From ce8138c68557a2d158424a8aa6056d73ff9cb1ba Mon Sep 17 00:00:00 2001
From: Laurenz <laurmaedje@gmail.com>
Date: Wed, 27 Jan 2021 11:50:51 +0100
Subject: [PATCH] =?UTF-8?q?Scope=20variables=20in=20blocks=20=F0=9F=8F=94?=
 =?UTF-8?q?=EF=B8=8F?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 src/eval/mod.rs    |  9 +++++++++
 src/eval/scope.rs  | 19 ++++++++++++++++---
 src/parse/mod.rs   | 10 +++++-----
 src/syntax/expr.rs |  2 ++
 4 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index e04879d28..cadffef5e 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -210,10 +210,19 @@ impl Eval for Spanned<&ExprBlock> {
     type Output = Value;
 
     fn eval(self, ctx: &mut EvalContext) -> Self::Output {
+        if self.v.scopes {
+            ctx.scopes.push();
+        }
+
         let mut output = Value::None;
         for expr in &self.v.exprs {
             output = expr.eval(ctx);
         }
+
+        if self.v.scopes {
+            ctx.scopes.pop();
+        }
+
         output
     }
 }
diff --git a/src/eval/scope.rs b/src/eval/scope.rs
index ed4edbb96..1ed34f866 100644
--- a/src/eval/scope.rs
+++ b/src/eval/scope.rs
@@ -4,7 +4,7 @@ use std::iter;
 
 use super::Value;
 
-/// A hierarchy of scopes.
+/// A stack of scopes.
 #[derive(Debug, Clone, PartialEq)]
 pub struct Scopes<'a> {
     /// The active scope.
@@ -21,6 +21,19 @@ impl<'a> Scopes<'a> {
         Self { top: Scope::new(), scopes: vec![], base }
     }
 
+    /// Push a new scope.
+    pub fn push(&mut self) {
+        self.scopes.push(std::mem::take(&mut self.top));
+    }
+
+    /// Pop the topmost scope.
+    ///
+    /// # Panics
+    /// Panics if no scope was pushed.
+    pub fn pop(&mut self) {
+        self.top = self.scopes.pop().expect("no pushed scope");
+    }
+
     /// Define a variable in the active scope.
     pub fn define(&mut self, var: impl Into<String>, value: impl Into<Value>) {
         self.top.define(var, value);
@@ -29,7 +42,7 @@ impl<'a> Scopes<'a> {
     /// Look up the value of a variable.
     pub fn get(&self, var: &str) -> Option<&Value> {
         iter::once(&self.top)
-            .chain(&self.scopes)
+            .chain(self.scopes.iter().rev())
             .chain(iter::once(self.base))
             .find_map(|scope| scope.get(var))
     }
@@ -37,7 +50,7 @@ impl<'a> Scopes<'a> {
     /// Get a mutable reference to a variable.
     pub fn get_mut(&mut self, var: &str) -> Option<&mut Value> {
         iter::once(&mut self.top)
-            .chain(&mut self.scopes)
+            .chain(self.scopes.iter_mut().rev())
             .find_map(|scope| scope.get_mut(var))
     }
 
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index 8687ac246..ff75a563d 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -52,7 +52,7 @@ fn node(p: &mut Parser, at_start: &mut bool) -> Option<Node> {
 
         // Code block.
         Token::LeftBrace => {
-            return Some(Node::Expr(block(p)?));
+            return Some(Node::Expr(block(p, false)?));
         }
 
         // Markup.
@@ -285,7 +285,7 @@ fn primary(p: &mut Parser) -> Option<Expr> {
 
         // Nested block.
         Some(Token::LeftBrace) => {
-            return block(p);
+            return block(p, true);
         }
 
         // Dictionary or just a parenthesized expression.
@@ -340,7 +340,7 @@ fn template(p: &mut Parser) -> Expr {
 }
 
 /// Parse a block expression: `{...}`.
-fn block(p: &mut Parser) -> Option<Expr> {
+fn block(p: &mut Parser, scopes: bool) -> Option<Expr> {
     p.start_group(Group::Brace, TokenMode::Code);
     let mut exprs = vec![];
     while !p.eof() {
@@ -355,7 +355,7 @@ fn block(p: &mut Parser) -> Option<Expr> {
         p.skip_white();
     }
     p.end_group();
-    Some(Expr::Block(ExprBlock { exprs }))
+    Some(Expr::Block(ExprBlock { exprs, scopes }))
 }
 
 /// Parse a parenthesized function call.
@@ -469,7 +469,7 @@ fn ident(p: &mut Parser) -> Option<Ident> {
 fn body(p: &mut Parser) -> Option<Expr> {
     match p.peek() {
         Some(Token::LeftBracket) => Some(template(p)),
-        Some(Token::LeftBrace) => block(p),
+        Some(Token::LeftBrace) => block(p, true),
         _ => {
             p.expected_at("body", p.last_end());
             None
diff --git a/src/syntax/expr.rs b/src/syntax/expr.rs
index 9f3bd9ad2..15c53cc0e 100644
--- a/src/syntax/expr.rs
+++ b/src/syntax/expr.rs
@@ -147,6 +147,8 @@ pub type ExprGroup = SpanBox<Expr>;
 pub struct ExprBlock {
     /// The list of expressions contained in the block.
     pub exprs: SpanVec<Expr>,
+    /// Whether the block should create a scope.
+    pub scopes: bool,
 }
 
 impl Pretty for ExprBlock {