Except, this isn't quite right. It's almost what we want, but not exactly. Can you see the flaw?
Except, this isn't quite right. It's almost what we want, but not exactly. Can you see the flaw?
-The flaw is this: inside `new_closure`, what is `'f;` bound to? It's bound by `savedg'` to `orig_closure`, which in turn leaves `'f'` free (or bound to whatever existing value it had according to `savedg`). This isn't what we want. It'll break if we need to make applications of `Variable 'f'` which recurse more than once.
+The flaw is this: inside `new_closure`, what is `'f'` bound to? It's bound by `savedg'` to `orig_closure`, which in turn leaves `'f'` free (or bound to whatever existing value it had according to `savedg`). This isn't what we want. It'll break if we need to make applications of `Variable 'f'` which recurse more than once.
What we really want is for `'f'` to be bound to `new_closure`, something like this:
What we really want is for `'f'` to be bound to `new_closure`, something like this:
@@ -222,12+222,12 @@ If we implemented assignments as functions rather than as lists of pairs, the co
| Let (var_to_bind, t1, t2) ->
let value1 = eval t1 g
| Let (var_to_bind, t1, t2) ->
let value1 = eval t1 g
- in let g' = fun var -> if var = var_to_bind value1 else g var
+ in let g' = fun var -> if var = var_to_bind then value1 else g var
in eval t2 g'
...
| Letrec (var_to_bind, t1, t2) ->
let Closure (arg_var, body, savedg) = eval t1 g
in eval t2 g'
...
| Letrec (var_to_bind, t1, t2) ->
let Closure (arg_var, body, savedg) = eval t1 g
- in let rec savedg' = fun var -> if var = var_to_bind Closure (arg_var, body, savedg') else savedg var
+ in let rec savedg' = fun var -> if var = var_to_bind then Closure (arg_var, body, savedg') else savedg var
in let g' = fun var -> if var = var_to_bind then Closure (arg_var, body, savedg') else g var
in eval t2 g';;
in let g' = fun var -> if var = var_to_bind then Closure (arg_var, body, savedg') else g var