Rename compute to eval ✏

4 files changed, 957 insertions, 0 deletions

diff --git a/src/eval/dict.rs b/src/eval/dict.rs
new file mode 100644
index 00000000..e6216572
--- /dev/null
+++ b/src/eval/dict.rs
@@ -0,0 +1,417 @@
+//! A key-value map that can also model array-like structures.
+
+use std::collections::BTreeMap;
+use std::fmt::{self, Debug, Display, Formatter};
+use std::ops::Index;
+
+use crate::syntax::{Span, Spanned};
+
+/// A dictionary data structure, which maps from integers (`u64`) or strings to
+/// a generic value type.
+///
+/// The dictionary can be used to model arrays by assigning values to successive
+/// indices from `0..n`. The `push` method offers special support for this
+/// pattern.
+#[derive(Clone)]
+pub struct Dict<V> {
+    nums: BTreeMap<u64, V>,
+    strs: BTreeMap<String, V>,
+    lowest_free: u64,
+}
+
+impl<V> Dict<V> {
+    /// Create a new empty dictionary.
+    pub fn new() -> Self {
+        Self {
+            nums: BTreeMap::new(),
+            strs: BTreeMap::new(),
+            lowest_free: 0,
+        }
+    }
+
+    /// The total number of entries in the dictionary.
+    pub fn len(&self) -> usize {
+        self.nums.len() + self.strs.len()
+    }
+
+    /// Whether the dictionary contains no entries.
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// The first number key-value pair (with lowest number).
+    pub fn first(&self) -> Option<(u64, &V)> {
+        self.nums.iter().next().map(|(&k, v)| (k, v))
+    }
+
+    /// The last number key-value pair (with highest number).
+    pub fn last(&self) -> Option<(u64, &V)> {
+        self.nums.iter().next_back().map(|(&k, v)| (k, v))
+    }
+
+    /// Get a reference to the value with the given key.
+    pub fn get<'a, K>(&self, key: K) -> Option<&V>
+    where
+        K: Into<RefKey<'a>>,
+    {
+        match key.into() {
+            RefKey::Num(num) => self.nums.get(&num),
+            RefKey::Str(string) => self.strs.get(string),
+        }
+    }
+
+    /// Borrow the value with the given key mutably.
+    pub fn get_mut<'a, K>(&mut self, key: K) -> Option<&mut V>
+    where
+        K: Into<RefKey<'a>>,
+    {
+        match key.into() {
+            RefKey::Num(num) => self.nums.get_mut(&num),
+            RefKey::Str(string) => self.strs.get_mut(string),
+        }
+    }
+
+    /// Insert a value into the dictionary.
+    pub fn insert<K>(&mut self, key: K, value: V)
+    where
+        K: Into<DictKey>,
+    {
+        match key.into() {
+            DictKey::Num(num) => {
+                self.nums.insert(num, value);
+                if self.lowest_free == num {
+                    self.lowest_free += 1;
+                }
+            }
+            DictKey::Str(string) => {
+                self.strs.insert(string, value);
+            }
+        }
+    }
+
+    /// Remove the value with the given key from the dictionary.
+    pub fn remove<'a, K>(&mut self, key: K) -> Option<V>
+    where
+        K: Into<RefKey<'a>>,
+    {
+        match key.into() {
+            RefKey::Num(num) => {
+                self.lowest_free = self.lowest_free.min(num);
+                self.nums.remove(&num)
+            }
+            RefKey::Str(string) => self.strs.remove(string),
+        }
+    }
+
+    /// Append a value to the dictionary.
+    ///
+    /// This will associate the `value` with the lowest free number key (zero if
+    /// there is no number key so far).
+    pub fn push(&mut self, value: V) {
+        while self.nums.contains_key(&self.lowest_free) {
+            self.lowest_free += 1;
+        }
+        self.nums.insert(self.lowest_free, value);
+        self.lowest_free += 1;
+    }
+
+    /// Iterator over all borrowed keys and values.
+    pub fn iter(&self) -> impl Iterator<Item = (RefKey, &V)> {
+        self.nums()
+            .map(|(&k, v)| (RefKey::Num(k), v))
+            .chain(self.strs().map(|(k, v)| (RefKey::Str(k), v)))
+    }
+
+    /// Iterate over all values in the dictionary.
