Implement mutablility

We'll need it eventually, and it lets me steal lox tests

src/ast.rs

@@ -5,6 +5,7 @@ use std::rc::Rc;
 crate enum Tree {
     Leaf(Literal),
     Define(String, Box<Tree>),
+    Set(String, Box<Tree>),
     If(Box<[Tree; 3]>),
     // Its easier to box the lambdas in the parser than the vm, as
     // here we see all of them exactly once

src/eval.rs

@@ -70,6 +70,11 @@ pub(crate) fn eval(t: &ast::Tree, env: Rc<RefCell<Env>>) -> Result<Value, RTErro
             env.borrow_mut().define(name.to_owned(), val);
             Value::Trap
         }
+        ast::Tree::Set(name, to) => {
+            let val = eval(to, Rc::clone(&env))?;
+            env.borrow_mut().set(name, val)?;
+            Value::Trap
+        }
         ast::Tree::If(box [cond, tcase, fcase]) => {
             let b = eval(cond, Rc::clone(&env))?.as_bool()?;
             let body = if b { tcase } else { fcase };
@@ -134,8 +139,20 @@ impl Env {
     pub(crate) fn define(&mut self, name: String, val: Value) {
         assert_ne!(val, Value::Trap); // TODO: Better error
 
+        // TODO: Error on previous def
         self.vars.insert(name, val);
     }
+
+    crate fn set(&mut self, name: &str, val: Value) -> Result<(), RTError> {
+        if let Some(loc) = self.vars.get_mut(name) {
+            *loc = val;
+            Ok(())
+        } else if let Some(parent) = &self.enclosing {
+            parent.borrow_mut().set(name, val)
+        } else {
+            err(format!("Tried to `set!` un `define`d var `{}`", name))
+        }
+    }
 }
 
 pub(crate) fn default_env() -> Env {

src/grammar.lalrpop

@@ -21,6 +21,7 @@ pub(crate) Tree: Tree = {
         => Tree::Define(name, Box::new(Tree::Func(Rc::new(Func{args, body})))),
     "(" "if" <Tree> <Tree> <Tree> ")" => Tree::If(Box::new([<>])),
     "(" "lambda (" <args:Sym*> ")" <body:Trees> ")" => Tree::Func(Rc::new(Func{<>})),
+    "(" "set!" <Sym> <BTree> ")" => Tree::Set(<>),
     Literal => Tree::Leaf(<>),
 }
 

src/tests.rs

@@ -18,7 +18,7 @@ upstream for this
 fn run_pass() {
     let manifest_dir = Path::new(env!("CARGO_MANIFEST_DIR"));
 
-    insta::glob!("test/run-pass/**.scm", |p| {
+    insta::glob!("test/run-pass/**/*.scm", |p| {
         let p = PathBuf::from(
             p.canonicalize()
                 .unwrap()