Problems with recursive function in OCaml

Question

i am doing a function in OCaml with i want to count the number of iterations in the variable n. This is the function:

let mapdoble f1 f2 l = let rec aux n f_1 f_2 l1 l2= match(n,f_1,f_2,l1,l2) with (n,_,_,[],l2) -> l2 | (n,f_1,_,h::t,l2) when n mod 2 = 0 -> aux n+1 f1 f2 t l2@[f_1 h] | (n,_,f_2,h::t,l2) when n mod 2 = 1 -> aux n+1 f1 f2 t l2@[f_2 h] in aux 0 f1 f2 l [];;

When i compile it i have this error, i don't know what it is:

Error: This expression has type 'a -> 'b -> 'c list -> 'd -> 'd but an expression was expected of type int

I think you're simply missing parentheses around the two instances of n+1. — Reimer Behrends
– Reimer Behrends, Commented Jan 22, 2016 at 12:20
@ReimerBehrends is write, you have to put parenthesis around n+1. You also have to match all cases, the compiler should warn you about that too with an example. — Çağdaş Bozman
– Çağdaş Bozman, Commented Jan 22, 2016 at 12:28

ivg · Accepted Answer · 2016-01-22 14:30:11Z

Function application, that is just written as a juxtaposition of function name and its arguments, binds tighter than infix operators, i.e., it has a higher precedence. To clarify the above said, let's go to an example. The expression:

aux n+1 f1 f2 t l2@[f_1 h]

is actually parsed by a compiler as:

(aux n) + (1 f1 f2 t l2) @ ([f_1 h])

now, I hope, it is obvious, why you got so strange compiler messages. In particular, you are trying to apply (+) operator to the result of aux n, which is actually a function of four arguments and definitely not an int.

So the correct version would be:

aux (n + 1) f1 f2 t (l2 @ [f_1 h])

You have also some issues with pattern matching, it is not irrefutable, but it is out of the scope of the question.

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