+    pub fn values(&self) -> impl Iterator<Item = &V> {
+        self.nums().map(|(_, v)| v).chain(self.strs().map(|(_, v)| v))
+    }
+
+    /// Iterate over the number key-value pairs.
+    pub fn nums(&self) -> std::collections::btree_map::Iter<u64, V> {
+        self.nums.iter()
+    }
+
+    /// Iterate over the string key-value pairs.
+    pub fn strs(&self) -> std::collections::btree_map::Iter<String, V> {
+        self.strs.iter()
+    }
+
+    /// Move into an owned iterator over owned keys and values.
+    pub fn into_iter(self) -> impl Iterator<Item = (DictKey, V)> {
+        self.nums
+            .into_iter()
+            .map(|(k, v)| (DictKey::Num(k), v))
+            .chain(self.strs.into_iter().map(|(k, v)| (DictKey::Str(k), v)))
+    }
+
+    /// Move into an owned iterator over all values in the dictionary.
+    pub fn into_values(self) -> impl Iterator<Item = V> {
+        self.nums
+            .into_iter()
+            .map(|(_, v)| v)
+            .chain(self.strs.into_iter().map(|(_, v)| v))
+    }
+
+    /// Iterate over the number key-value pairs.
+    pub fn into_nums(self) -> std::collections::btree_map::IntoIter<u64, V> {
+        self.nums.into_iter()
+    }
+
+    /// Iterate over the string key-value pairs.
+    pub fn into_strs(self) -> std::collections::btree_map::IntoIter<String, V> {
+        self.strs.into_iter()
+    }
+}
+
+impl<'a, K, V> Index<K> for Dict<V>
+where
+    K: Into<RefKey<'a>>,
+{
+    type Output = V;
+
+    fn index(&self, index: K) -> &Self::Output {
+        self.get(index).expect("key not in dict")
+    }
+}
+
+impl<V> Default for Dict<V> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<V: Eq> Eq for Dict<V> {}
+
+impl<V: PartialEq> PartialEq for Dict<V> {
+    fn eq(&self, other: &Self) -> bool {
+        self.iter().eq(other.iter())
+    }
+}
+
+impl<V: Debug> Debug for Dict<V> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        if self.is_empty() {
+            return f.write_str("()");
+        }
+
+        let mut builder = f.debug_tuple("");
+
+        struct Entry<'a>(bool, &'a dyn Display, &'a dyn Debug);
+        impl<'a> Debug for Entry<'a> {
+            fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+                if self.0 {
+                    f.write_str("\"")?;
+                }
+                self.1.fmt(f)?;
+                if self.0 {
+                    f.write_str("\"")?;
+                }
+                if f.alternate() {
+                    f.write_str(" = ")?;
+                } else {
+                    f.write_str("=")?;
+                }
+                self.2.fmt(f)
+            }
+        }
+
+        for (key, value) in self.nums() {
+            builder.field(&Entry(false, &key, &value));
+        }
+
+        for (key, value) in self.strs() {
+            builder.field(&Entry(key.contains(' '), &key, &value));
+        }
+
+        builder.finish()
+    }
+}
+
+/// The owned variant of a dictionary key.
+#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)]
+pub enum DictKey {
+    Num(u64),
+    Str(String),
+}
+
+impl From<&Self> for DictKey {
+    fn from(key: &Self) -> Self {
+        key.clone()
+    }
+}
+
+impl From<RefKey<'_>> for DictKey {
+    fn from(key: RefKey<'_>) -> Self {
+        match key {
+            RefKey::Num(num) => Self::Num(num),
+            RefKey::Str(string) => Self::Str(string.to_string()),
+        }
+    }
+}
+
+impl From<u64> for DictKey {
+    fn from(num: u64) -> Self {
+        Self::Num(num)
+    }
+}
+
+impl From<String> for DictKey {
+    fn from(string: String) -> Self {
+        Self::Str(string)
+    }
+}
+
+impl From<&'static str> for DictKey {
+    fn from(string: &'static str) -> Self {
+        Self::Str(string.to_string())
+    }
+}
+
+/// The borrowed variant of a dictionary key.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+pub enum RefKey<'a> {
+    Num(u64),
+    Str(&'a str),
+}
+
+impl From<u64> for RefKey<'static> {
+    fn from(num: u64) -> Self {
+        Self::Num(num)
+    }
+}
+
+impl<'a> From<&'a String> for RefKey<'a> {
+    fn from(string: &'a String) -> Self {
+        Self::Str(&string)
+    }
+}
+
+impl<'a> From<&'a str> for RefKey<'a> {
+    fn from(string: &'a str) -> Self {
+        Self::Str(string)
+    }
+}
+
+/// A dictionary entry which tracks key and value span.
