On Sun, 14 Dec 2008, Mario Bla?evi? wrote:
I'll take a swing at this one:
instance Container (Maybe x) [x] where
wrapper = isNothing
. . .
That isn't a sensible definition of 'wrapper', but I believe without
trying to compile it is completely legal. Which wrapper do you use?
You /don't/ have a different matching Container instance, but without the
functional dependency you /might/, and ghc barfs.
But liftWrap doesn't require any particular instance, it's a
generic function accepting any pair of types for which there is
an instance of Container. Instance selection (as I understand it)
shouldn't come into play until one applies liftWrap to a
particular type, and indeed it does cause problems there: note
the type annotations on the last line. That part I understand
and accept, or at least have learned to live with.
Yes, that is an intuitive understanding to us humans, but the instance
selection applies seperately for 'wrapper' and 'listWrap'; ghc doesn't
automatically prefer a particular instance just because it is in the
context of the calling function.
--L
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