[Haskell-cafe] Use of uninstantiated type class

[Ryan Ingram]

On Fri, Mar 4, 2011 at 3:45 PM, Yves Parès <limestrael at gmail.com> wrote:

> 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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