--- /dev/null
+(* calc6.ml, enhanced with Aliases and Passing by Reference *)
+
+ 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)
+ | Alias of (char * char * term)
+ | Applyalias of (term * char)
+ ;;
+
+ 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''
+ | Alias (var_to_bind, orig_var, t3) ->
+ (* we don't handle cases where g doesn't bind orig_var to any value *)
+ let index = List.assoc orig_var g
+ (* bind var_to_bind to the same index in the store *)
+ in let g' = ((var_to_bind, index) :: g)
+ in eval t3 g' s
+ | Applyalias (t1, var) ->
+ (* 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
+ (* we don't handle cases where g doesn't bind var to any value *)
+ in let index = List.assoc var g
+ (* evaluate body under savedg, except with arg_var bound to existing index *)
+ in let savedg' = (arg_var, index) :: savedg
+ in eval body savedg' s'
+ ;;
+