Implement set! resolves #3

src/environment.rs

@@ -63,6 +63,26 @@ pub fn define_variable(var: &Datum, val: Datum, env: &mut Env) -> Datum {
     var.clone()
 }
 
+pub fn set_variable(var: &Datum, val: Datum, env: &mut Env) -> Datum {
+    if let Datum::Symbol(symbol_name) = var {
+        for mapping in &mut env.mappings {
+            if let (Datum::Symbol(current_name), _) = mapping {
+                if symbol_name == current_name {
+                    mapping.1 = val;
+                    return var.clone();
+                }
+            }
+        }
+        match env.outer {
+            // XXX: search outer environments here
+            // Some(outer_env) => set_variable(var, val, outer_env),
+            _ => panic!("SET-VARIABLE -- unbound-var {:?}", var),
+        }
+    } else {
+        panic!("DEFINE-VARIABLE -- not-a-symbol {:?}", var)
+    }
+}
+
 pub fn extend<'a>(vars: Datum, vals: Datum, env: &'a Env) -> Env<'a> {
     let mut mappings = vec![];
     if let (Datum::List(mut vars), Datum::List(mut vals)) = (vars, vals) {

src/interpreter.rs

@@ -42,6 +42,17 @@ fn interpret_if(args: &Vec<Datum>, env: &mut Env) -> Datum {
     }
 }
 
+fn interpret_set(args: &Vec<Datum>, env: &mut Env) -> Datum {
+    match &args[1] {
+        Symbol(_) => {
+            let var = &args[1];
+            let val = interpret(&args[2], env);
+            environment::set_variable(var, val, env)
+        }
+        _ => panic!("INTERPRET-SET -- cannot set {:?}", args),
+    }
+}
+
 fn interpret_define(args: &Vec<Datum>, env: &mut Env) -> Datum {
     match &args[1] {
         Symbol(_) => {
@@ -119,7 +130,7 @@ pub fn interpret(exp: &Datum, env: &mut Env) -> Datum {
         Unspecified => Unspecified,
         Symbol(_) => environment::lookup_variable_value(exp, env),
         List(v) if has_tag(v, "if") => interpret_if(v, env),
-        List(v) if has_tag(v, "set!") => panic!("assignment-not-supported"),
+        List(v) if has_tag(v, "set!") => interpret_set(v, env),
         List(v) if has_tag(v, "define") => interpret_define(v, env),
         List(v) if has_tag(v, "cond") => panic!("cond-not-supported"),
         List(v) if has_tag(v, "let") => panic!("let-not-supported"),

src/lexer.rs

@@ -135,4 +135,4 @@ const SPECIAL_INITIAL: &[char] = &[
     '!', '$', '%', '&', '*', '/', ':', '<', '=', '>', '?', '^', '_', '~', '+', '-',
 ];
 
-const SPECIAL_SUBSEQUENT: &[char] = &['+', '-', '.', '@'];
+const SPECIAL_SUBSEQUENT: &[char] = &['+', '-', '.', '@', '!'];