OCaml type error for lists

Question

I want to write a function that will combine all the elements in a list in OCaml. Simple enough, so far I have a working function:

let rec comAll (f : 'a -> 'a -> 'a) (r : 'a) (l : 'a list) : 'a =
  match l with [] -> r
  | hd::tl -> let comAll2 = comAll f r tl in f hd comAll2
;;

The First argument is a function, the second is a default value to return when/if the input list is empty, and the third argument is the list itself.

This function works as intended, except when I try to call it using another function, for example:

let inList (l : 'a list) (e : 'a) : bool = comAll (fun x y -> if x == e then true else y) false l ;;

which will pass as a function, returning inList : bool list -> bool -> bool = <fun> however, I want to instead have it return inList : 'a list -> 'a -> bool = <fun>

I have tried instead defining comAll as: let rec comAll f r l =... and that works, but I want to declare the function with explicit types.

Any help or guidance on what I am doing incorrect here?

Jeffrey Scofield Jeffrey Scofield · Accepted Answer · 2016-09-24T02:00:20

You are declaring your parameter f to be of type 'a -> 'a -> 'a but you want to pass a function of type 'a -> bool -> bool. In other words, you want to have two different types. You should declare f to be of type 'a -> 'b -> 'b. The rest will follow.

# let rec comAll (f : 'a -> 'b -> 'b) (r : 'b) (l : 'a list) : 'b =
  match l with [] -> r
  | hd::tl -> let comAll2 = comAll f r tl in f hd comAll2  ;;
val comAll : ('a -> 'b -> 'b) -> 'b -> 'a list -> 'b = <fun>
# let inList (l : 'a list) (e : 'a) : bool =
      comAll (fun x y -> if x == e then true else y) false l;;
val inList : 'a list -> 'a -> bool = <fun>

OCaml type error for lists

1 Answers