Compile failure with base-4.5

[hvr]

The cabal file states to support base < 5, however at least `base-4.5 does not work:

GHC-7.4/persistable-record-0.5.0.0

Configuring component lib from persistable-record-0.5.0.0...
Preprocessing library persistable-record-0.5.0.0...

src/Database/Record/FromSql.hs:34:8:
    Could not find module `GHC.Generics'
    It is a member of the hidden package `ghc-prim'.
    Perhaps you need to add `ghc-prim' to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.

[khibino]

The cabal flag ghc74-generic enables ghc-prim-0.2.0.0 which avoids this failure with GHC 7.4.x and base-4.5.

https://travis-ci.org/khibino/haskell-relational-record/jobs/219966118

[hvr]

@khibino Oh, I see... ok, there's still a problem though: the way the .cabal file is written, cabal's solver has no indication that it needs to toggle the flag for GHC 7.4! In fact, future cabal versions may warn about the way that flag was used. In order to do this properly, you'd have to formulate the conditional in such a way, that the flag setting has no degree-of-freedom anymore (i.e. the constraints resulting from each if-branch must be mutually exclusive in some way).

However, in this specific case, there's an easier way to handle this; get rid of the flag and write instead

  if impl(ghc == 7.4.*)
     build-depends: ghc-prim == 0.2.*

I use a variant of this technique in e.g. http://hackage.haskell.org/package/deepseq-1.4.2.0/dependencies

[hvr]

@khibino I went ahead and replaced all flags by the more idiomatic impl()-based pattern, see #53

[khibino]

@hvr If it's alright with you, please tell me your situation using HRR with GHC 7.4.

[hvr]

@khibino well, looking at http://104.239.175.197:8080/package/persistable-record the latest release looks fine

[khibino]

I see. Thanks!