Introduce `EvalResult` for control flow

2 files changed, 89 insertions, 49 deletions

diff --git a/src/diag.rs b/src/diag.rs
index 329e9a8a..9d1ec160 100644
--- a/src/diag.rs
+++ b/src/diag.rs
@@ -7,7 +7,7 @@ use crate::syntax::{Span, Spanned};
 /// Early-return with a vec-boxed [`Error`].
 macro_rules! bail {
     ($span:expr, $message:expr $(,)?) => {
-        return Err($crate::diag::Error::boxed($span, $message))
+        return Err($crate::diag::Error::boxed($span, $message).into())
     };
 
     ($span:expr, $fmt:expr, $($arg:expr),+ $(,)?) => {
@@ -16,7 +16,10 @@ macro_rules! bail {
 }
 
 /// The result type for typesetting and all its subpasses.
-pub type TypResult<T> = Result<T, Box<Vec<Error>>>;
+pub type TypResult<T> = Result<T, TypError>;
+
+/// The error type for typesetting and all its subpasses.
+pub type TypError = Box<Vec<Error>>;
 
 /// A result type with a string error message.
 pub type StrResult<T> = Result<T, String>;
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index bc6c8fc8..be168d63 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -35,7 +35,7 @@ pub use value::*;
 
 use unicode_segmentation::UnicodeSegmentation;
 
-use crate::diag::{At, Error, StrResult, Trace, Tracepoint, TypResult};
+use crate::diag::{At, Error, StrResult, Trace, Tracepoint, TypError, TypResult};
 use crate::geom::{Angle, Fractional, Length, Relative};
 use crate::library;
 use crate::syntax::ast::*;
@@ -49,13 +49,46 @@ pub trait Eval {
     type Output;
 
     /// Evaluate the expression to the output value.
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output>;
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output>;
+}
+
+/// The result type for evaluating a syntactic construct.
+pub type EvalResult<T> = Result<T, Control>;
+
+/// A control flow event that occurred during evaluation.
+#[derive(Clone, Debug, PartialEq)]
+pub enum Control {
+    Break(Span),
+    Continue(Span),
+    Return(Option<Value>, Span),
+    Err(TypError),
+}
+
+impl From<TypError> for Control {
+    fn from(error: TypError) -> Self {
+        Self::Err(error)
+    }
+}
+
+impl From<Control> for TypError {
+    fn from(control: Control) -> Self {
+        match control {
+            Control::Break(span) => Error::boxed(span, "cannot break outside of loop"),
+            Control::Continue(span) => {
+                Error::boxed(span, "cannot continue outside of loop")
+            }
+            Control::Return(_, span) => {
+                Error::boxed(span, "cannot return outside of function")
+            }
+            Control::Err(e) => e,
+        }
+    }
 }
 
 impl Eval for Markup {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         eval_markup(ctx, scp, &mut self.nodes())
     }
 }
@@ -65,7 +98,7 @@ fn eval_markup(
     ctx: &mut Context,
     scp: &mut Scopes,
     nodes: &mut impl Iterator<Item = MarkupNode>,
-) -> TypResult<Template> {
+) -> EvalResult<Template> {
     let mut seq = Vec::with_capacity(nodes.size_hint().1.unwrap_or_default());
 
     while let Some(node) = nodes.next() {
@@ -93,7 +126,7 @@ fn eval_markup(
 impl Eval for MarkupNode {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(match self {
             Self::Space => Template::Space,
             Self::Linebreak => Template::Linebreak,
@@ -114,7 +147,7 @@ impl Eval for MarkupNode {
 impl Eval for StrongNode {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(Template::show(library::StrongNode(
             self.body().eval(ctx, scp)?,
         )))
@@ -124,7 +157,7 @@ impl Eval for StrongNode {
 impl Eval for EmphNode {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(Template::show(library::EmphNode(
             self.body().eval(ctx, scp)?,
         )))
@@ -134,7 +167,7 @@ impl Eval for EmphNode {
 impl Eval for RawNode {
     type Output = Template;
 
-    fn eval(&self, _: &mut Context, _: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, _: &mut Context, _: &mut Scopes) -> EvalResult<Self::Output> {
         let template = Template::show(library::RawNode {
             text: self.text.clone(),
             block: self.block,
@@ -149,7 +182,7 @@ impl Eval for RawNode {
 impl Eval for MathNode {
     type Output = Template;
 
