Alexander Solla wrote:
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)
As I understand, Sjoerd expects this to be done at the definition of (.)
in the type class RCategory, so that an instance method can relay on the
constraints collected by it:
class RCategory (~>) where
id :: Suitable2 (~>) a a => a ~> a
(.) :: (Suitable2 (~>) b c, Suitable2 (~>) a b, Suitable2 (~>) a c) => b ~> c -> a
~> b -> a ~> c
A simple example:
class Show el=> ExceptionNote el where
comment:: Show exception=> exception-> el-> String
instance ExceptionNote Int where
comment exception refId = show refId ++ ": " ++ show exception
Here you don't need to constrain ?exception? to be of ?Show? at the
instance declaration. So it does not appear wrong for Sjoerd to expect f
and g to already be of Suitable2...
This is exciting stuff, I am really a little astonished about the giant
leap Haskell has made since my efforts to translate the examples of
Rydeheart & Burstall, which actually was my intro to categories, from ML
to Haskell. This looks very elegant... Maybe it's time for a second
edition of the unique approach of Rydeheart & Burstall on basis of
Haskell? Wow, really cool stuff... :-)
Cheers,
Nick
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