Re: [Haskell-cafe] Use of uninstantiated type class

On 5 March 2011 10:45, Yves Parès 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 ?

Provide an explicit type signature for either runFoo or someAction; this is the same problem as doing "show . read" in that GHC can't tell which instance to use. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com