OCaml explicit type signatures

Question

In Haskell, it is considered good practice to explicitly declare the type signature of your functions, even though it can (usually) be inferred. It seems like this isn't even possible in OCaml, e.g.

val add : int -> int -> int ;;

gives me an error. (Although I can make type modules which give only signatures.)

1. Am I correct in that this isn't possible to do in OCaml?
2. If so, why? The type system of OCaml doesn't seem that incredibly different from Haskell.

Solution

OCaml has two ways of specifying types, they can be done inline:

let intEq (x : int) (y : int) : bool = ...

or they can be placed in an interface file, as you have done:

val intEq : int -> int -> bool

I believe the latter is preferred, since it more cleanly separates the specification (type) from the implementation (code).

---

References: OCaml for Haskellers

Comments

[Ignacio Tiraboschi]

The latter option is no longer a valid option. At least OCaml 4.04.0 doesn't support it. Everyone should use the first option.

[Perry]

That's not at all correct. OCaml 4.04.0 and above support interface files. .mli files are a preferred way to declare interfaces to modules.