On Fri, Mar 4, 2011 at 3:45 PM, Yves Parès
Hello,
For testing purposes, I am trying to make an overlay to IO which carries a phantom type to ensure a context. I define contexts using empty type classes :
class CtxFoo c class CtxBar c
The overlay :
newtype MyIO c a = MyIO (IO a)
Then I define some methods that run only a specific context :
runFoo :: (CtxFoo c) => MyIO c a -> IO a runFoo (MyIO x) = x
runBar :: (CtxBar c) => MyIO c a -> IO a runBar (MyIO x) = x
And then an action that runs in context 'Foo' :
someAction :: (CtxFoo c) => MyIO c () someAction = putStrLn "FOO"
Then I run it :
main = runFoo someAction
But obiously, GHC complains that my type 'c' remains uninstantiated :
Ambiguous type variable `c' in the constraint: (CtxFoo c) arising from a use of `runFoo' Probable fix: add a type signature that fixes these type variable(s) In the expression: runFoo someAction In an equation for `main': main = runFoo someAction
Is there a way to deal with this ? The interest of using type classes and not empty types to represent the contexts is that it stays simple, and that I can do that :
someAction2 :: (CtxFoo c, CtxBar c) => MyIO c () someAction2 = putStrLn "FOO and BAR"
... a function that can run in both contexts.
data X instance CtxFoo X runFoo (someAction :: MyIO X ()) data Y instance CtxFoo Y instance CtxBar Y runFoo (someAction2 :: MyIO Y ()) runBar (someAction2 :: MyIO Y ()) runFoo (someAction :: MyIO Y ()) -- also works since Y provides both a Foo and Bar context) -- ryan
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