Rework environment mode fixes #8

src/environment.rs

@@ -4,81 +4,84 @@ use crate::primitives;
 use std::mem;
 
 type Mapping = (Datum, Datum);
-type Frame = Vec<Mapping>;
-pub type Env = Vec<Frame>;
 
-pub fn get_global_environment() -> Env {
-    vec![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)),
-    ]]
+pub struct Frame<'a> {
+    mappings: Vec<Mapping>,
+    outer: Option<&'a Frame<'a>>,
+}
+
+pub type Env<'a> = Frame<'a>;
+
+pub fn get_global_environment<'a>() -> Env<'a> {
+    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)),
+        ],
+        outer: None,
+    }
 }
 
 pub fn lookup_variable_value(var: &Datum, env: &Env) -> Datum {
     if let Datum::Symbol(symbol_name) = var {
-        for i in (0..env.len()).rev() {
-            let frame = &env[i];
-            for mapping in frame {
-                if let (Datum::Symbol(current_name), datum) = mapping {
-                    if symbol_name == current_name {
-                        return datum.clone();
-                    }
+        for mapping in &env.mappings {
+            if let (Datum::Symbol(current_name), datum) = mapping {
+                if symbol_name == current_name {
+                    return datum.clone();
                 }
             }
         }
+        match env.outer {
+            Some(outer_env) => lookup_variable_value(var, outer_env),
+            None => panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable-error {:?}", var),
+        }
     } else {
         panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", var)
     }
-    panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable-error {:?}", var)
 }
 
 pub fn define_variable(var: &Datum, val: Datum, env: &mut Env) -> Datum {
     if let Datum::Symbol(symbol_name) = var {
         // Assign to variable if it already exits
-        if let Some(frame) = env.last_mut() {
-            for mapping in frame {
-                if let (Datum::Symbol(current_name), _) = mapping {
-                    if symbol_name == current_name {
-                        mapping.1 = val;
-                        return Datum::Unspecified;
-                    }
+        for mapping in &mut env.mappings {
+            if let (Datum::Symbol(current_name), _) = mapping {
+                if symbol_name == current_name {
+                    mapping.1 = val;
+                    return var.clone();
                 }
             }
-        } 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);
-        }
+        let pair = (var.clone(), val);
+        env.mappings.push(pair);
     } else {
         panic!("DEFINE-VARIABLE -- not-a-symbol {:?}", var)
     }
-    Datum::Unspecified
+    var.clone()
 }
 
-pub fn extend_environment(vars: Datum, vals: Datum, env: &mut Env) -> Datum {
+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) {
         if vars.len() != vals.len() {
             panic!("EXTEND-ENVIRONMENT -- length of vars and vals does not match")
         }
-        let mut frame = vec![];
         for i in 0..vars.len() {
             let pair = (mem::take(&mut vars[i]), mem::take(&mut vals[i]));
-            frame.push(pair);
+            mappings.push(pair);
         }
-        env.push(frame);
     } else {
         panic!("EXTEND-ENVIRONMENT -- vars or vals not lists")
     }
-    Datum::Unspecified
+    Frame {
+        mappings: mappings,
+        outer: Some(env),
+    }
 }
 
-pub fn shrink_environment(env: &mut Env) -> Datum {
-    env.pop();
-    Datum::Unspecified
-}
+// pub fn shrink_environment(env: &mut Env) -> Datum {
+//     env.pop();
+//     Datum::Unspecified
+// }

src/interpreter.rs

@@ -96,16 +96,15 @@ fn interpret_application(vec: &Vec<Datum>, env: &mut Env) -> Datum {
             let lambda_body = &v[2];
             let lambda_args = args[1..].to_vec();
 
-            // FIXME: this does not optimize tail-calls
-            environment::extend_environment(lambda_parameters, List(lambda_args), env);
-            let mut r = Datum::Unspecified;
+            let mut new_env = environment::extend(lambda_parameters, List(lambda_args), env);
             if let List(seq) = lambda_body {
-                for i in 0..seq.len() {
-                    r = interpret(&seq[i], env);
+                for i in 0..(seq.len() - 1) {
+                    interpret(&seq[i], &mut new_env);
                 }
+                interpret(&seq[seq.len() - 1], &mut new_env)
+            } else {
+                panic!("INTERPRET-APPLICATION -- no-list {:?}", lambda_body)
             }
-            environment::shrink_environment(env);
-            r
         }
         _ => panic!("INTERPRET-APPLICATION -- not-aplicable {:?}", args[0]),
     }

src/main.rs

@@ -4,11 +4,11 @@ mod lexer;
 mod parser;
 mod primitives;
 
-use std::path::Path;
-use std::fs;
 use std::env;
+use std::fs;
 use std::io;
 use std::io::prelude::*;
+use std::path::Path;
 
 fn repl() {
     let mut env = environment::get_global_environment();
@@ -44,7 +44,7 @@ fn interpret_file(filename: &Path) {
             let mut env = environment::get_global_environment();
             let result = interpreter::interpret(&datum, &mut env);
             println!("{:?}", result);
-        },
+        }
         Err(error) => println!("error: {} {:?}", error, filename),
     }
 }