What steps will reproduce the problem? 1. Install GHC 7.8-rc2 2. cabal install --with-compiler=ghc-7.8.0.20140228 xmonad-contrib

What is the expected output? What do you see instead?

I expect to see a successful compile. Instead I see:

Downloading xmonad-contrib-0.11.2... Configuring xmonad-contrib-0.11.2... Building xmonad-contrib-0.11.2... Preprocessing library xmonad-contrib-0.11.2... [ 1 of 226] Compiling XMonad.Util.StringProp ( XMonad/Util/StringProp.hs, dist/build/XMonad/Util/StringProp.o ) [ 2 of 226] Compiling XMonad.Util.Replace ( XMonad/Util/Replace.hs, dist/build/XMonad/Util/Replace.o ) [ 3 of 226] Compiling XMonad.Util.CustomKeys ( XMonad/Util/CustomKeys.hs, dist/build/XMonad/Util/CustomKeys.o ) [ 4 of 226] Compiling XMonad.Layout.ThreeColumns ( XMonad/Layout/ThreeColumns.hs, dist/build/XMonad/Layout/ThreeColumns.o ) [ 5 of 226] Compiling XMonad.Layout.StackTile ( XMonad/Layout/StackTile.hs, dist/build/XMonad/Layout/StackTile.o ) [ 6 of 226] Compiling XMonad.Layout.Spiral ( XMonad/Layout/Spiral.hs, dist/build/XMonad/Layout/Spiral.o ) [ 7 of 226] Compiling XMonad.Layout.Roledex ( XMonad/Layout/Roledex.hs, dist/build/XMonad/Layout/Roledex.o ) [ 8 of 226] Compiling XMonad.Layout.ResizableTile ( XMonad/Layout/ResizableTile.hs, dist/build/XMonad/Layout/ResizableTile.o ) [ 9 of 226] Compiling XMonad.Layout.OneBig ( XMonad/Layout/OneBig.hs, dist/build/XMonad/Layout/OneBig.o ) [ 10 of 226] Compiling XMonad.Layout.MultiToggle ( XMonad/Layout/MultiToggle.hs, dist/build/XMonad/Layout/MultiToggle.o )

XMonad/Layout/MultiToggle.hs:162:9: Could not deduce (HList b w0) arising from the ambiguity check for ‘??’ from the context (HList b w) bound by the type signature for (??) :: HList b w => a -> b -> HCons a b at XMonad/Layout/MultiToggle.hs:162:9-42 The type variable ‘w0’ is ambiguous In the ambiguity check for: forall a b w. HList b w => a -> b -> HCons a b To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature for ‘??’: (??) :: HList b w => a -> b -> HCons a b cabal: Error: some packages failed to install: xmonad-contrib-0.11.2 failed during the building phase. The exception was: ExitFailure 1

What version of the product are you using? On what operating system?

xmonad-contrib-0.11.2

Please provide any additional information below.

There are two things that need to be done. Write down the incompatibility in the .cabal file for xmonad-contrib and fix this in a later version.