On Fri, Jun 5, 2009 at 7:25 AM, Claus Reinke
If ProjectPackage actually depends on the existence of those orphan instances, plan B is to delay instance resolution, from library to clients, so instead of importing the orphan instances
module ProjectPackage where import MyMonadT_Parsec f .. = .. orphan instances are available, use them ..
(which leads to the dreaded implicit export), you'd just assert their existence:
module ProjectPackage where f :: .. Orphan x => .. ; f .. = .. use orphan instances ..
That gets awkward if you're dealing with a concrete type. Consider,
class C a where
foo :: a -> a
data T = T
bar :: C T => T
bar = foo T
I was able to get GHCi to accept this with FlexibleContexts, but it
obviously doesn't like it.
*Main> :bro Main
class C a where foo :: a -> a
data T = T
bar :: (C T) => T
*Main> :t bar
<interactive>:1:0:
No instance for (C T)
arising from a use of `bar' at <interactive>:1:0-2
Possible fix: add an instance declaration for (C T)
In the expression: bar
But at least it's possible. And the problems go away once the instance
is in scope.
--
Dave Menendez