Question

I’m just starting to learn haskell. I have custom class

class MyClass a where
  doSomething :: a -> ()

Can I write a data with polymorphic callback in constructor?

data MyData = MyConstructor {
  callback :: (MyClass m) => m -> () -- error
}

I want to make constraint for function argument.

Answer 1

In short, Yes you can by using Rank2Types or RankNTypes pragma.

As your data type is existentially quantified. It is actually

data MyData = MyConstructor (forall m. MyClass m => m -> ())

for which you require higher rank extension.