Show for higher-order nested datatypes

[Ian Bayley]

Is "deriving Show" meant to work for higher-order nested datatypes ?
Hugs hangs when loading in the following file:

type SqMat a = SM Nil a
data SM f a = ZeroS (f (f a)) | SuccS (SM (Cons f) a) 
                       deriving Show


data Nil a = MkNil
                       deriving Show
data Cons f a = MkCons a (f a)
                       deriving Show

... but it loads successfully if the first "deriving Show" line is commented out