[xmonad] Testing GHC HEAD

[adam vogt]

On Sun, Feb 17, 2013 at 7:30 AM, Carsten Mattner
<carstenmattner at gmail.com> wrote:
> xmonad built fine with GHC HEAD but XMonadContrib failed with the following
> build error:
>
> 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
>     In the type signature for `??':
>       ?? :: HList b w => a -> b -> HCons a b
>
> Can anyone suggest a fix?

Hi Carsten,

You could remove that context completely:

(??) ::  a -> b -> HCons a b

Unfortunately, this makes it possible to construct HLists that might
be invalid for the rest of the code. For example with the context
there I have:

 :t \a b -> a ?? b ?? ()

<interactive>:1:16:
    No instance for (HList () w0)
      arising from a use of `??'
    Possible fix: add an instance declaration for (HList () w0)
    In the second argument of `(??)', namely `b ?? ()'
    In the expression: a ?? b ?? ()
    In the expression: \ a b -> a ?? b ?? ()

With my suggested change, you are allowed to end the list with ()

\a b -> a ?? b ?? () :: a -> a1 -> HCons a (HCons a1 ())


Other things to try, that probably won't work, is to try something
that might make ghc be more lazy about finding the instance:

(??) :: (b2 ~ b, HList b2 w) => a -> b -> HCons a b

Also, you might also try adding a fundep to the HList class, which
will fix the 'w' given the 'b'.

class HList c a | c -> a where


--
Adam