
+/// The first argument is a table containing the arguments passed to the
+/// function. The function may be asynchronous (as such it returns a dynamic
+/// future) and it may emit diagnostics, which are contained in the returned
+/// `Pass`. In the end, the function must evaluate to `Value`. Your typical
+/// typesetting function will return a `Commands` value which will instruct the
+/// layouting engine to do what the function pleases.
+///
+/// The dynamic function object is wrapped in an `Rc` to keep `Value` clonable.
+pub type FuncValue = Rc<
+    dyn Fn(TableValue, LayoutContext<'_>) -> DynFuture<Pass<Value>>
+>;
+
+/// A table of values.
+///
+/// # Example
+/// ```typst
+/// (false, 12cm, greeting="hi")
+/// ```
+pub type TableValue = Table<SpannedEntry<Value>>;
+
+impl TableValue {
+    /// Retrieve and remove the matching value with the lowest number key,
+    /// skipping and ignoring all non-matching entries with lower keys.
+    pub fn take<T: TryFromValue>(&mut self) -> Option<T> {
+        for (&key, entry) in self.nums() {
+            let expr = entry.val.as_ref();
+            if let Some(val) = T::try_from_value(expr, &mut Feedback::new()) {
+                self.remove(key);
+                return Some(val);
+            }
+        }
+        None
+    }
+
+    /// Retrieve and remove the matching value with the lowest number key,
+    /// removing and generating errors for all non-matching entries with lower
+    /// keys.
+    pub fn expect<T: TryFromValue>(&mut self, f: &mut Feedback) -> Option<T> {
+        while let Some((num, _)) = self.first() {
+            let entry = self.remove(num).unwrap();
+            if let Some(val) = T::try_from_value(entry.val.as_ref(), f) {
+                return Some(val);
+            }
+        }
+        None
+    }
+
+    /// Retrieve and remove a matching value associated with the given key if
+    /// there is any.
+    ///
+    /// Generates an error if the key exists but the value does not match.
+    pub fn take_with_key<'a, K, T>(&mut self, key: K, f: &mut Feedback) -> Option<T>
+    where
+        K: Into<BorrowedKey<'a>>,
+        T: TryFromValue,
+    {
+        self.remove(key).and_then(|entry| {
+            let expr = entry.val.as_ref();
+            T::try_from_value(expr, f)
+        })
+    }
+
+    /// Retrieve and remove all matching pairs with number keys, skipping and
+    /// ignoring non-matching entries.
+    ///
+    /// The pairs are returned in order of increasing keys.
+    pub fn take_all_num<'a, T>(&'a mut self) -> impl Iterator<Item = (u64, T)> + 'a
+    where
+        T: TryFromValue,
+    {
+        let mut skip = 0;
+        std::iter::from_fn(move || {
+            for (&key, entry) in self.nums().skip(skip) {
+                let expr = entry.val.as_ref();
+                if let Some(val) = T::try_from_value(expr, &mut Feedback::new()) {
+                    self.remove(key);
+                    return Some((key, val));
+                }
+                skip += 1;
+            }
+
+            None
+        })
+    }
+
+
+    /// Retrieve and remove all matching values with number keys, skipping and
+    /// ignoring non-matching entries.
+    ///
+    /// The values are returned in order of increasing keys.
+    pub fn take_all_num_vals<'a, T: 'a>(&'a mut self) -> impl Iterator<Item = T> + 'a
+    where
+        T: TryFromValue,
+    {
+        self.take_all_num::<T>().map(|(_, v)| v)
+    }
+
+    /// Retrieve and remove all matching pairs with string keys, skipping and
+    /// ignoring non-matching entries.
+    ///
+    /// The pairs are returned in order of increasing keys.
+    pub fn take_all_str<'a, T>(&'a mut self) -> impl Iterator<Item = (String, T)> + 'a
+    where
+        T: TryFromValue,
+    {
+        let mut skip = 0;
+        std::iter::from_fn(move || {
+            for (key, entry) in self.strs().skip(skip) {
+                let expr = entry.val.as_ref();
+                if let Some(val) = T::try_from_value(expr, &mut Feedback::new()) {
+                    let key = key.clone();
+                    self.remove(&key);
+                    return Some((key, val));
+                }
+                skip += 1;
+            }
+
+            None
+        })
+    }
+
+    /// Generated `"unexpected argument"` errors for all remaining entries.
