Better fix for #19

[hvr]

I noticed #19, and currently fgl.cabal contains

    build-depends:    base < 5
                    , transformers
                    , containers
                    , array

    if impl(ghc >= 7.4)
       build-depends:
            deepseq >= 1.1.0.0 && < 1.5.0.0

However this underspecified, as in the ghc>=7.4 you clearly require also containers >= 0.4.2, so a better fix would be e.g

    build-depends:    base < 5
                    , transformers
                    , containers >= 0.4.2
                    , array
                    , deepseq >= 1.1.0.0 && < 1.5

or alternatively (but this leaves us with an API that conditionally provides the NFData feature depending on the containers version):

flag containers042
   manual: false
   default: true

library
-- ....
    build-depends:    base < 5
                    , transformers
                    , containers
                    , array

    if flag(containers042)
       build-depends:
            containers >= 0.4.2
            deepseq >= 1.1.0.0 && < 1.5
    else
       build-depends: containers < 0.4.2

PS: it's unsound to write an upper bound with trailing zeroes like <1.5.0.0 since 1.5 < 1.5.0 < 1.5.0.0

[ivan-m]

You added a minimum bound to transformers in your second code block; was that meant to be for containers instead?

I'm not sure I like the flag approach (though I can see now why the version of containers should depend upon whether deepseq is being used or not; I just blindly went by what GHC has rather than being compiler-agnostic).

I suppose the upper bound (if any) on containers would then go in the main build-depends block (or should it be taken out of there and only mentioned within the flag bit)?

[hvr]

You added a minimum bound to transformers in your second code block; was that meant to be for containers instead?

Yes, sorry... I've fixed the example

I'm not sure I like the flag approach (though I can see now why the version of containers should depend upon whether deepseq is being used or not; I just blindly went by what GHC has rather than being compiler-agnostic).

Just because GHC ships with a specific containers version doesn't guarantee cabal will pick that version for you (the solver has a soft preference for installed packages though). In fact, here's a compatibilty matrix:

I suppose the upper bound (if any) on containers would then go in the main build-depends block (or should it be taken out of there and only mentioned within the flag bit)?

I'd put a global upper bound in the main build-depends as that's less error-prone
(and btw, the build-depends property is a conjunctive Monoid)

[ivan-m]

Whilst this works, it results in older GHC's having newer containers built on them on Travis-CI; making the default False results in containers being downgraded on 7.4.2.

Any idea how this can be resolved?