With expressions

8 files changed, 135 insertions, 48 deletions

diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 116d3c12..a9fff56b 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -263,7 +263,8 @@ impl Eval for Expr {
             Self::Group(ref v) => v.eval(ctx),
             Self::Block(ref v) => v.eval(ctx),
             Self::Call(ref v) => v.eval(ctx),
-            Self::Closure(ref v) => Value::Func(v.eval(ctx)),
+            Self::Closure(ref v) => v.eval(ctx),
+            Self::With(ref v) => v.eval(ctx),
             Self::Unary(ref v) => v.eval(ctx),
             Self::Binary(ref v) => v.eval(ctx),
             Self::Let(ref v) => v.eval(ctx),
@@ -498,11 +499,13 @@ impl Eval for CallArg {
     fn eval(&self, ctx: &mut EvalContext) -> Self::Output {
         match self {
             Self::Pos(expr) => FuncArg {
+                span: self.span(),
                 name: None,
                 value: Spanned::new(expr.eval(ctx), expr.span()),
             },
             Self::Named(Named { name, expr }) => FuncArg {
-                name: Some(Spanned::new(name.string.clone(), name.span)),
+                span: self.span(),
+                name: Some(name.string.clone()),
                 value: Spanned::new(expr.eval(ctx), expr.span()),
             },
         }
@@ -510,7 +513,7 @@ impl Eval for CallArg {
 }
 
 impl Eval for ClosureExpr {
-    type Output = FuncValue;
+    type Output = Value;
 
     fn eval(&self, ctx: &mut EvalContext) -> Self::Output {
         let params = Rc::clone(&self.params);
@@ -524,7 +527,7 @@ impl Eval for ClosureExpr {
         };
 
         let name = self.name.as_ref().map(|id| id.to_string());
-        FuncValue::new(name, move |ctx, args| {
+        Value::Func(FuncValue::new(name, move |ctx, args| {
             // Don't leak the scopes from the call site. Instead, we use the
             // scope of captured variables we collected earlier.
             let prev = mem::take(&mut ctx.scopes);
@@ -539,7 +542,40 @@ impl Eval for ClosureExpr {
             let value = body.eval(ctx);
             ctx.scopes = prev;
             value
-        })
+        }))
+    }
+}
+
+impl Eval for WithExpr {
+    type Output = Value;
+
+    fn eval(&self, ctx: &mut EvalContext) -> Self::Output {
+        let callee = self.callee.eval(ctx);
+        if let Some(func) = ctx.cast::<FuncValue>(callee, self.callee.span()) {
+            let applied = self.args.eval(ctx);
+            let name = func.name().map(|s| s.to_string());
+            Value::Func(FuncValue::new(name, move |ctx, args| {
+                // Remove named arguments that were overridden.
+                let kept: Vec<_> = applied
+                    .items
+                    .iter()
+                    .filter(|arg| {
+                        arg.name.is_none()
+                            || args.items.iter().all(|other| arg.name != other.name)
+                    })
+                    .cloned()
+                    .collect();
+
+                // Preprend the applied arguments so that the positional arguments
+                // are in the right order.
+                args.items.splice(.. 0, kept);
+
+                // Call the original function.
+                func(ctx, args)
+            }))
+        } else {
+            Value::Error
+        }
     }
 }
 
diff --git a/src/eval/value.rs b/src/eval/value.rs
index e8ecffa2..28c7d588 100644
--- a/src/eval/value.rs
+++ b/src/eval/value.rs
@@ -274,7 +274,7 @@ impl Debug for FuncValue {
 pub struct FuncArgs {
     /// The span of the whole argument list.
     pub span: Span,
-    /// The arguments.
+    /// The positional arguments.
     pub items: Vec<FuncArg>,
 }
 
@@ -346,7 +346,7 @@ impl FuncArgs {
         let index = self
             .items
             .iter()
-            .position(|arg| arg.name.as_ref().map(|s| s.v.as_str()) == Some(name))?;
+            .position(|arg| arg.name.as_ref().map_or(false, |other| name == other))?;
 
         let value = self.items.remove(index).value;
         let span = value.span;
@@ -373,7 +373,7 @@ impl FuncArgs {
     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"));
+                ctx.diag(error!(arg.span, "unexpected argument"));
             }
         }
     }
@@ -382,22 +382,14 @@ impl FuncArgs {
 /// 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<Spanned<String>>,
+    pub name: Option<String>,
     /// The value of the argument.
     pub value: Spanned<Value>,
 }
 
-impl FuncArg {
-    /// The source code location.
-    pub fn span(&self) -> Span {
-        match &self.name {
-            Some(name) => name.span.join(self.value.span),
-            None => self.value.span,
-        }
-    }
-}
-
 /// A wrapper around a dynamic value.
 pub struct AnyValue(Box<dyn Bounds>);
 
