1 (* calc7.ml: calc6.ml enhanced with Aliases and Passing by Reference *)
5 | Multiplication of (term * term)
6 | Addition of (term * term)
8 | Let of (char * term * term)
10 | If of (term * term * term)
11 | Makepair of (term * term)
13 | Lambda of (char * term)
14 | Apply of (term * term)
15 | Letrec of (char * term * term)
16 | Change of (char * term * term)
17 | Alias of (char * char * term)
18 | Applyalias of (term * char)
23 type bound_value = index;;
24 type assignment = (char * bound_value) list;;
25 type expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment;;
27 type store = expressed_value list;;
29 let rec eval (t : term) (g : assignment) (s : store) = match t with
30 Intconstant x -> (Int x, s)
31 | Multiplication (t1, t2) ->
32 (* we don't handle cases where the subterms don't evaluate to Ints *)
33 let (Int i1, s') = eval t1 g s
34 in let (Int i2, s'') = eval t2 g s'
35 (* Multiplication (t1, t2) should evaluate to an Int *)
36 in (Int (i1 * i2), s'')
37 | Addition (t1, t2) ->
38 let (Int i1, s') = eval t1 g s
39 in let (Int i2, s'') = eval t2 g s'
40 in (Int (i1 + i2), s'')
42 (* we don't handle cases where g doesn't bind var to any value *)
43 let index = List.assoc var g
44 (* get value stored at location index in s *)
45 in let value = List.nth s index
47 | Let (var_to_bind, t2, t3) ->
48 let (value2, s') = eval t2 g s
49 (* note that s' may be different from s, if t2 itself contained any mutation operations *)
50 (* get next free index in s' *)
51 in let new_index = List.length s'
52 (* now we want to insert value2 there; the following is an easy but inefficient way to do it *)
53 in let s'' = List.append s' [value2]
54 (* bind var_to_bind to location new_index in the store *)
55 in let g' = ((var_to_bind, new_index) :: g)
58 (* we don't handle cases where t1 doesn't evaluate to an Int *)
59 let (Int i1, s') = eval t1 g s
60 (* Iszero t1 should evaluate to a Bool *)
61 in (Bool (i1 = 0), s')
63 (* we don't handle cases where t1 doesn't evaluate to a boolean *)
64 let (Bool b1, s') = eval t1 g s
65 (* note we thread s' through only one of the then/else clauses *)
66 in if b1 then eval t2 g s'
68 | Makepair (t1, t2) ->
69 let (value1, s') = eval t1 g s
70 in let (value2, s'') = eval t2 g s'
71 in (Pair (value1, value2), s'')
73 (* we don't handle cases where t1 doesn't evaluate to a Pair *)
74 let (Pair (value1, value2), s') = eval t1 g s
76 | Lambda (arg_var, t2) -> (Closure (arg_var, t2, g), s)
78 (* we don't handle cases where t1 doesn't evaluate to a function value *)
79 let (Closure (arg_var, body, savedg), s') = eval t1 g s
80 in let (value2, s'') = eval t2 g s'
81 (* evaluate body under savedg, except with arg_var bound to a new location containing value2 *)
82 in let new_index = List.length s''
83 in let s''' = List.append s'' [value2]
84 in let savedg' = (arg_var, new_index) :: savedg
85 in eval body savedg' s'''
86 | Letrec (var_to_bind, t2, t3) ->
87 (* we don't handle cases where t2 doesn't evaluate to a function value *)
88 let (Closure (arg_var, body, savedg), s') = eval t2 g s
89 in let new_index = List.length s'
90 in let savedg' = (var_to_bind, new_index) :: savedg
91 in let new_closure = Closure (arg_var, body, savedg')
92 in let s'' = List.append s' [new_closure]
93 in let g' = (var_to_bind, new_index) :: g
95 | Change (var, t2, t3) ->
96 (* we don't handle cases where g doesn't bind var to any value *)
97 let index = List.assoc var g
98 in let (value2, s') = eval t2 g s
99 (* note that s' may be different from s, if t2 itself contained any mutation operations *)
100 (* now we create a list which is just like s' except it has value2 at index *)
101 in let rec replace_nth lst m =
103 | [] -> failwith "list too short"
104 | x::xs when m = 0 -> value2 :: xs
105 | x::xs -> x :: replace_nth xs (m - 1)
106 in let s'' = replace_nth s' index
107 (* evaluate t3 using original assignment function and new store *)
109 | Alias (var_to_bind, orig_var, t3) ->
110 (* we don't handle cases where g doesn't bind orig_var to any value *)
111 let index = List.assoc orig_var g
112 (* bind var_to_bind to the same index in the store *)
113 in let g' = ((var_to_bind, index) :: g)
115 | Applyalias (t1, var) ->
116 (* we don't handle cases where t1 doesn't evaluate to a function value *)
117 let (Closure (arg_var, body, savedg), s') = eval t1 g s
118 (* we don't handle cases where g doesn't bind var to any value *)
119 in let index = List.assoc var g
120 (* evaluate body under savedg, except with arg_var bound to existing index *)
121 in let savedg' = (arg_var, index) :: savedg
122 in eval body savedg' s'