+    pub fn unexpected(&self, f: &mut Feedback) {
+        for entry in self.values() {
+            let span = Span::merge(entry.key, entry.val.span);
+            error!(@f, span, "unexpected argument");
+        }
+    }
+}
+
+/// A trait for converting values into more specific types.
+pub trait TryFromValue: Sized {
+    // This trait takes references because we don't want to move the value
+    // out of its origin in case this returns `None`. This solution is not
+    // perfect because we need to do some cloning in the impls for this trait,
+    // but we haven't got a better solution, for now.
+
+    /// Try to convert a value to this type.
+    ///
+    /// Returns `None` and generates an appropriate error if the value is not
+    /// valid for this type.
+    fn try_from_value(value: Spanned<&Value>, f: &mut Feedback) -> Option<Self>;
+}
+
+macro_rules! impl_match {
+    ($type:ty, $name:expr, $($p:pat => $r:expr),* $(,)?) => {
+        impl TryFromValue for $type {
+            fn try_from_value(value: Spanned<&Value>, f: &mut Feedback) -> Option<Self> {
+                #[allow(unreachable_patterns)]
+                match value.v {
+                    $($p => Some($r)),*,
+                    other => {
+                        error!(
+                            @f, value.span,
+                            "expected {}, found {}", $name, other.name()
+                        );
+                        None
+                    }
+                }
+            }
+        }
+    };
+}
+
+macro_rules! impl_ident {
+    ($type:ty, $name:expr, $parse:expr) => {
+        impl TryFromValue for $type {
+            fn try_from_value(value: Spanned<&Value>, f: &mut Feedback) -> Option<Self> {
+                if let Value::Ident(ident) = value.v {
+                    let val = $parse(ident.as_str());
+                    if val.is_none() {
+                        error!(@f, value.span, "invalid {}", $name);
+                    }
+                    val
+                } else {
+                    error!(
+                        @f, value.span,
+                        "expected {}, found {}", $name, value.v.name()
+                    );
+                    None
+                }
+            }
+        }
+    };
+}
+
+impl<T: TryFromValue> TryFromValue for Spanned<T> {
+    fn try_from_value(value: Spanned<&Value>, f: &mut Feedback) -> Option<Self> {
+        let span = value.span;
+        T::try_from_value(value, f).map(|v| Spanned { v, span })
+    }
+}
+
+impl_match!(Value, "value", v => v.clone());
+impl_match!(Ident, "identifier", Value::Ident(i) => i.clone());
+impl_match!(String, "string", Value::Str(s) => s.clone());
+impl_match!(bool, "bool", &Value::Bool(b) => b);
+impl_match!(f64, "number", &Value::Number(n) => n);
+impl_match!(Length, "length", &Value::Length(l) => l);
+impl_match!(SyntaxTree, "tree", Value::Tree(t) => t.clone());
+impl_match!(TableValue, "table", Value::Table(t) => t.clone());
+impl_match!(FuncValue, "function", Value::Func(f) => f.clone());
+impl_match!(ScaleLength, "number or length",
+    &Value::Length(length) => ScaleLength::Absolute(length),
+    &Value::Number(scale) => ScaleLength::Scaled(scale),
+);
+
+/// A value type that matches identifiers and strings and implements
+/// `Into<String>`.
