Basic let bindings 🎞

8 files changed, 135 insertions, 9 deletions

diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 32a7d6ba..0e6fa79f 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -46,6 +46,17 @@ pub trait Eval {
     fn eval(self, ctx: &mut EvalContext) -> Self::Output;
 }
 
+impl<'a, T> Eval for &'a Spanned<T>
+where
+    Spanned<&'a T>: Eval,
+{
+    type Output = <Spanned<&'a T> as Eval>::Output;
+
+    fn eval(self, ctx: &mut EvalContext) -> Self::Output {
+        self.as_ref().eval(ctx)
+    }
+}
+
 impl Eval for &[Spanned<Node>] {
     type Output = ();
 
@@ -163,6 +174,14 @@ impl Eval for Spanned<&Expr> {
             Expr::Template(v) => Value::Template(v.clone()),
             Expr::Group(v) => v.as_ref().with_span(self.span).eval(ctx),
             Expr::Block(v) => v.as_ref().with_span(self.span).eval(ctx),
+            Expr::Let(v) => {
+                let value = match &v.expr {
+                    Some(expr) => expr.as_ref().eval(ctx),
+                    None => Value::None,
+                };
+                Rc::make_mut(&mut ctx.state.scope).set(v.pat.v.as_str(), value);
+                Value::None
+            }
         }
     }
 }
@@ -171,7 +190,7 @@ impl Eval for Spanned<&ExprUnary> {
     type Output = Value;
 
     fn eval(self, ctx: &mut EvalContext) -> Self::Output {
-        let value = (*self.v.expr).as_ref().eval(ctx);
+        let value = self.v.expr.as_ref().eval(ctx);
 
         if let Value::Error = value {
             return Value::Error;
@@ -189,8 +208,8 @@ impl Eval for Spanned<&ExprBinary> {
     type Output = Value;
 
     fn eval(self, ctx: &mut EvalContext) -> Self::Output {
-        let lhs = (*self.v.lhs).as_ref().eval(ctx);
-        let rhs = (*self.v.rhs).as_ref().eval(ctx);
+        let lhs = self.v.lhs.as_ref().eval(ctx);
+        let rhs = self.v.rhs.as_ref().eval(ctx);
 
         if lhs == Value::Error || rhs == Value::Error {
             return Value::Error;
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index a66660e5..4483ed76 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -82,6 +82,11 @@ fn node(p: &mut Parser, at_start: bool) -> Option<Node> {
         Token::Raw(t) => Node::Raw(raw(p, t)),
         Token::UnicodeEscape(t) => Node::Text(unicode_escape(p, t)),
 
+        // Keywords.
+        Token::Let => {
+            return Some(Node::Expr(expr_let(p)?));
+        }
+
         // Comments.
         Token::LineComment(_) | Token::BlockComment(_) => {
             p.eat();
@@ -329,7 +334,7 @@ fn value(p: &mut Parser) -> Option<Expr> {
         Some(Token::Angle(val, unit)) => Expr::Angle(val, unit),
         Some(Token::Percent(p)) => Expr::Percent(p),
         Some(Token::Hex(hex)) => Expr::Color(color(p, hex)),
-        Some(Token::Str(token)) => Expr::Str(str(p, token)),
+        Some(Token::Str(token)) => Expr::Str(string(p, token)),
 
         // No value.
         _ => {
@@ -377,7 +382,7 @@ fn color(p: &mut Parser, hex: &str) -> RgbaColor {
 }
 
 /// Parse a string.
-fn str(p: &mut Parser, token: TokenStr) -> String {
+fn string(p: &mut Parser, token: TokenStr) -> String {
     if !token.terminated {
         p.diag_expected_at("quote", p.peek_span().end);
     }
@@ -385,5 +390,33 @@ fn str(p: &mut Parser, token: TokenStr) -> String {
     resolve::resolve_string(token.string)
 }
 
