CODEWITHSUNDEEP
haskellffi
Elise Huard
Elise Huard

Reputation: 241

Haskell Data instance for opaque data type

I'm writing an open source patch to use a font library, or rather the haskell bindings to a font library in C (FTGL). I'm pointing to the Font type in one of the data structures, which is defined as follows:

type Font = Ptr Font_Opaque
data Font_Opaque

Unfortunately, to fit into the data structure of the library I'm patching, this type needs to be an instance of Data. Ptr already is, but Font_Opaque obviously isn't, so the compiler complains.

As it's an opaque type I'm not sure how to proceed ... how to implement Data Font_Opaque in a more or less sensible way? Is there a sensible way?

Upvotes: 16

Views: 923

Answers (1)

Elise Huard
Elise Huard

Reputation: 241

As the comment by András Kovács suggests, using the StandaloneDeriving language extension

{-# LANGUAGE StandaloneDeriving -#}

and then:

deriving instance Data Font_Opaque

did the trick, at least where the compiler is concerned. I'll report back if this affects the program in any way. Thanks!

Upvotes: 5

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