+pub struct StringLike(pub String);
+
+impl From<StringLike> for String {
+    fn from(like: StringLike) -> String {
+        like.0
+    }
+}
+
+impl_match!(StringLike, "identifier or string",
+    Value::Ident(Ident(s)) => StringLike(s.clone()),
+    Value::Str(s) => StringLike(s.clone()),
+);
+
+impl_ident!(Dir, "direction", |s| match s {
+    "ltr" => Some(Self::LTR),
+    "rtl" => Some(Self::RTL),
+    "ttb" => Some(Self::TTB),
+    "btt" => Some(Self::BTT),
+    _ => None,
+});
+
+impl_ident!(SpecAlign, "alignment", |s| match s {
+    "left" => Some(Self::Left),
+    "right" => Some(Self::Right),
+    "top" => Some(Self::Top),
+    "bottom" => Some(Self::Bottom),
+    "center" => Some(Self::Center),
+    _ => None,
+});
+
+impl_ident!(FontStyle, "font style", FontStyle::from_name);
+impl_ident!(Paper, "paper", Paper::from_name);
+
+impl TryFromValue for FontWeight {
+    fn try_from_value(value: Spanned<&Value>, f: &mut Feedback) -> Option<Self> {
+        match value.v {
+            &Value::Number(weight) => {
+                const MIN: u16 = 100;
+                const MAX: u16 = 900;
+
+                Some(Self(if weight < MIN as f64 {
+                    error!(@f, value.span, "the minimum font weight is {}", MIN);
+                    MIN
+                } else if weight > MAX as f64 {
+                    error!(@f, value.span, "the maximum font weight is {}", MAX);
+                    MAX
+                } else {
+                    weight.round() as u16
+                }))
+            }
+            Value::Ident(ident) => {
+                let weight = Self::from_name(ident.as_str());
+                if weight.is_none() {
+                    error!(@f, value.span, "invalid font weight");
+                }
+                weight
+            }
+            other => {
+                error!(
+                    @f, value.span,
+                    "expected font weight (name or number), found {}",
+                    other.name(),
+                );
+                None
+            }
+        }
+    }
+}
+
+impl TryFromValue for FontWidth {
+    fn try_from_value(value: Spanned<&Value>, f: &mut Feedback) -> Option<Self> {
+        match value.v {
+            &Value::Number(width) => {
+                const MIN: u16 = 1;
+                const MAX: u16 = 9;
+
+                Self::new(if width < MIN as f64 {
+                    error!(@f, value.span, "the minimum font width is {}", MIN);
+                    MIN
+                } else if width > MAX as f64 {
+                    error!(@f, value.span, "the maximum font width is {}", MAX);
+                    MAX
+                } else {
+                    width.round() as u16
+                })
+            }
+            Value::Ident(ident) => {
+                let width = Self::from_name(ident.as_str());
+                if width.is_none() {
+                    error!(@f, value.span, "invalid font width");
+                }
+                width
+            }
+            other => {
+                error!(
+                    @f, value.span,
+                    "expected font width (name or number), found {}",
+                    other.name(),
+                );
+                None
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn entry(value: Value) -> SpannedEntry<Value> {
+        SpannedEntry::val(Spanned::zero(value))
+    }
+
+    #[test]
+    fn test_table_take_removes_correct_entry() {
+        let mut table = Table::new();
+        table.insert(1, entry(Value::Bool(false)));
+        table.insert(2, entry(Value::Str("hi".to_string())));
+        assert_eq!(table.take::<String>(), Some("hi".to_string()));
+        assert_eq!(table.len(), 1);
+        assert_eq!(table.take::<bool>(), Some(false));
+        assert!(table.is_empty());
+    }
+
+    #[test]
+    fn test_table_expect_errors_about_previous_entries() {
+        let mut f = Feedback::new();
+        let mut table = Table::new();
+        table.insert(1, entry(Value::Bool(false)));
+        table.insert(3, entry(Value::Str("hi".to_string())));
+        table.insert(5, entry(Value::Bool(true)));
+        assert_eq!(table.expect::<String>(&mut f), Some("hi".to_string()));
+        assert_eq!(f.diagnostics, [error!(Span::ZERO, "expected string, found bool")]);
+        assert_eq!(table.len(), 1);
+    }
+
+    #[test]
+    fn test_table_take_with_key_removes_the_entry() {
+        let mut f = Feedback::new();
+        let mut table = Table::new();
+        table.insert(1, entry(Value::Bool(false)));
+        table.insert("hi", entry(Value::Bool(true)));
+        assert_eq!(table.take_with_key::<_, bool>(1, &mut f), Some(false));
+        assert_eq!(table.take_with_key::<_, f64>("hi", &mut f), None);
+        assert_eq!(f.diagnostics, [error!(Span::ZERO, "expected number, found bool")]);
+        assert!(table.is_empty());
+    }
+
+    #[test]
+    fn test_table_take_all_removes_the_correct_entries() {
+        let mut table = Table::new();
+        table.insert(1, entry(Value::Bool(false)));
+        table.insert(3, entry(Value::Number(0.0)));
+        table.insert(7, entry(Value::Bool(true)));
+        assert_eq!(
+            table.take_all_num::<bool>().collect::<Vec<_>>(),
+            [(1, false), (7, true)],
+        );
+        assert_eq!(table.len(), 1);
+        assert_eq!(table[3].val.v, Value::Number(0.0));
+    }
+}