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index d8000d8c..381d44e2 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -245,6 +245,10 @@ fn expr_with(p: &mut Parser, atomic: bool, min_prec: usize) -> Option<Expr> {
             continue;
         }
 
+        if p.eat_if(Token::With) {
+            lhs = with_expr(p, lhs);
+        }
+
         if atomic {
             break;
         }
@@ -545,6 +549,19 @@ fn args(p: &mut Parser) -> CallArgs {
     CallArgs { span: p.span(start), items }
 }
 
+/// Parse a with expression.
+fn with_expr(p: &mut Parser, callee: Expr) -> Expr {
+    p.start_group(Group::Paren, TokenMode::Code);
+    let args = args(p);
+    p.end_group();
+
+    Expr::With(WithExpr {
+        span: p.span(callee.span().start),
+        callee: Box::new(callee),
+        args,
+    })
+}
+
 /// Parse a let expression.
 fn let_expr(p: &mut Parser) -> Option<Expr> {
     let start = p.next_start();
@@ -552,32 +569,37 @@ fn let_expr(p: &mut Parser) -> Option<Expr> {
 
     let mut let_expr = None;
     if let Some(binding) = ident(p) {
-        // If a parenthesis follows, this is a function definition.
-        let mut params = None;
-        if p.peek_direct() == Some(Token::LeftParen) {
-            p.start_group(Group::Paren, TokenMode::Code);
-            let items = collection(p).0;
-            params = Some(idents(p, items));
-            p.end_group();
-        }
-
         let mut init = None;
-        if p.eat_if(Token::Eq) {
-            init = expr(p);
-        } else if params.is_some() {
-            // Function definitions must have a body.
-            p.expected_at("body", p.prev_end());
-        }
 
-        // Rewrite into a closure expression if it's a function definition.
-        if let Some(params) = params {
-            let body = init?;
-            init = Some(Expr::Closure(ClosureExpr {
-                span: binding.span.join(body.span()),
-                name: Some(binding.clone()),
-                params: Rc::new(params),
-                body: Rc::new(body),
-            }));
+        if p.eat_if(Token::With) {
+            init = Some(with_expr(p, Expr::Ident(binding.clone())));
+        } else {
+            // If a parenthesis follows, this is a function definition.
+            let mut params = None;
+            if p.peek_direct() == Some(Token::LeftParen) {
+                p.start_group(Group::Paren, TokenMode::Code);
+                let items = collection(p).0;
+                params = Some(idents(p, items));
+                p.end_group();
+            }
+
+            if p.eat_if(Token::Eq) {
+                init = expr(p);
+            } else if params.is_some() {
+                // Function definitions must have a body.
+                p.expected_at("body", p.prev_end());
+            }
+
+            // Rewrite into a closure expression if it's a function definition.
+            if let Some(params) = params {
+                let body = init?;
+                init = Some(Expr::Closure(ClosureExpr {
+                    span: binding.span.join(body.span()),
+                    name: Some(binding.clone()),
+                    params: Rc::new(params),
+                    body: Rc::new(body),
+                }));
+            }
         }
 
         let_expr = Some(Expr::Let(LetExpr {
diff --git a/src/parse/tokens.rs b/src/parse/tokens.rs
index a496010e..abc3d6a6 100644
--- a/src/parse/tokens.rs
+++ b/src/parse/tokens.rs
@@ -366,9 +366,6 @@ impl<'s> Tokens<'s> {
     fn ident(&mut self, start: usize) -> Token<'s> {
         self.s.eat_while(is_id_continue);
         match self.s.eaten_from(start) {
-            "not" => Token::Not,
-            "and" => Token::And,
-            "or" => Token::Or,
             "none" => Token::None,
             "auto" => Token::Auto,
             "true" => Token::Bool(true),
@@ -489,6 +486,10 @@ impl Debug for Tokens<'_> {
 
 fn keyword(id: &str) -> Option<Token<'static>> {
     Some(match id {
+        "not" => Token::Not,
+        "and" => Token::And,
+        "or" => Token::Or,
+        "with" => Token::With,
         "let" => Token::Let,
         "if" => Token::If,
         "else" => Token::Else,
@@ -777,6 +778,7 @@ mod tests {
     fn test_tokenize_keywords() {
         // A list of a few (not all) keywords.
         let list = [
+            ("not", Not),
             ("let", Let),
             ("if", If),
             ("else", Else),
diff --git a/src/pretty.rs b/src/pretty.rs
index e2942f6f..216a6370 100644
--- a/src/pretty.rs
+++ b/src/pretty.rs
@@ -217,6 +217,7 @@ impl Pretty for Expr {
             Self::Binary(v) => v.pretty(p),
             Self::Call(v) => v.pretty(p),
             Self::Closure(v) => v.pretty(p),
+            Self::With(v) => v.pretty(p),
             Self::Let(v) => v.pretty(p),
             Self::If(v) => v.pretty(p),
             Self::While(v) => v.pretty(p),
@@ -370,6 +371,15 @@ impl Pretty for ClosureExpr {
     }
 }
 