+#[derive(Clone, PartialEq)]
+pub struct SpannedEntry<V> {
+    pub key: Span,
+    pub val: Spanned<V>,
+}
+
+impl<V> SpannedEntry<V> {
+    /// Create a new entry.
+    pub fn new(key: Span, val: Spanned<V>) -> Self {
+        Self { key, val }
+    }
+
+    /// Create an entry with the same span for key and value.
+    pub fn val(val: Spanned<V>) -> Self {
+        Self { key: val.span, val }
+    }
+
+    /// Convert from `&SpannedEntry<T>` to `SpannedEntry<&T>`
+    pub fn as_ref(&self) -> SpannedEntry<&V> {
+        SpannedEntry { key: self.key, val: self.val.as_ref() }
+    }
+
+    /// Map the entry to a different value type.
+    pub fn map<U>(self, f: impl FnOnce(V) -> U) -> SpannedEntry<U> {
+        SpannedEntry { key: self.key, val: self.val.map(f) }
+    }
+}
+
+impl<V: Debug> Debug for SpannedEntry<V> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        if f.alternate() {
+            f.write_str("key")?;
+            self.key.fmt(f)?;
+            f.write_str(" ")?;
+        }
+        self.val.fmt(f)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Dict;
+
+    #[test]
+    fn test_dict_different_key_types_dont_interfere() {
+        let mut dict = Dict::new();
+        dict.insert(10, "hello");
+        dict.insert("twenty", "there");
+        assert_eq!(dict.len(), 2);
+        assert_eq!(dict[10], "hello");
+        assert_eq!(dict["twenty"], "there");
+    }
+
+    #[test]
+    fn test_dict_push_skips_already_inserted_keys() {
+        let mut dict = Dict::new();
+        dict.insert(2, "2");
+        dict.push("0");
+        dict.insert(3, "3");
+        dict.push("1");
+        dict.push("4");
+        assert_eq!(dict.len(), 5);
+        assert_eq!(dict[0], "0");
+        assert_eq!(dict[1], "1");
+        assert_eq!(dict[2], "2");
+        assert_eq!(dict[3], "3");
+        assert_eq!(dict[4], "4");
+    }
+
+    #[test]
+    fn test_dict_push_remove_push_reuses_index() {
+        let mut dict = Dict::new();
+        dict.push("0");
+        dict.push("1");
+        dict.push("2");
+        dict.remove(1);
+        dict.push("a");
+        dict.push("3");
+        assert_eq!(dict.len(), 4);
+        assert_eq!(dict[0], "0");
+        assert_eq!(dict[1], "a");
+        assert_eq!(dict[2], "2");
+        assert_eq!(dict[3], "3");
+    }
+
+    #[test]
+    fn test_dict_first_and_last_are_correct() {
+        let mut dict = Dict::new();
+        assert_eq!(dict.first(), None);
+        assert_eq!(dict.last(), None);
+        dict.insert(4, "hi");
+        dict.insert("string", "hi");
+        assert_eq!(dict.first(), Some((4, &"hi")));
+        assert_eq!(dict.last(), Some((4, &"hi")));
+        dict.insert(2, "bye");
+        assert_eq!(dict.first(), Some((2, &"bye")));
+        assert_eq!(dict.last(), Some((4, &"hi")));
+    }
+
+    #[test]
+    fn test_dict_format_debug() {
+        let mut dict = Dict::new();
+        assert_eq!(format!("{:?}", dict), "()");
+        assert_eq!(format!("{:#?}", dict), "()");
+
+        dict.insert(10, "hello");
+        dict.insert("twenty", "there");
+        dict.insert("sp ace", "quotes");
+        assert_eq!(
+            format!("{:?}", dict),
+            r#"(10="hello", "sp ace"="quotes", twenty="there")"#,
+        );
+        assert_eq!(format!("{:#?}", dict).lines().collect::<Vec<_>>(), [
+            "(",
+            r#"    10 = "hello","#,
+            r#"    "sp ace" = "quotes","#,
+            r#"    twenty = "there","#,
+            ")",
+        ]);
+    }
+}
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
new file mode 100644
index 00000000..65fc71c4
--- /dev/null
+++ b/src/eval/mod.rs
@@ -0,0 +1,9 @@
+//! Evaluation of syntax trees.
