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[Caml-list] Using %identity for casting types
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Date: -- (:)
From: Jacques Garrigue <garrigue@k...>
Subject: Re: [Caml-list] Using %identity for casting types
From: Jérôme Marant <>
>   I'm doing some experiments in interfacing OCaml with C++ and
>   it is sometimes necessary to cast one type to another.
>   I've seen in lablgtk something like:
>     external unsafe_cast : 'a obj -> 'b obj = "%identity"
>   It is meant to be used by the end user?

Well, not really, but if you're interfacing with C++, it's not more
dangerous to do the cast in ocaml than in C++ (except if you use

The above idiom is equivalent to

    let unsafe_cast : 'a obj -> 'b obj = Obj.magic

It may be more efficient, as the compiler knows that unsafe_cast is
the identity, and can discard it. No code is generated.

I also tend to prefer it because it makes clear the need to properly
write types: such casts only make sense at a precise type, while
Obj.magic is 'a -> 'b, which allows too much.

And, as always, only use unsafe casts for non-ocaml data.
Ocaml datatypes are expressive enough that you don't need unsafe
casts for them.

Jacques Garrigue-------------------
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