[Haskell-cafe] Use of uninstantiated type class

[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.
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