(* calc1.ml: original calculator from Week7, enhanced with Booleans and Immutable Pairs *)
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
;;
type expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value;;
type bound_value = expressed_value;;
type assignment = (char * bound_value) list;;
let rec eval (t : term) (g : assignment) = match t with
Intconstant x -> Int x
| Multiplication (t1, t2) ->
(* we don't handle cases where the subterms don't evaluate to Ints *)
let Int i1 = eval t1 g
in let Int i2 = eval t2 g
(* Multiplication (t1, t2) should evaluate to an Int *)
in Int (i1 * i2)
| Addition (t1, t2) ->
let Int i1 = eval t1 g
in let Int i2 = eval t2 g
in Int (i1 + i2)
| Variable (var) ->
(* we don't handle cases where g doesn't bind var to any value *)
List.assoc var g
| Let (var_to_bind, t2, t3) ->
(* evaluate t3 under a new assignment where var_to_bind has been bound to
the result of evaluating t2 under the current assignment *)
let value2 = eval t2 g
in let g' = (var_to_bind, value2) :: g
in eval t3 g'
| Iszero (t1) ->
(* we don't handle cases where t1 doesn't evaluate to an Int *)
let Int i1 = eval t1 g
(* Iszero t1 should evaluate to a Bool *)
in Bool (i1 = 0)
| If (t1, t2, t3) ->
(* we don't handle cases where t1 doesn't evaluate to a boolean *)
let Bool b1 = eval t1 g
in if b1 then eval t2 g
else eval t3 g
| Makepair (t1, t2) ->
let value1 = eval t1 g
in let value2 = eval t2 g
in Pair (value1, value2)
| First (t1) ->
(* we don't handle cases where t1 doesn't evaluate to a Pair *)
let Pair (value1, value2) = eval t1 g
in value1
;;