On Feb 20, 2010, at 10:29 AM, Sjoerd Visscher wrote:
I don't understand this, as I thought the constraints the error is complaining about is just what withConstraintsOf g should provide. I guess there's something about the Suitable trick that I don't understand, or possibly the type families Fst and Snd are biting me.
Who can help me out? Thanks.
You specifically ask withConstraintsOf to accept only Suitable2's when you say
withConstraintsOf :: Suitable2 m a b => m a b -> (Constraints m a b -
k) -> k
But you aren't saying that the argument of withConstraintsOf IS a Suitable2, when you say:
instance (RCategory c1, RCategory c2) => RCategory (c1 :***: c2) where id = withResConstraints $ \ProdConstraints -> id :***: id -- f@(f1 :***: f2) . g@(g1 :***: g2) = -- withResConstraints $ \ProdConstraints -> -- withConstraintsOf f $ \ProdConstraints -> -- withConstraintsOf g $ \ProdConstraints -> -- (f1 . g1) :***: (f2 . g2)
You need to make a type class instance for Suitable2 for whatever type \ProdConstraints -> ... represents. For comparison, try:
data Unordered = A | B | C
A <= B No instance for (Ord Unordered) arising from a use of `<=' at <interactive>:1:0-5 Possible fix: add an instance declaration for (Ord Unordered) In the expression: A <= B In the definition of `it': it = A <= B
and now:
data Ordered = A | B | C deriving (Show, Eq, Ord) -- (easier than writing an instance for Eq, Ord) A <= B True