GADT as an optional argument

[wokalski]

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Begginer looking to provide a default parameter without changing the signature of a function

Hi! I’m just starting out so please let me know if I’m doing anything particularly silly: I have defined a function: let try_next ~f s = try Some (Stream.next s |> f) with Stream.Failure -> None ;; val try_next : f:('a -> 'b) -> 'a Stream.t -> 'b option = <fun> But when I try to provide the identity function as a default for f I find that the signature for the function changes; presumably because the type-checker can see that Fn.id is of 'a -> 'a: let try_next ?(f=Fn.id) s = …