+/// Parse a let expresion.
+fn expr_let(p: &mut Parser) -> Option<Expr> {
+    p.push_mode(TokenMode::Code);
+    p.start_group(Group::Terminated);
+    p.eat_assert(Token::Let);
+
+    let pat = p.span_if(ident);
+    let mut rhs = None;
+
+    if pat.is_some() {
+        if p.eat_if(Token::Eq) {
+            if let Some(expr) = p.span_if(expr) {
+                rhs = Some(Box::new(expr));
+            }
+        }
+    } else {
+        p.diag_expected("identifier");
+    }
+
+    while !p.eof() {
+        p.diag_unexpected();
+    }
+
+    p.pop_mode();
+    p.end_group();
+    pat.map(|pat| Expr::Let(ExprLet { pat, expr: rhs }))
+}
+
 #[cfg(test)]
 mod tests;
diff --git a/src/parse/parser.rs b/src/parse/parser.rs
index 0d3761df..2b5fe720 100644
--- a/src/parse/parser.rs
+++ b/src/parse/parser.rs
@@ -117,6 +117,7 @@ impl<'s> Parser<'s> {
             Group::Bracket => self.eat_assert(Token::LeftBracket),
             Group::Brace => self.eat_assert(Token::LeftBrace),
             Group::Subheader => {}
+            Group::Terminated => {}
         }
 
         self.groups.push(group);
@@ -139,6 +140,7 @@ impl<'s> Parser<'s> {
             Group::Bracket => Some(Token::RightBracket),
             Group::Brace => Some(Token::RightBrace),
             Group::Subheader => None,
+            Group::Terminated => Some(Token::Semicolon),
         };
 
         if let Some(token) = end {
@@ -290,6 +292,7 @@ impl<'s> Parser<'s> {
             Some(Token::RightBracket) => Group::Bracket,
             Some(Token::RightBrace) => Group::Brace,
             Some(Token::Pipe) => Group::Subheader,
+            Some(Token::Semicolon) => Group::Terminated,
             _ => return,
         }) {
             self.peeked = None;
@@ -316,4 +319,6 @@ pub enum Group {
     /// A group ended by a chained subheader or a closing bracket:
     /// `... >>`, `...]`.
     Subheader,
+    /// A group ended by a semicolon: `;`.
+    Terminated,
 }
diff --git a/src/parse/tests.rs b/src/parse/tests.rs
index b9a3d301..9460db6b 100644
--- a/src/parse/tests.rs
+++ b/src/parse/tests.rs
@@ -202,6 +202,21 @@ macro_rules! Block {
     };
 }
 
+macro_rules! Let {
+    (@$pat:expr $(=> $expr:expr)?) => {{
+        #[allow(unused)]
+        let mut expr = None;
+        $(expr = Some(Box::new(into!($expr)));)?
+        Expr::Let(ExprLet {
+            pat: into!($pat).map(|s: &str| Ident(s.into())),
+            expr
+        })
+    }};
+    ($($tts:tt)*) => {
+        Node::Expr(Let!(@$($tts)*))
+    };
+}
+
 #[test]
 fn test_parse_comments() {
     // In markup.
@@ -651,3 +666,26 @@ fn test_parse_values() {
         nodes: [],
         errors: [S(1..3, "expected expression, found invalid token")]);
 }
+
+#[test]
+fn test_parse_let_bindings() {
+    // Basic let.
+    t!("#let x;" Let!("x"));
+    t!("#let _y=1;" Let!("_y" => Int(1)));
+
+    // Followed by text.
+    t!("#let x = 1\n+\n2;\nHi there"
+        Let!("x" => Binary(Int(1), Add, Int(2))),
+        Space, Text("Hi"), Space, Text("there"));
+
+    // Missing semicolon.
+    t!("#let x = a\nHi"
+        nodes: [Let!("x" => Id("a"))],
+        errors: [S(11..13, "unexpected identifier"),
+                 S(13..13, "expected semicolon")]);
+
+    // Missing identifier.
+    t!("#let 1;"
+        nodes: [],
+        errors: [S(5..6, "expected identifier, found integer")])
+}
diff --git a/src/parse/tokens.rs b/src/parse/tokens.rs
index d16cf2ce..32cc11d9 100644
--- a/src/parse/tokens.rs
+++ b/src/parse/tokens.rs
@@ -60,7 +60,7 @@ impl<'s> Iterator for Tokens<'s> {
         loop {
             // Common elements.
             return Some(match c {
-                // Functions and blocks.
+                // Functions, blocks and terminators.
                 '[' => Token::LeftBracket,
                 ']' => Token::RightBracket,
                 '{' => Token::LeftBrace,
@@ -112,6 +112,7 @@ impl<'s> Iterator for Tokens<'s> {
 
