Show instance in System.Event.Manager causes conflicts

Conal Elliott conal at
Mon Apr 4 19:18:15 CEST 2011

System.Event.Manager [1] contains the following type alias and Show

-- | Callback invoked on I/O events.
type IOCallback = FdKey -> Event -> IO ()

instance Show IOCallback where
    show _ = "IOCallback"

This specialized instance conflicts with the more general instance Show (a
-> b) defined in Text.Show.Functions.

Stefan found this conflict when trying to compile the checkers package on
ghc 7.0.3. I suspect it will arise with other packages as well.

My suggestion is to remove the specialized Show instance and instead import
Text.Show.Functions as needed.

Other thoughts?


  - Conal
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