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"
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