-    fn eval(&self, _: &mut Context, _: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, _: &mut Context, _: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(Template::show(library::MathNode {
             formula: self.formula.clone(),
             display: self.display,
@@ -160,7 +193,7 @@ impl Eval for MathNode {
 impl Eval for HeadingNode {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(Template::show(library::HeadingNode {
             body: self.body().eval(ctx, scp)?,
             level: self.level(),
@@ -171,7 +204,7 @@ impl Eval for HeadingNode {
 impl Eval for ListNode {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(Template::List(library::ListItem {
             number: None,
             body: Box::new(self.body().eval(ctx, scp)?),
@@ -182,7 +215,7 @@ impl Eval for ListNode {
 impl Eval for EnumNode {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(Template::Enum(library::ListItem {
             number: self.number(),
             body: Box::new(self.body().eval(ctx, scp)?),
@@ -193,7 +226,7 @@ impl Eval for EnumNode {
 impl Eval for Expr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         match self {
             Self::Lit(v) => v.eval(ctx, scp),
             Self::Ident(v) => v.eval(ctx, scp),
@@ -211,6 +244,7 @@ impl Eval for Expr {
             Self::Set(_) | Self::Show(_) | Self::Wrap(_) => {
                 Err("set, show and wrap are only allowed directly in markup")
                     .at(self.span())
+                    .map_err(Into::into)
             }
             Self::If(v) => v.eval(ctx, scp),
             Self::While(v) => v.eval(ctx, scp),
@@ -227,7 +261,7 @@ impl Eval for Expr {
 impl Eval for Lit {
     type Output = Value;
 
-    fn eval(&self, _: &mut Context, _: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, _: &mut Context, _: &mut Scopes) -> EvalResult<Self::Output> {
         Ok(match self.kind() {
             LitKind::None => Value::None,
             LitKind::Auto => Value::Auto,
@@ -246,7 +280,7 @@ impl Eval for Lit {
 impl Eval for Ident {
     type Output = Value;
 
-    fn eval(&self, _: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, _: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         match scp.get(self) {
             Some(slot) => Ok(slot.read().unwrap().clone()),
             None => bail!(self.span(), "unknown variable"),
@@ -257,7 +291,7 @@ impl Eval for Ident {
 impl Eval for ArrayExpr {
     type Output = Array;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         self.items().map(|expr| expr.eval(ctx, scp)).collect()
     }
 }
@@ -265,7 +299,7 @@ impl Eval for ArrayExpr {
 impl Eval for DictExpr {
     type Output = Dict;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         self.items()
             .map(|x| Ok((x.name().take(), x.expr().eval(ctx, scp)?)))
             .collect()
@@ -275,7 +309,7 @@ impl Eval for DictExpr {
 impl Eval for TemplateExpr {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         scp.enter();
         let template = self.body().eval(ctx, scp)?;
         scp.exit();
@@ -286,7 +320,7 @@ impl Eval for TemplateExpr {
 impl Eval for GroupExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         self.expr().eval(ctx, scp)
     }
 }
@@ -294,7 +328,7 @@ impl Eval for GroupExpr {
 impl Eval for BlockExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         scp.enter();
 
         let mut output = Value::None;
@@ -312,21 +346,21 @@ impl Eval for BlockExpr {
 impl Eval for UnaryExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let value = self.expr().eval(ctx, scp)?;
         let result = match self.op() {
             UnOp::Pos => ops::pos(value),
             UnOp::Neg => ops::neg(value),
             UnOp::Not => ops::not(value),
         };
-        result.at(self.span())
+        Ok(result.at(self.span())?)
     }
 }
 
 impl Eval for BinaryExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         match self.op() {
             BinOp::Add => self.apply(ctx, scp, ops::add),
             BinOp::Sub => self.apply(ctx, scp, ops::sub),
@@ -356,7 +390,7 @@ impl BinaryExpr {
         ctx: &mut Context,
         scp: &mut Scopes,
         op: fn(Value, Value) -> StrResult<Value>,
-    ) -> TypResult<Value> {
+    ) -> EvalResult<Value> {
         let lhs = self.lhs().eval(ctx, scp)?;
 
         // Short-circuit boolean operations.
@@ -367,7 +401,7 @@ impl BinaryExpr {
         }
 
         let rhs = self.rhs().eval(ctx, scp)?;
-        op(lhs, rhs).at(self.span())
+        Ok(op(lhs, rhs).at(self.span())?)
     }
 
     /// Apply an assignment operation.
@@ -376,7 +410,7 @@ impl BinaryExpr {
         ctx: &mut Context,
         scp: &mut Scopes,
         op: fn(Value, Value) -> StrResult<Value>,
-    ) -> TypResult<Value> {
+    ) -> EvalResult<Value> {
         let rhs = self.rhs().eval(ctx, scp)?;
         self.lhs().access(
             ctx,
@@ -394,12 +428,12 @@ impl BinaryExpr {
 impl Eval for CallExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let span = self.callee().span();
         let callee = self.callee().eval(ctx, scp)?;
         let args = self.args().eval(ctx, scp)?;
 
