hello, I have a question about context in type signature. I would like to write a function, say (f :: T -> T) which also relies on an instance of class C being defined for T. The problem is, I don't want this instance defined at the same time f is defined, instead I would like to defer this definition until f is called (in some other module). Naively the code would look like this: ================================= module DefineF where class C a where fC :: a -> a data T = T f :: (C T) => T -> T f T = fC T ================================= module CallF where instance C T where fC = id call = f T ================================= The definition of f in DefineF won't compile because "All of the type variables .. are already in scope". Could you recommend any other way to achieve what I am trying to do? An interesting thing is that I can easily make the code compile with the same meaning by changing the declaration of T to include a dummy type variable:
data T a = T it's just a pity that I have to trick the compiler in such an ugly way.
Cheers, Misha