free-5 doesn't build on old GHCs with transformers-0.4

[RyanGlScott]

As noticed in this Travis build.

$ cabal new-build -w /opt/ghc/7.8.4/bin/ghc --constraint=transformers==0.4.3.0
Resolving dependencies...
In order, the following will be built (use -v for more details):
 - free-5 (lib) (first run)
Preprocessing library for free-5..
Building library for free-5..
[ 9 of 20] Compiling Control.Monad.Trans.Free.Church ( src/Control/Monad/Trans/Free/Church.hs, /home/rgscott/Documents/Hacking/Haskell/free/dist-newstyle/build/x86_64-linux/ghc-7.8.4/free-5/build/Control/Monad/Trans/Free/Church.o ) [Control.Monad.Cont.Class changed]

src/Control/Monad/Trans/Free/Church.hs:84:13:
    Could not deduce (Functor m) arising from a use of ‘eq1’
    from the context (Functor f, Monad m, Eq1 f, Eq1 m)
      bound by the instance declaration
      at src/Control/Monad/Trans/Free/Church.hs:83:10-59
    or from (Eq a)
      bound by the type signature for
                 eq1 :: Eq a => FT f m a -> FT f m a -> Bool
      at src/Control/Monad/Trans/Free/Church.hs:84:3-5
    Possible fix:
      add (Functor m) to the context of
        the type signature for eq1 :: Eq a => FT f m a -> FT f m a -> Bool
        or the instance declaration
    In the expression: eq1 (fromFT x) (fromFT y)
    In an equation for ‘eq1’: eq1 x y = eq1 (fromFT x) (fromFT y)
    In the instance declaration for ‘Eq1 (FT f m)’

src/Control/Monad/Trans/Free/Church.hs:87:18:
    Could not deduce (Functor m) arising from a use of ‘compare1’
    from the context (Eq1 (FT f m), Functor f, Monad m, Ord1 f, Ord1 m)
      bound by the instance declaration
      at src/Control/Monad/Trans/Free/Church.hs:86:10-62
    or from (Ord a)
      bound by the type signature for
                 compare1 :: Ord a => FT f m a -> FT f m a -> Ordering
      at src/Control/Monad/Trans/Free/Church.hs:87:3-10
    Possible fix:
      add (Functor m) to the context of
        the type signature for
          compare1 :: Ord a => FT f m a -> FT f m a -> Ordering
        or the instance declaration
    In the expression: compare1 (fromFT x) (fromFT y)
    In an equation for ‘compare1’:
        compare1 x y = compare1 (fromFT x) (fromFT y)
    In the instance declaration for ‘Ord1 (FT f m)’

[hvr]

@RyanGlScott are hackage revisions needed?

[RyanGlScott]

Maybe. But I can't possibly fathom what the revision would need to be, as this error was an obscure combination of transformers, base, and GHC.

In any case, this point is a bit moot now that free-5.0.1 has been released, which has the same version bounds as free-5 does.

[hvr]

@RyanGlScott unfortunately it isn't completely moot... nor does free-5.0.1 have the same bounds as free-5... and so I noticed install-plans where the solver was justified to backtrack into free-5 rather than free-5.0.1

So the way I understand it, the configuration that needs to be excluded is

• free == 5
• transformers == 0.4.*
• base < 4.8

?