-        match callee {
+        Ok(match callee {
             Value::Array(array) => {
                 array.get(args.into_index()?).map(Value::clone).at(self.span())
             }
@@ -423,14 +457,14 @@ impl Eval for CallExpr {
                 "expected callable or collection, found {}",
                 v.type_name(),
             ),
-        }
+        }?)
     }
 }
 
 impl Eval for CallArgs {
     type Output = Args;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let mut items = Vec::new();
 
         for arg in self.items() {
@@ -482,7 +516,7 @@ impl Eval for CallArgs {
 impl Eval for ClosureExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         // The closure's name is defined by its let binding if there's one.
         let name = self.name().map(Ident::take);
 
@@ -529,7 +563,7 @@ impl Eval for ClosureExpr {
 impl Eval for WithExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let callee = self.callee();
         let func = callee.eval(ctx, scp)?.cast::<Func>().at(callee.span())?;
         let args = self.args().eval(ctx, scp)?;
@@ -540,7 +574,7 @@ impl Eval for WithExpr {
 impl Eval for LetExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let value = match self.init() {
             Some(expr) => expr.eval(ctx, scp)?,
             None => Value::None,
@@ -553,18 +587,18 @@ impl Eval for LetExpr {
 impl Eval for SetExpr {
     type Output = StyleMap;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let class = self.class();
         let class = class.eval(ctx, scp)?.cast::<Class>().at(class.span())?;
         let args = self.args().eval(ctx, scp)?;
-        class.set(args)
+        Ok(class.set(args)?)
     }
 }
 
 impl Eval for ShowExpr {
     type Output = StyleMap;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let class = self.class();
         let class = class.eval(ctx, scp)?.cast::<Class>().at(class.span())?;
         let closure = self.closure();
@@ -578,7 +612,7 @@ impl Eval for ShowExpr {
 impl Eval for IfExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let condition = self.condition();
         if condition.eval(ctx, scp)?.cast::<bool>().at(condition.span())? {
             self.if_body().eval(ctx, scp)
@@ -593,7 +627,7 @@ impl Eval for IfExpr {
 impl Eval for WhileExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let mut output = Value::None;
 
         let condition = self.condition();
@@ -610,7 +644,7 @@ impl Eval for WhileExpr {
 impl Eval for ForExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         macro_rules! iter {
             (for ($($binding:ident => $value:ident),*) in $iter:expr) => {{
                 let mut output = Value::None;
@@ -673,7 +707,7 @@ impl Eval for ForExpr {
 impl Eval for ImportExpr {
     type Output = Value;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let span = self.path().span();
         let path = self.path().eval(ctx, scp)?.cast::<EcoString>().at(span)?;
         let module = import(ctx, &path, span)?;
@@ -702,7 +736,7 @@ impl Eval for ImportExpr {
 impl Eval for IncludeExpr {
     type Output = Template;
 
-    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> TypResult<Self::Output> {
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let span = self.path().span();
         let path = self.path().eval(ctx, scp)?.cast::<EcoString>().at(span)?;
         let module = import(ctx, &path, span)?;
@@ -735,24 +769,27 @@ fn import(ctx: &mut Context, path: &str, span: Span) -> TypResult<Module> {
 impl Eval for BreakExpr {
     type Output = Value;
 
-    fn eval(&self, _: &mut Context, _: &mut Scopes) -> TypResult<Self::Output> {
-        Err("break is not yet implemented").at(self.span())
+    fn eval(&self, _: &mut Context, _: &mut Scopes) -> EvalResult<Self::Output> {
+        Err(Control::Break(self.span()))
     }
 }
 
 impl Eval for ContinueExpr {
     type Output = Value;
 
-    fn eval(&self, _: &mut Context, _: &mut Scopes) -> TypResult<Self::Output> {
-        Err("continue is not yet implemented").at(self.span())
+    fn eval(&self, _: &mut Context, _: &mut Scopes) -> EvalResult<Self::Output> {
+        Err(Control::Continue(self.span()))
     }
 }
 
 impl Eval for ReturnExpr {
     type Output = Value;
 
-    fn eval(&self, _: &mut Context, _: &mut Scopes) -> TypResult<Self::Output> {
-        Err("return is not yet implemented").at(self.span())
+    fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
+        Err(Control::Return(
+            self.body().map(|body| body.eval(ctx, scp)).transpose()?,
+            self.span(),
+        ))
     }
 }