Question

So I wrote a function “filter” that is applied to the given predicate to each element in the input list and checks whether it should be included in the output.

let rec filer (pred: 'a -> bool) (l: 'a list) : 'a list =
begin match l with
| [] -> []
| hd :: rest -> if (pred hd) then hd :: (filter pred rest) else filter pred rest
end

Now I’m writing a predicate that can be passed to filter to keep just multiples of five.

let multiples_of_five_pred: int -> bool =
filter (fun (x: int) -> x mod 5)

I don’t know what I’m missing here…I’m getting a syntax error that says “this expression has type int but an expression was expected of type bool”.

Answer 1

The function given to filter needs to produce an boolean, but as the error message is telling you, the function you give to filter produces an integer, not a boolean. You need to change it to produce a boolean instead.

Once you fix that, you’ll have another type error: Applying filter to a function that takes an integer will produce a function of type int list -> int list, but according to your type signature, you want the type int -> bool.

Also note that you say that multiples_of_five_prod should be a function that is given to filter, but you actually call filter inside the function. That doesn’t fit. You should remove the call to filter.