Unbound Value in recursive function ocaml

Question

I'm trying to create a filter function which recieves a function as parameter and a list and returns another list with the filtering result.

let rec filter bool_func l r = 
    match l with
    | [] -> r
    | h::t -> if bool_func h then filter bool_func t (h::r)
    else filter bool_func t r in filter bool_func t [];;

Answer 1

The keyword in goes with a previous let to make a local definition for use in the expression following in . So you have a local function definition of filter followed by the expression filter bool_func t . And indeed, outside the let there are no values (that you are showing us) named bool_func or t .