1 files changed, 176 insertions, 0 deletions
diff --git a/src/eval/function.rs b/src/eval/function.rs
new file mode 100644
index 00000000..c986d71a
--- /dev/null
+++ b/src/eval/function.rs
@@ -0,0 +1,176 @@
+use std::fmt::{self, Debug, Formatter};
+use std::ops::Deref;
+use std::rc::Rc;
+
+use super::{Cast, CastResult, EvalContext, Value};
+use crate::eco::EcoString;
+use crate::syntax::{Span, Spanned};
+
+/// An evaluatable function.
+#[derive(Clone)]
+pub struct Function {
+    /// The name of the function.
+    ///
+    /// The string is boxed to make the whole struct fit into 24 bytes, so that
+    /// a value fits into 32 bytes.
+    name: Option<Box<EcoString>>,
+    /// The closure that defines the function.
+    f: Rc<dyn Fn(&mut EvalContext, &mut FuncArgs) -> Value>,
+}
+
+impl Function {
+    /// Create a new function from a rust closure.
+    pub fn new<F>(name: Option<EcoString>, f: F) -> Self
+    where
+        F: Fn(&mut EvalContext, &mut FuncArgs) -> Value + 'static,
+    {
+        Self { name: name.map(Box::new), f: Rc::new(f) }
+    }
+
+    /// The name of the function.
+    pub fn name(&self) -> Option<&EcoString> {
+        self.name.as_ref().map(|s| &**s)
+    }
+}
+
+impl Deref for Function {
+    type Target = dyn Fn(&mut EvalContext, &mut FuncArgs) -> Value;
+
+    fn deref(&self) -> &Self::Target {
+        self.f.as_ref()
+    }
+}
+
+impl Debug for Function {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.debug_struct("ValueFunc").field("name", &self.name).finish()
+    }
+}
+
+impl PartialEq for Function {
+    fn eq(&self, other: &Self) -> bool {
+        // We cast to thin pointers because we don't want to compare vtables.
+        Rc::as_ptr(&self.f) as *const () == Rc::as_ptr(&other.f) as *const ()
+    }
+}
+
+/// Evaluated arguments to a function.
+#[derive(Debug, Clone, PartialEq)]
+pub struct FuncArgs {
+    /// The span of the whole argument list.
+    pub span: Span,
+    /// The positional arguments.
+    pub items: Vec<FuncArg>,
+}
+
+/// An argument to a function call: `12` or `draw: false`.
+#[derive(Debug, Clone, PartialEq)]
+pub struct FuncArg {
+    /// The span of the whole argument.
+    pub span: Span,
+    /// The name of the argument (`None` for positional arguments).
+    pub name: Option<EcoString>,
+    /// The value of the argument.
+    pub value: Spanned<Value>,
+}
+
+impl FuncArgs {
+    /// Find and consume the first castable positional argument.
+    pub fn eat<T>(&mut self, ctx: &mut EvalContext) -> Option<T>
+    where
+        T: Cast<Spanned<Value>>,
+    {
+        (0 .. self.items.len()).find_map(|index| {
+            let slot = &mut self.items[index];
+            if slot.name.is_some() {
+                return None;
+            }
+
+            let value = std::mem::replace(&mut slot.value, Spanned::zero(Value::None));
+            let span = value.span;
+
+            match T::cast(value) {
+                CastResult::Ok(t) => {
+                    self.items.remove(index);
+                    Some(t)
+                }
+                CastResult::Warn(t, m) => {
+                    self.items.remove(index);
+                    ctx.diag(warning!(span, "{}", m));
+                    Some(t)
+                }
+                CastResult::Err(value) => {
+                    slot.value = value;
+                    None
+                }
+            }
+        })
+    }
+
+    /// Find and consume the first castable positional argument, producing a
+    /// `missing argument: {what}` error if no match was found.
+    pub fn expect<T>(&mut self, ctx: &mut EvalContext, what: &str) -> Option<T>
+    where
+        T: Cast<Spanned<Value>>,
+    {
+        let found = self.eat(ctx);
+        if found.is_none() {
+            ctx.diag(error!(self.span, "missing argument: {}", what));
+        }
+        found
+    }
+
+    /// Find, consume and collect all castable positional arguments.
+    ///
+    /// This function returns a vector instead of an iterator because the
+    /// iterator would require unique access to the context, rendering it rather
+    /// unusable. If you need to process arguments one-by-one, you probably want
+    /// to use a while-let loop together with [`eat()`](Self::eat).
+    pub fn all<T>(&mut self, ctx: &mut EvalContext) -> Vec<T>
+    where
+        T: Cast<Spanned<Value>>,
+    {
+        std::iter::from_fn(|| self.eat(ctx)).collect()
+    }
+
+    /// Cast and remove the value for the given named argument, producing an
+    /// error if the conversion fails.
+    pub fn named<T>(&mut self, ctx: &mut EvalContext, name: &str) -> Option<T>
+    where
+        T: Cast<Spanned<Value>>,
+    {
+        let index = self
+            .items
+            .iter()
+            .position(|arg| arg.name.as_ref().map_or(false, |other| other == name))?;
+
+        let value = self.items.remove(index).value;
+        let span = value.span;
+
+        match T::cast(value) {
+            CastResult::Ok(t) => Some(t),
+            CastResult::Warn(t, m) => {
+                ctx.diag(warning!(span, "{}", m));
+                Some(t)
+            }
+            CastResult::Err(value) => {
+                ctx.diag(error!(
+                    span,
+                    "expected {}, found {}",
+                    T::TYPE_NAME,
+                    value.v.type_name(),
+                ));
+                None
+            }
+        }
+    }
+
+    /// Produce "unexpected argument" errors for all remaining arguments.
+    pub fn finish(self, ctx: &mut EvalContext) {
+        for arg in &self.items {
+            if arg.value.v != Value::Error {
+                ctx.diag(error!(arg.span, "unexpected argument"));
+            }
+        }
+    }
+}