Question

I am able to export simple haskell functions through FFI containing standard data types. However, I do not know how to export a function that takes data type other than standard data types.

E.g.

data SomeType a = SomeType a
data SomeOtherType b = SomeOtherType b

doSomething:: SomeType a -> SomeOtherType b

How can I export function doSomething?

The existing documents talks about very simple examples.

Answer 1

Short answer is, you can’t.

You need to pick an instance of the function and export that.

e.g. doSomething :: SomeType Int -> SomeOtherType Int is exportable. I wrote a longer answer here that might be helpful

The reason is that the Haskell side needs to know how to marshall the structure, how much memory to allocate etc.