+
+mod dict;
+mod scope;
+mod value;
+
+pub use dict::*;
+pub use scope::*;
+pub use value::*;
diff --git a/src/eval/scope.rs b/src/eval/scope.rs
new file mode 100644
index 00000000..8e6576d1
--- /dev/null
+++ b/src/eval/scope.rs
@@ -0,0 +1,35 @@
+//! Mapping from identifiers to functions.
+
+use std::collections::HashMap;
+use std::fmt::{self, Debug, Formatter};
+
+use super::value::FuncValue;
+
+/// A map from identifiers to functions.
+pub struct Scope {
+    functions: HashMap<String, FuncValue>,
+}
+
+impl Scope {
+    // Create a new empty scope with a fallback function that is invoked when no
+    // match is found.
+    pub fn new() -> Self {
+        Self { functions: HashMap::new() }
+    }
+
+    /// Associate the given name with the function.
+    pub fn insert(&mut self, name: impl Into<String>, function: FuncValue) {
+        self.functions.insert(name.into(), function);
+    }
+
+    /// Return the function with the given name if there is one.
+    pub fn func(&self, name: &str) -> Option<&FuncValue> {
+        self.functions.get(name)
+    }
+}
+
+impl Debug for Scope {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.debug_set().entries(self.functions.keys()).finish()
+    }
+}
diff --git a/src/eval/value.rs b/src/eval/value.rs
new file mode 100644
index 00000000..cfbc302c
--- /dev/null
+++ b/src/eval/value.rs
@@ -0,0 +1,496 @@
+//! Computational values: Syntactical expressions can be evaluated into these.
+
+use std::fmt::{self, Debug, Formatter};
+use std::ops::Deref;
+use std::rc::Rc;
+
+use fontdock::{FontStretch, FontStyle, FontWeight};
+
+use super::dict::{Dict, SpannedEntry};
+use crate::color::RgbaColor;
+use crate::layout::{Command, Commands, Dir, LayoutContext, SpecAlign};
+use crate::length::{Length, ScaleLength};
+use crate::paper::Paper;
+use crate::syntax::{Ident, Span, SpanWith, Spanned, SynNode, SynTree};
+use crate::{DynFuture, Feedback, Pass};
+
+/// 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 dictionary value: `(false, 12cm, greeting="hi")`.
+    Dict(DictValue),
+    /// A syntax tree containing typesetting content.
+    Tree(SynTree),
+    /// 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",
+            Dict(_) => "dict",
+            Tree(_) => "syntax tree",
+            Func(_) => "function",
+            Commands(_) => "commands",
+        }
+    }
+}
+
+impl Spanned<Value> {
+    /// Transform this value into something layoutable.
+    ///
+    /// If this is already a command-value, it is simply unwrapped, otherwise
+    /// the value is represented as layoutable content in a reasonable way.
+    pub fn into_commands(self) -> Commands {
+        match self.v {
+            Value::Commands(commands) => commands,
+            Value::Tree(tree) => vec![Command::LayoutSyntaxTree(tree)],
+
+            // Forward to each entry, separated with spaces.
+            Value::Dict(dict) => {
+                let mut commands = vec![];
+                let mut end = None;
+                for entry in dict.into_values() {
+                    if let Some(last_end) = end {
+                        let span = Span::new(last_end, entry.key.start);
+                        let tree = vec![SynNode::Spacing.span_with(span)];
+                        commands.push(Command::LayoutSyntaxTree(tree));
+                    }
+
+                    end = Some(entry.val.span.end);
+                    commands.extend(entry.val.into_commands());
+                }
+                commands
+            }
+
+            // Format with debug.
+            val => {
+                let fmt = format!("{:?}", val);
+                let tree = vec![SynNode::Text(fmt).span_with(self.span)];
+                vec![Command::LayoutSyntaxTree(tree)]
+            }
+        }
+    }
+}
+
+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),
+            Dict(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,
+            (Dict(a), Dict(b)) => a == b,
+            (Tree(a), Tree(b)) => a == b,
+            (Func(a), Func(b)) => Rc::ptr_eq(a, b),
+            (Commands(a), Commands(b)) => a == b,
+            _ => false,
+        }
+    }
+}
+
+/// An executable function value.
