CODEWITHSUNDEEP
purescript
WHITECOLOR
WHITECOLOR

Reputation: 26182

Pass polymorphic record to foreign function

I have R1 and R2 record types and I need to pass either R1 or R2 to a foreign function (it can handle either r1 or r2 record structure) is it possible to do this (I thought maybe via conversion to Foreign Object)?

Or do I need to declare two different foreign imports (with different type signatures for passing R1 and R2) pointing to the same js function?

Another way I found using unsafeCoerce for type conversion:

foreign import data R1orR2 ∷ Type

fromR1 :: R1 -> R1orR2
fromR1 = unsafeCoerce

fromR2 :: R2 -> R1orR2
fromR2 = unsafeCoerce

So maybe there some other ways.

Upvotes: 1

Views: 54

Answers (1)

Fyodor Soikin
Fyodor Soikin

Reputation: 80870

When writing FFI bindings, unsafeCoerce is quite ok: after all, foreign import has all the same drawbacks already, so you're not really losing anything.

And yes, what you came up with - R1orR2 - is the right approach that's used quite frequently in FFI bindings.

You may also want to check out undefined-is-not-a-problem and untagged-union libraries. They offer some more advanced and generalized techniques in this area.

Upvotes: 3

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