Brian Troutwine wrote:
Do you have any reason not to do the above?
Yes, the subset types that I wish to define are not clean partitions, though my example does suggest this. Let's say that the definition of Foo is now
data Foo = One | Two | Three | Four | Five | Six
while Odd and Even remain the same. I would further like to define Triangular, which I will do incorrectly for consistency.
data Triangular = One | Three | Six
I could not accommodate this definition using your scheme, correct?
A variation on scheme proposed by Ross Mellgren earlier in this thread. It's a bit tedious but allows for definition of arbitrary subsets thus it may work for you: {-# LANGUAGE GADTs, EmptyDataDecls #-} data One data Two data Three data Four data Foo a where FOne :: Foo One FTwo :: Foo Two FThree :: Foo Three FFour :: Foo Four class IsEven a instance IsEven Two instance IsEven Four class IsOdd a instance IsOdd One instance IsOdd Three class IsLessThanThree a instance IsLessThanThree One instance IsLessThanThree Two quux :: IsEven a => Foo a -> String quux FTwo = "2" quux FFour = "4" bzzt :: IsLessThanThree a => Foo a -> String bzzt FOne = "1" bzzt FTwo = "2"