OCaml explicit type signatures

In Haskell, it is considered good practice to explicitly declare the type signature of your开发者_运维问答 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.

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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).

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References: OCaml for Haskellers

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In general, the syntax to let-bind a value with a constrained type is:

let identifier_or_pattern : constraint = e ...

Applied to a function, you can specify the signature as follows:

let add : int -> int -> int = fun x y -> ...

This is analogous to the syntax required to constrain a module to a signature:

module Mod
  : sig    ... end
  = struct ... end