haskell cannot construct the infinite type

Question

I thought the following code should work:

sum_up x = loop_it 0 x
    where loop_it sum i | i > 0     = loop_it sum+i i-1
                        | i == 0    = sum

But I'm getting this error:

<interactive>:3:15: error:
    • Occurs check: cannot construct the infinite type:
        t2 ~ (t0 -> t2) -> t2
      Expected type: t2 -> t2
        Actual type: t2 -> (t0 -> t2) -> t2
    • In an equation for ‘sum_up’:
          sum_up x
            = loop_it 0 x
            where
                loop_it sum i
                  | i > 0 = loop_it sum + i i - 1
                  | i == 0 = sum
    • Relevant bindings include
        loop_it :: t2 -> t2 (bound at <interactive>:3:15)

Why doesn't this compile?

Answer 1

You need parentheses around the arguments of the recursive call to loop_it:

sum_up x = loop_it 0 x
    where loop_it sum i | i > 0     = loop_it (sum+i) (i-1)  -- <- Here
                        | i == 0    = sum

If you don't group it like that, the compiler would implicitly group it like this:

((loop_it sum)+(i i))-1

... which is probably not what you wanted, since that means: "apply loop_it to sum, then add that to i i (i.e. i applied to itself), then subtract 1.

This happens because function application has highest precedence in Haskell, so function application binds more tightly than arithmetic.