                 // Length one.
                 ',' => Token::Comma,
+                ';' => Token::Semicolon,
                 ':' => Token::Colon,
                 '|' => Token::Pipe,
                 '+' => Token::Plus,
@@ -575,6 +576,7 @@ mod tests {
     fn test_tokenize_code_symbols() {
         // Test all symbols.
         t!(Code: ","        => Comma);
+        t!(Code: ";"        => Semicolon);
         t!(Code: ":"        => Colon);
         t!(Code: "|"        => Pipe);
         t!(Code: "+"        => Plus);
@@ -682,7 +684,7 @@ mod tests {
 
         // Test code symbols in text.
         t!(Markup[" /"]: "a():\"b" => Text("a():\"b"));
-        t!(Markup[" /"]: ":,=|/+-" => Text(":,=|/+-"));
+        t!(Markup[" /"]: ";:,=|/+-" => Text(";:,=|/+-"));
 
         // Test text ends.
         t!(Markup[""]: "hello " => Text("hello"), Space(0));
diff --git a/src/syntax/expr.rs b/src/syntax/expr.rs
index 09472df4..b758a849 100644
--- a/src/syntax/expr.rs
+++ b/src/syntax/expr.rs
@@ -44,6 +44,8 @@ pub enum Expr {
     Group(Box<Expr>),
     /// A block expression: `{1 + 2}`.
     Block(Box<Expr>),
+    /// A let expression: `let x = 1`.
+    Let(ExprLet),
 }
 
 impl Pretty for Expr {
@@ -79,6 +81,7 @@ impl Pretty for Expr {
                 v.pretty(p);
                 p.push_str("}");
             }
+            Self::Let(v) => v.pretty(p),
         }
     }
 }
@@ -300,6 +303,26 @@ impl Pretty for ExprDict {
 /// A template expression: `[*Hi* there!]`.
 pub type ExprTemplate = Tree;
 
+/// A let expression: `let x = 1`.
+#[derive(Debug, Clone, PartialEq)]
+pub struct ExprLet {
+    /// The pattern to assign to.
+    pub pat: Spanned<Ident>,
+    /// The expression to assign to the pattern.
+    pub expr: Option<Box<Spanned<Expr>>>,
+}
+
+impl Pretty for ExprLet {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str("#let ");
+        p.push_str(&self.pat.v);
+        if let Some(expr) = &self.expr {
+            p.push_str(" = ");
+            expr.v.pretty(p);
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::super::tests::test_pretty;
@@ -336,6 +359,9 @@ mod tests {
         // Parens and blocks.
         test_pretty("{(1)}", "{(1)}");
         test_pretty("{{1}}", "{{1}}");
+
+        // Let binding.
+        test_pretty("#let x=1+2", "#let x = 1 + 2");
     }
 
     #[test]
diff --git a/src/syntax/token.rs b/src/syntax/token.rs
index 7055d61a..43415198 100644
--- a/src/syntax/token.rs
+++ b/src/syntax/token.rs
@@ -27,6 +27,8 @@ pub enum Token<'s> {
     Backslash,
     /// A comma: `,`.
     Comma,
+    /// A semicolon: `;`.
+    Semicolon,
     /// A colon: `:`.
     Colon,
     /// A pipe: `|`.
@@ -201,6 +203,7 @@ impl<'s> Token<'s> {
             Self::Tilde => "tilde",
             Self::Backslash => "backslash",
             Self::Comma => "comma",
+            Self::Semicolon => "semicolon",
             Self::Colon => "colon",
             Self::Pipe => "pipe",
             Self::Plus => "plus",
diff --git a/tests/typeset.rs b/tests/typeset.rs
index 735a46c0..e586ae1a 100644
--- a/tests/typeset.rs
+++ b/tests/typeset.rs
@@ -137,7 +137,7 @@ fn test(
         output: frames,
         feedback: Feedback { mut diags, .. },
     } = typeset(&src, Rc::clone(env), state);
-    diags.sort();
+    diags.sort_by_key(|d| d.span);
 
     let env = env.borrow();
     let canvas = draw(&frames, &env, 2.0);
@@ -215,7 +215,7 @@ fn parse_metadata(src: &str, map: &LineMap) -> (SpanVec<Diag>, bool) {
         diags.push(Diag::new(level, s.rest().trim()).with_span(start .. end));
     }
 
-    diags.sort();
+    diags.sort_by_key(|d| d.span);
 
     (diags, compare_ref)
 }