* TP
Hi all,
I wonder if there is some means to get a type variable value in the body of a class instance definition. It seems it is not possible (a workaround has already been given on this list), but I would like a confirmation. For example, imagine that I have a Tensor type constructor, that takes a type-level integer (representing the Tensor order) to make a concrete type:
--------- instance Show (Tensor order) where show TensorVar str = show "tensor " ++ str ++ " of order " ++ (show (c2num order)) ----------
where c2num transforms a type-level integer to an integer, through typeclasses (see http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue5/Number_Param_Type...)
I obtain a compilation error: order is not known in the body of the instance. Putting ScopedTypeVariable as extension does not change anything (http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/other-type-exten...).
You are confusing type and value variables. c2num order means apply the function 'c2num' to the value variable 'order', which is not defined. To get from a type to a value you can use a type class 'Sing' (for 'singleton') class Sing a where sing :: a instance Sing Zero where sing = Zero instance Sing a => Sing (Succ a) where sing = Succ sing After adding the appropriate constraint to the instance, you can write show $ c2num $ (sing :: order) Roman