Implement let-expressions resolves #2

src/environment.rs

@@ -1,6 +1,5 @@
 use crate::parser::make_symbol;
 use crate::parser::Datum;
-use crate::primitives;
 use std::mem;
 
 type Mapping = (Datum, Datum);
@@ -14,23 +13,22 @@ pub struct Frame<'a> {
 pub type Env<'a> = Frame<'a>;
 
 pub fn get_global_environment<'a>() -> Env<'a> {
+    use crate::parser::Datum::Primitive;
+    use crate::primitives::*;
     Frame {
         mappings: vec![
-            (make_symbol("+"), Datum::Procedure(primitives::add)),
-            (make_symbol("*"), Datum::Procedure(primitives::mul)),
-            (make_symbol("-"), Datum::Procedure(primitives::sub)),
-            (make_symbol("="), Datum::Procedure(primitives::eq)),
-            (make_symbol("<"), Datum::Procedure(primitives::lt)),
-            (make_symbol("list"), Datum::Procedure(primitives::list)),
-            (make_symbol("cons"), Datum::Procedure(primitives::cons)),
-            (make_symbol("car"), Datum::Procedure(primitives::car)),
-            (make_symbol("cdr"), Datum::Procedure(primitives::cdr)),
-            (make_symbol("pair?"), Datum::Procedure(primitives::pair)),
-            (make_symbol("null?"), Datum::Procedure(primitives::null)),
-            (
-                make_symbol("display"),
-                Datum::Procedure(primitives::display),
-            ),
+            (make_symbol("+"), Primitive(add)),
+            (make_symbol("*"), Primitive(mul)),
+            (make_symbol("-"), Primitive(sub)),
+            (make_symbol("="), Primitive(eq)),
+            (make_symbol("<"), Primitive(lt)),
+            (make_symbol("list"), Primitive(list)),
+            (make_symbol("cons"), Primitive(cons)),
+            (make_symbol("car"), Primitive(car)),
+            (make_symbol("cdr"), Primitive(cdr)),
+            (make_symbol("pair?"), Primitive(pair)),
+            (make_symbol("null?"), Primitive(null)),
+            (make_symbol("display"), Primitive(display)),
         ],
         outer: None,
     }

src/interpreter.rs

@@ -109,16 +109,30 @@ fn interpret_lambda(vec: &Vec<Datum>, _env: &mut Env) -> Datum {
             }
         }
     }
-    let body;
-    if let List(insts) = &vec[2] {
-        body = insts.to_vec();
-    } else {
-        panic!("INTERPRET-LAMBDA - body not a list {:?}", vec[1]);
-    }
+    let body = vec[2..].to_vec();
+    Lambda { parameters, body }
+}
 
-    Lambda {
-        parameters: parameters,
-        body: body,
+fn transform_let_to_lambda(vec: &Vec<Datum>) -> Datum {
+    let mut args = vec![];
+    let mut parameters = vec![];
+    let body = vec[2..].to_vec();
+
+    if let List(bindings) = &vec[1] {
+        for binding in bindings {
+            if let List(binding) = binding {
+                parameters.push(binding[0].clone());
+                args.push(binding[1].clone());
+            } else {
+                panic!("TRANSFORM-LET-TO-LAMBDA -- invalid binding {:?}", binding);
+            }
+        }
+        let lambda = Lambda { parameters, body };
+        let mut result = vec![lambda];
+        result.append(&mut args);
+        List(result)
+    } else {
+        panic!("TRANSFORM-LET-TO-LAMBDA -- invalid bindings {:?}", vec[1]);
     }
 }
 
@@ -142,7 +156,7 @@ fn interpret_application(vec: &Vec<Datum>, env: &mut Env) -> Datum {
     }
 
     match &args[0] {
-        Procedure(proc) => proc(args),
+        Primitive(proc) => proc(args),
         Lambda { parameters, body } => {
             let lambda_args = args[1..].to_vec();
             let mut new_env =
@@ -167,10 +181,11 @@ pub fn interpret(exp: &Datum, env: &mut Env) -> Datum {
         List(v) if has_tag(v, "define") => interpret_define(v, env),
         List(v) if has_tag(v, "cond") => interpret_cond(v, env),
         List(v) if has_tag(v, "lambda") => interpret_lambda(v, env),
-        List(v) if has_tag(v, "let") => panic!("let-not-supported"),
+        List(v) if has_tag(v, "let") => interpret(&transform_let_to_lambda(v), env),
         List(v) if has_tag(v, "begin") => interpret_begin(v, env),
         List(v) if has_tag(v, "quote") => v[1].clone(),
         List(v) => interpret_application(v, env),
+        Lambda { .. } => exp.clone(),
         Unspecified => Unspecified,
         _ => panic!("unknown-expression {:?}", exp),
     }

src/main.rs

@@ -52,7 +52,7 @@ fn interpret_file(filename: &Path) {
 fn main() {
     let args: Vec<String> = env::args().collect();
     // let scm_code = "(begin (define (f n) (if (= n 1) 1 (* n (f (- n 1))))) (f 10))";
-    let scm_code = "";
+    let scm_code = "(let ((a 3) (b 4)) (define c (+ a b)) (* c c))";
 
     if args.len() == 2 {
         let arg_path = Path::new(&args[1]);

src/parser.rs

@@ -11,7 +11,7 @@ pub enum Datum {
     Symbol(String),
     Pair(Box<(Datum, Datum)>),
     List(Vec<Datum>),
-    Procedure(fn(Vec<Datum>) -> Datum),
+    Primitive(fn(Vec<Datum>) -> Datum),
     Lambda {
         parameters: Vec<Datum>,
         body: Vec<Datum>,