Why does this give syntax error under TypeError?

Question

let n1 = eval_expr e1 in
let n2 = eval_expr e2 in
(match n1, n2 with
 | Int_Val i, Int_Val j -> Int_Val i, Int_Val j
 | _ -> raise TypeError) in

let n3 = (n1 + n2) in
n3

It says syntax error under TypeError, why?

Answer 1

(If possible you should provide the full error message and show the associated position in the text.)

The error is actually located on the symbol in on the following line:

| _ -> raise TypeError) in 

In fact at that point there is no let for the in to be associated wtih. You have had two let s and two in s before that.

It seems to me that you're trying to extract the integer values i and j . You can do that something like this:

let i, j =
    match n1, n2 with
    | Int_val i, Int_val j -> i, j
    | _ -> raise TypeError
in
i + j

Your code applies + to n1 and n2 , which doesn't make sense since they appear to be values of some algebraic type (not int).