+++ /dev/null
-(* calc6.ml: calc3.ml enhanced with Mutable Variables *)
-
- type term =
- Intconstant of int
- | Multiplication of (term * term)
- | Addition of (term * term)
- | Variable of char
- | Let of (char * term * term)
- | Iszero of term
- | If of (term * term * term)
- | Makepair of (term * term)
- | First of term
- | Lambda of (char * term)
- | Apply of (term * term)
- | Letrec of (char * term * term)
- | Change of (char * term * term)
- ;;
-
- type index = int;;
-
- type bound_value = index;;
- type assignment = (char * bound_value) list;;
- type expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment;;
-
- type store = expressed_value list;;
-
- let rec eval (t : term) (g : assignment) (s : store) = match t with
- Intconstant x -> (Int x, s)
- | Multiplication (t1, t2) ->
- (* we don't handle cases where the subterms don't evaluate to Ints *)
- let (Int i1, s') = eval t1 g s
- in let (Int i2, s'') = eval t2 g s'
- (* Multiplication (t1, t2) should evaluate to an Int *)
- in (Int (i1 * i2), s'')
- | Addition (t1, t2) ->
- let (Int i1, s') = eval t1 g s
- in let (Int i2, s'') = eval t2 g s'
- in (Int (i1 + i2), s'')
- | Variable (var) ->
- (* we don't handle cases where g doesn't bind var to any value *)
- let index = List.assoc var g
- (* get value stored at location index in s *)
- in let value = List.nth s index
- in (value, s)
- | Let (var_to_bind, t2, t3) ->
- let (value2, s') = eval t2 g s
- (* note that s' may be different from s, if t2 itself contained any mutation operations *)
- (* get next free index in s' *)
- in let new_index = List.length s'
- (* now we want to insert value2 there; the following is an easy but inefficient way to do it *)
- in let s'' = List.append s' [value2]
- (* bind var_to_bind to location new_index in the store *)
- in let g' = ((var_to_bind, new_index) :: g)
- in eval t3 g' s''
- | Iszero (t1) ->
- (* we don't handle cases where t1 doesn't evaluate to an Int *)
- let (Int i1, s') = eval t1 g s
- (* Iszero t1 should evaluate to a Bool *)
- in (Bool (i1 = 0), s')
- | If (t1, t2, t3) ->
- (* we don't handle cases where t1 doesn't evaluate to a boolean *)
- let (Bool b1, s') = eval t1 g s
- (* note we thread s' through only one of the then/else clauses *)
- in if b1 then eval t2 g s'
- else eval t3 g s'
- | Makepair (t1, t2) ->
- let (value1, s') = eval t1 g s
- in let (value2, s'') = eval t2 g s'
- in (Pair (value1, value2), s'')
- | First (t1) ->
- (* we don't handle cases where t1 doesn't evaluate to a Pair *)
- let (Pair (value1, value2), s') = eval t1 g s
- in (value1, s')
- | Lambda (arg_var, t2) -> (Closure (arg_var, t2, g), s)
- | Apply (t1, t2) ->
- (* we don't handle cases where t1 doesn't evaluate to a function value *)
- let (Closure (arg_var, body, savedg), s') = eval t1 g s
- in let (value2, s'') = eval t2 g s'
- (* evaluate body under savedg, except with arg_var bound to a new location containing value2 *)
- in let new_index = List.length s''
- in let s''' = List.append s'' [value2]
- in let savedg' = (arg_var, new_index) :: savedg
- in eval body savedg' s'''
- | Letrec (var_to_bind, t2, t3) ->
- (* we don't handle cases where t2 doesn't evaluate to a function value *)
- let (Closure (arg_var, body, savedg), s') = eval t2 g s
- in let new_index = List.length s'
- in let savedg' = (var_to_bind, new_index) :: savedg
- in let new_closure = Closure (arg_var, body, savedg')
- in let s'' = List.append s' [new_closure]
- in let g' = (var_to_bind, new_index) :: g
- in eval t3 g' s''
- | Change (var, t2, t3) ->
- (* we don't handle cases where g doesn't bind var to any value *)
- let index = List.assoc var g
- in let (value2, s') = eval t2 g s
- (* note that s' may be different from s, if t2 itself contained any mutation operations *)
- (* now we create a list which is just like s' except it has value2 at index *)
- in let rec replace_nth lst m =
- match lst with
- | [] -> failwith "list too short"
- | x::xs when m = 0 -> value2 :: xs
- | x::xs -> x :: replace_nth xs (m - 1)
- in let s'' = replace_nth s' index
- (* evaluate t3 using original assignment function and new store *)
- in eval t3 g s''
- ;;
-