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Asked: 2026-05-29T19:12:27+00:00

I am building two libraries in OCaml which contain the same variant type. The

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I am building two libraries in OCaml which contain the same variant type. The detail of the variant is not too important, other than that it is really large, and it would be annoying to manually write conversion functions for it. (It’s actually the bfd_architecture enumeration converted from C using ocamlidl).

I’m now writing a program using the two libraries. In particular, I’m calling A.f which returns a value of A.variant_type, and I need to use that value in a call to B.g, which takes a value of B.variant_type as an input.

Is there any way to tell OCaml that A.variant_type and B.variant_type are really the same type, and thus it’s okay to convert a value from one to the other? The libraries are independent, so they should not reference each other. Right now I’m using Obj.magic to do the conversion, but this is a hack.

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  1. Editorial Team

    You dont have to use Obj.magic, although you do have to use some “magic”,

    external convertAB : A.t -> B.t = "%identity"
external convertBA : B.t -> A.t = "%identity"

    is enough to do the conversion. Of course, I understand your concern; both types must be exactly the same else you’ll be dealing with run-time errors. If it’s at all possible and relevant, converting those variants to Polymorphic Variants would solve the problem.

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