+///
+/// The first argument is a dictionary 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(Span, DictValue, LayoutContext<'_>) -> DynFuture<Pass<Value>>>;
+
+/// A dictionary of values.
+///
+/// # Example
+/// ```typst
+/// (false, 12cm, greeting="hi")
+/// ```
+pub type DictValue = Dict<SpannedEntry<Value>>;
+
+impl DictValue {
+    /// 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.
+    ///
+    /// Generates an error at `err_span` when no matching value was found.
+    pub fn expect<T: TryFromValue>(
+        &mut self,
+        name: &str,
+        span: Span,
+        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);
+            }
+        }
+        error!(@f, span, "missing argument: {}", name);
+        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_key<T>(&mut self, key: &str, f: &mut Feedback) -> Option<T>
+    where
+        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);
+                    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!(SynTree, "tree", Value::Tree(t) => t.clone());
+impl_match!(DictValue, "dict", Value::Dict(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 Deref for StringLike {
+    type Target = str;
+
+    fn deref(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
+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", Self::from_str);
+impl_ident!(FontStretch, "font stretch", Self::from_str);
+impl_ident!(Paper, "paper", Self::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;
+
+                if weight < MIN as f64 {
+                    error!(@f, value.span, "the minimum font weight is {}", MIN);
+                    Some(Self::THIN)
+                } else if weight > MAX as f64 {
+                    error!(@f, value.span, "the maximum font weight is {}", MAX);
+                    Some(Self::BLACK)
+                } else {
+                    FontWeight::from_number(weight.round() as u16)
+                }
+            }
+            Value::Ident(ident) => {
+                let weight = Self::from_str(ident);
+                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
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn entry(value: Value) -> SpannedEntry<Value> {
+        SpannedEntry::val(Spanned::zero(value))
+    }
+
+    #[test]
+    fn test_dict_take_removes_correct_entry() {
+        let mut dict = Dict::new();
+        dict.insert(1, entry(Value::Bool(false)));
+        dict.insert(2, entry(Value::Str("hi".to_string())));
+        assert_eq!(dict.take::<String>(), Some("hi".to_string()));
+        assert_eq!(dict.len(), 1);
+        assert_eq!(dict.take::<bool>(), Some(false));
+        assert!(dict.is_empty());
+    }
+
+    #[test]
+    fn test_dict_expect_errors_about_previous_entries() {
+        let mut f = Feedback::new();
+        let mut dict = Dict::new();
+        dict.insert(1, entry(Value::Bool(false)));
+        dict.insert(3, entry(Value::Str("hi".to_string())));
+        dict.insert(5, entry(Value::Bool(true)));
+        assert_eq!(
+            dict.expect::<String>("", Span::ZERO, &mut f),
+            Some("hi".to_string())
+        );
+        assert_eq!(f.diagnostics, [error!(
+            Span::ZERO,
+            "expected string, found bool"
+        )]);
+        assert_eq!(dict.len(), 1);
+    }
+
+    #[test]
+    fn test_dict_take_with_key_removes_the_entry() {
+        let mut f = Feedback::new();
+        let mut dict = Dict::new();
+        dict.insert(1, entry(Value::Bool(false)));
+        dict.insert("hi", entry(Value::Bool(true)));
+        assert_eq!(dict.take::<bool>(), Some(false));
+        assert_eq!(dict.take_key::<f64>("hi", &mut f), None);
+        assert_eq!(f.diagnostics, [error!(
+            Span::ZERO,
+            "expected number, found bool"
+        )]);
+        assert!(dict.is_empty());
+    }
+
+    #[test]
+    fn test_dict_take_all_removes_the_correct_entries() {
+        let mut dict = Dict::new();
+        dict.insert(1, entry(Value::Bool(false)));
+        dict.insert(3, entry(Value::Number(0.0)));
+        dict.insert(7, entry(Value::Bool(true)));
+        assert_eq!(dict.take_all_num::<bool>().collect::<Vec<_>>(), [
+            (1, false),
+            (7, true)
+        ],);
+        assert_eq!(dict.len(), 1);
+        assert_eq!(dict[3].val.v, Value::Number(0.0));
+    }
+}