Assume I have a complex GADT which with many hidden type parameters as constructors:
data T where
A :: Num n => n -> T
B :: (Num n, Integral m) => n -> m -> T
C :: Floating a => [a] -> T
-- and so on
Z :: Num n => n -> n -> T
I want to make this datatype showable without having to manually write the instance. Problem is, that since Show isn't a superclass of Num anymore, adding a simple deriving instance Show T isn't enough for the compiler to infer that it has to add Show constraints to all the internal hidden type parameters.
For each hidden type parameter, it outputs something like
Could not deduce (Show n) arising from a use of 'showsPrec'
from the context Num n
bound by a pattern with constructor
A :: forall n. Num n => n -> T
...
Possible fix:
add (Show n) to the context of the data constructor 'A'
Adding a Show constraint to the datatype isn't an option either, since it limits the possible inhabitants of T. It seems like deriving instanec Show T should introduce the constraint Show on the hidden data types, although I am not sure.
How can I go about this?