-(* calc6.ml, enhanced with Aliases and Passing by Reference *)
+(* calc7.ml: calc6.ml enhanced with Aliases and Passing by Reference *)
- type term =
+ type term =
Intconstant of int
| Multiplication of (term * term)
| Addition of (term * term)
| Applyalias of (term * char)
;;
- type index = int;;
+ 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 expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment;;
- type store = expressed_value list;;
+ 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 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 *)
+ | 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]
+ 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
+ | 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 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 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 *)
+ | 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)
+ (* 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 *)
+ | 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 *)
+ | 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'
+ (* evaluate body under savedg, except with arg_var bound to existing index *)
+ in let savedg' = (arg_var, index) :: savedg
+ in eval body savedg' s'
;;