+impl Pretty for WithExpr {
+    fn pretty(&self, p: &mut Printer) {
+        self.callee.pretty(p);
+        p.push_str(" with (");
+        self.args.pretty(p);
+        p.push(')');
+    }
+}
+
 impl Pretty for LetExpr {
     fn pretty(&self, p: &mut Printer) {
         p.push_str("let ");
@@ -539,7 +549,7 @@ impl Pretty for FuncArgs {
 impl Pretty for FuncArg {
     fn pretty(&self, p: &mut Printer) {
         if let Some(name) = &self.name {
-            p.push_str(&name.v);
+            p.push_str(&name);
             p.push_str(": ");
         }
         self.value.v.pretty(p);
diff --git a/src/syntax/expr.rs b/src/syntax/expr.rs
index 24850d65..62f02399 100644
--- a/src/syntax/expr.rs
+++ b/src/syntax/expr.rs
@@ -52,6 +52,8 @@ pub enum Expr {
     Call(CallExpr),
     /// A closure expression: `(x, y) => z`.
     Closure(ClosureExpr),
+    /// A with expression: `f with (x, y: 1)`.
+    With(WithExpr),
     /// A let expression: `let x = 1`.
     Let(LetExpr),
     /// An if-else expression: `if x { y } else { z }`.
@@ -91,6 +93,7 @@ impl Expr {
             Self::Binary(ref v) => v.span,
             Self::Call(ref v) => v.span,
             Self::Closure(ref v) => v.span,
+            Self::With(ref v) => v.span,
             Self::Let(ref v) => v.span,
             Self::If(ref v) => v.span,
             Self::While(ref v) => v.span,
@@ -383,7 +386,7 @@ pub enum Associativity {
 pub struct CallExpr {
     /// The source code location.
     pub span: Span,
-    /// The callee of the function.
+    /// The function to call.
     pub callee: Box<Expr>,
     /// The arguments to the function.
     pub args: CallArgs,
@@ -435,6 +438,19 @@ pub struct ClosureExpr {
     pub body: Rc<Expr>,
 }
 
+/// A with expression: `f with (x, y: 1)`.
+///
+/// Applies arguments to a function.
+#[derive(Debug, Clone, PartialEq)]
+pub struct WithExpr {
+    /// The source code location.
+    pub span: Span,
+    /// The function to apply the arguments to.
+    pub callee: Box<Expr>,
+    /// The arguments to apply to the function.
+    pub args: CallArgs,
+}
+
 /// A let expression: `let x = 1`.
 #[derive(Debug, Clone, PartialEq)]
 pub struct LetExpr {
diff --git a/src/syntax/token.rs b/src/syntax/token.rs
index 254a56a2..3f07bb33 100644
--- a/src/syntax/token.rs
+++ b/src/syntax/token.rs
@@ -74,6 +74,8 @@ pub enum Token<'s> {
     And,
     /// The `or` operator.
     Or,
+    /// The `with` operator.
+    With,
     /// The none literal: `none`.
     None,
     /// The auto literal: `auto`.
@@ -241,6 +243,7 @@ impl<'s> Token<'s> {
             Self::Not => "operator `not`",
             Self::And => "operator `and`",
             Self::Or => "operator `or`",
+            Self::With => "operator `with`",
             Self::None => "`none`",
             Self::Auto => "`auto`",
             Self::Let => "keyword `let`",
diff --git a/src/syntax/visit.rs b/src/syntax/visit.rs
index a1a848ef..cf819ef0 100644
--- a/src/syntax/visit.rs
+++ b/src/syntax/visit.rs
@@ -99,6 +99,7 @@ visit! {
             Expr::Binary(e) => v.visit_binary(e),
             Expr::Call(e) => v.visit_call(e),
             Expr::Closure(e) => v.visit_closure(e),
+            Expr::With(e) => v.visit_with(e),
             Expr::Let(e) => v.visit_let(e),
             Expr::If(e) => v.visit_if(e),
             Expr::While(e) => v.visit_while(e),
@@ -176,6 +177,11 @@ visit! {
         }
     }
 
+    fn visit_with(v, node: &WithExpr) {
+        v.visit_expr(&node.callee);
+        v.visit_args(&node.args);
+    }
+
     fn visit_let(v, node: &LetExpr) {
         v.visit_binding(&node.binding);
         if let Some(init) = &node.init {