Capture variables in templates 🔍

1 files changed, 56 insertions, 0 deletions

diff --git a/src/eval/template.rs b/src/eval/template.rs
new file mode 100644
index 00000000..040685f8
--- /dev/null
+++ b/src/eval/template.rs
@@ -0,0 +1,56 @@
+use super::*;
+use crate::syntax::visit::*;
+
+impl Eval for Spanned<&ExprTemplate> {
+    type Output = Value;
+
+    fn eval(self, ctx: &mut EvalContext) -> Self::Output {
+        let mut template = self.v.clone();
+        let mut visitor = CapturesVisitor::new(ctx);
+        visitor.visit_template(&mut template);
+        Value::Template(template)
+    }
+}
+
+/// A visitor that replaces all captured variables with their values.
+struct CapturesVisitor<'a> {
+    external: &'a Scopes<'a>,
+    internal: Scopes<'a>,
+}
+
+impl<'a> CapturesVisitor<'a> {
+    fn new(ctx: &'a EvalContext) -> Self {
+        Self {
+            external: &ctx.scopes,
+            internal: Scopes::default(),
+        }
+    }
+}
+
+impl<'a> Visitor<'a> for CapturesVisitor<'a> {
+    fn visit_scope_pre(&mut self) {
+        self.internal.push();
+    }
+
+    fn visit_scope_post(&mut self) {
+        self.internal.pop();
+    }
+
+    fn visit_def(&mut self, id: &mut Ident) {
+        self.internal.define(id.as_str(), Value::None);
+    }
+
+    fn visit_expr(&mut self, expr: &'a mut Expr) {
+        if let Expr::Ident(ident) = expr {
+            // Find out whether the identifier is not locally defined, but
+            // captured, and if so, replace it with it's value.
+            if self.internal.get(ident).is_none() {
+                if let Some(value) = self.external.get(ident) {
+                    *expr = Expr::CapturedValue(value.clone());
+                }
+            }
+        } else {
+            walk_expr(self, expr);
+        }
+    }
+}