Simple OCaml exercise

Question

I'm trying to teach myself OCaml through Jason Hickey notes and the following exercise got me stumped. Question: Write a function sum that given two integer bounds m,n and a function f computes a summation. I'm trying this:

     let rec sum m n f=
     if m>n then 0
     else if m=n then f n
     else f m + sum n m+1 f

but it doesn't work, producing a type error.

Type error _ sum n m_ `Error: This expression has type ('a -> int) -> 'b but an expression was expected of type int'. I already got an answer but I guess if I read this more carefully I had a chance to figure it out myself. — Torus

Keith Irwin Keith Irwin · Accepted Answer · 2011-10-15T01:17:00

You need some parentheses.

let rec sum m n f=
     if m>n then 0
     else if m=n then f n
     else f m + sum n (m+1) f

(Although for readability, I would usually parenthesize the last line as else (f m) + (sum n (m+1) f). ) What's happening without the parentheses is that it's treating it as (f m) + (sum n m) + (1 f) which is producing the error that sum n m doesn't have type int, as it's a partial function application with a more complex type.

As a general rule, when an expression is being passed as an argument to a function, it always needs to be parenthesized. On a related note, if you ever actually wanted to pass the plus function as an argument, you would put it in parentheses (for example: sum m n (+) (although that wouldn't type check in this case, since + expects two numbers)).

Simple OCaml exercise

2 Answers