Multiparameter classes in HUGS and GHC
Wed, 30 Apr 2003 21:26:06 +0100
At 14:45 30/04/2003 -0400, Dean Herington wrote:
>Again the kinds are wrong. However, you can't make a Pair instance out of
>MyPair4 because the latter is
>a `type` rather than `newtype` or `data`.
>Hope this helps.
Yes, very much, thank you.
If I now have this right, it's the distinction between a parametric
polymorphic type and an algebraic type constructor that I had failed to
properly appreciate. Now I'm alerted to it, I don't know why I didn't
realize sooner that something there was wrong.
I discover that I can also use:
type MyPair5 = (,)
instance Pair MyPair5 Int String where
newPair = id
getPair = id
though I'm not sure how MyPair5 is interpreted as a type synonym ;-)
There's a small matter that still puzzles me a little. In:
data (Eq a, Show a) => MyPair3 a b = P3 a b
instance Pair MyPair3 Int String where
newPair (x,y) = P3 x y
getPair (P3 x y) = (x,y)
(which seems OK), "Pair3" is a type _expression_ of kind (* -> * -> *), the
corresponding _constructor_ for which is P3. Yet, according to , "(,)"
is a type _constructor_. I guess there may be some terminological
crossover here as type constructors often use the same name as the
corresponding type expression?
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