Ocaml: This expression has type 'a list * 'a list -> bool but an expression was expected of type bool

Question

I just started learning Ocaml and am playing around with recursive functions. Ocaml Compiler is telling me that recursively calling helper in "if h1=h2 then helper t1 t2" causes an error: This expression has type 'a list * 'a list -> bool but an expression was expected of type bool. I understand that it is telling me that the compiler is expecting a boolean but instead gets a function that returns the boolean. But I have no idea how I can fix this. Any help is appreciated

let rec a_func l =
  let rec helper tmp l1 = function
      | [], [] -> true
      | _, [] -> false
      | h1::t1, h2::t2 -> if h1=h2 then helper t1 t2 else helper [h2]@l1 t2
  in helper [] l

Isabelle Newbie Isabelle Newbie · Accepted Answer · 2017-02-13T10:30:24

Your definition let rec helper tmp l1 = function ... defines a function helper which takes three arguments: tmp, l1, and an anonymous argument which is used in the pattern match. This is because the function keyword introduces an additional argument, it does not mean a pattern mantch on previous arguments.

It looks like you want to match on tmp and l1. In that case, you can write let rec helper tmp l1 = match tmp, l1 with .... You can also write let rec helper = function ..., but that defines a function that takes a pair, so you would have to call it as helper (t1, t2), helper ([], l) etc.

You will also need to put parentheses around [h2]@l1. Note that a more idiomatic way of writing this is h2::l1, but you will still need the parentheses. Finally, OCaml will warn you that you have no case for the pattern ([], _::_).

Ocaml: This expression has type 'a list * 'a list -> bool but an expression was expected of type bool

1 Answers