Implement begin and define

src/environment.rs

@@ -13,8 +13,8 @@ pub fn get_global_environment() -> Env {
               (make_symbol("-"), Procedure(primitives::sub))]]
 }
 
-pub fn lookup_variable_value(exp: &Datum, env: &Env) -> Datum {
-    if let Symbol(symbol_name) = exp {
+pub fn lookup_variable_value(var: &Datum, env: &Env) -> Datum {
+    if let Symbol(symbol_name) = var {
         for frame in env {
             for mapping in frame {
                 if let (Symbol(current_name), datum) = mapping {
@@ -25,7 +25,34 @@ pub fn lookup_variable_value(exp: &Datum, env: &Env) -> Datum {
             }
         }
     } else {
-        panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", exp)
+        panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", var)
     }
-    panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable-error {:?}", exp)
+    panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable-error {:?}", var)
+}
+
+pub fn define_variable(var: &Datum, val: Datum, env: &mut Env) -> Datum {
+    if let Symbol(symbol_name) = var {
+        // Assign to variable if it already exits
+        if let Some(frame) = env.last_mut() {
+            for mapping in frame {
+                if let (Symbol(current_name), _) = mapping {
+                    if symbol_name == current_name {
+                        mapping.1 = val;
+                        return Datum::Unspecified;
+                    }
+                }
+            }
+        } else {
+            panic!("DEFINE-VARIABLE -- no-frames {:?}", env)
+        }
+
+        // Create variable because it does not exist yet
+        if let Some(frame) = env.last_mut() {
+            let pair = (var.clone(), val);
+            frame.push(pair);
+        }
+    } else {
+        panic!("DEFINE-VARIABLE -- not-a-symbol {:?}", var)
+    }
+    Datum::Unspecified
 }

src/interpreter.rs

@@ -13,29 +13,6 @@ fn is_true(exp: &Datum) -> bool {
     }
 }
 
-fn application(exp: &Datum, env: &Env) -> Datum {
-    if let List(vector) = exp {
-        if vector.len() == 0 {
-            panic!("APPLICATION -- no-procedure")
-        }
-        
-        let mut args = vec![];
-        for v in vector {
-            let a = interpret(v, env);
-            args.push(a)
-        }
-
-        // let mut args: Vec<Datum> = vector.into_iter().map(interpret).collect();
-        if let Procedure(proc) = mem::take(&mut args[0]) {
-            proc(args)
-        } else {
-            panic!("APPLICATION -- not-aplicable {:?}", args[0])
-        }
-    } else {
-        panic!("APPLICATION -- cannot-apply {:?}", exp)
-    }
-}
-
 fn has_tag(exp: &Datum, tag: &str) -> bool {
     if let List(l) = exp {
         if l.len() == 0 {
@@ -50,39 +27,76 @@ fn has_tag(exp: &Datum, tag: &str) -> bool {
     false
 }
 
-fn interpret_if(args: &Vec<Datum>, env: &Env) -> Datum {
+fn interpret_if(args: &Vec<Datum>, env: &mut Env) -> Datum {
     let predicate = &args[1];
     let alternative = &args[2];
 
-    if is_true(&predicate) {
-        interpret(&alternative, env)
+    if is_true(predicate) {
+        interpret(alternative, env)
     } else {
         if args.len() == 4 {
             let consequent = &args[3];
-            interpret(&consequent, env)
+            interpret(consequent, env)
         } else {
             Unspecified
         }
     }
 }
 
-fn interpret_define(_args: &Vec<Datum>) -> Datum {
-
-    Number(42)
+fn interpret_define(args: &Vec<Datum>, env: &mut Env) -> Datum {
+    match args[1] {
+        Symbol(_) => {
+            let var = &args[1];
+            let val = interpret(&args[2], env);
+            environment::define_variable(var, val, env)
+        },
+        List(_) => {
+            println!("procedure definition");
+            Number(42)
+        }
+        _ => panic!("INTERPRET-DEFINE -- cannot define {:?}", args)
+    }
 }
 
-pub fn interpret(exp: &Datum, env: &Env) -> Datum {
+fn interpret_begin(vec: &Vec<Datum>, env: &mut Env) -> Datum {
+    let mut r = Datum::Unspecified;
+    for i in 1..vec.len() {
+        r = interpret(&vec[i], env);
+    }
+    r
+}
+
+fn interpret_application(vec: &Vec<Datum>, env: &mut Env) -> Datum {
+    if vec.len() == 0 {
+        panic!("APPLICATION -- no-procedure")
+    }
+
+    let mut args = vec![];
+    for v in vec {
+        let a = interpret(v, env);
+        args.push(a)
+    }
+
+    if let Procedure(proc) = mem::take(&mut args[0]) {
+        proc(args)
+    } else {
+        panic!("APPLICATION -- not-aplicable {:?}", args[0])
+    }
+}
+
+pub fn interpret(exp: &Datum, env: &mut Env) -> Datum {
     match exp {
-        Boolean(b) => Boolean(*b),
-        Number(n) => Number(*n),
-        Symbol(_) => environment::lookup_variable_value(&exp, &env),
-        List(v) if has_tag(&exp, "if") => interpret_if(v, &env),
+        Boolean(_) => exp.clone(),
+        Number(_) => exp.clone(),
+        Unspecified => Unspecified,
+        Symbol(_) => environment::lookup_variable_value(exp, env),
+        List(v) if has_tag(&exp, "if") => interpret_if(v, env),
         List(_) if has_tag(&exp, "set!") => panic!("assignment-not-supported"),
-        List(v) if has_tag(&exp, "define") => interpret_define(v),
+        List(v) if has_tag(&exp, "define") => interpret_define(v, env),
         List(_) if has_tag(&exp, "cond") => panic!("cond-not-supported"),
         List(_) if has_tag(&exp, "let") => panic!("let-not-supported"),
-        List(_) if has_tag(&exp, "begin") => panic!("begin-not-supported"),
-        List(_) => application(exp, env),
+        List(v) if has_tag(&exp, "begin") => interpret_begin(v, env),
+        List(v) => interpret_application(v, env),
         // TODO: quoted
         _ => panic!("unknown-expression {:?}", exp),
     }

src/main.rs

@@ -5,12 +5,12 @@ mod primitives;
 mod environment;
 
 fn main() {
-    let scm_code = "(if #f (+ 1 1) (* 2 3 4))";
-    // let scm_code = "(define a (+ 3 3)";
-    // let scm_code = "(- (* 3 3) 4)";
-    let env = environment::get_global_environment();
+    // let scm_code = "(if #t (+ 1 1) (* 2 3 4))";
+    // let scm_code = "(begin (define a (+ 2 2)) (* 3 2) (* a a a))";
+    let scm_code = "(begin (define (f n) (* n n)) (* 3 2))";
     let tokens = lexer::read(scm_code);
     let datum = parser::parse(tokens);
-    let result = interpreter::interpret(&datum, &env);
+    let mut env = environment::get_global_environment();
+    let result = interpreter::interpret(&datum, &mut env);
     println!("{:?}", result);
 }