gistfile1.ml

type Exp = 
	Num of int 
	| Neg of Exp
	| Sum of Exp * Exp
	| Diff of Exp * Exp
	| Prod of Exp * Exp
	| Quot of Exp * Exp

let rec evaluate = function
| Num n -> Some n
| Neg n -> 	match evaluate n with
			| Some x -> Some (-x)
			| _ -> None
| Sum (l,r) -> 	match (evaluate l, evaluate r) with
				| (Some le, Some re) -> Some (le+re)
				| _ -> None
| Diff (l,r) -> match (evaluate l, evaluate r) with
				| (Some le,Some re) -> Some (le-re)
				| _ -> None
| Prod (l,r) -> match (evaluate l, evaluate r) with
				| (Some le, Some re) -> Some (le*re)
				| _ -> None
| Quot (l,r) -> match (evaluate l, evaluate r) with
				| (Some le, Some re) when re=0 -> None
				| (Some le, Some re) -> Some (le/re)
				| _ -> None