Will be fixed in 7.4.2:

http://hackage.haskell.org/trac/ghc/ticket/5878

Cheers,
        Simon

On 16/02/2012 05:10, Evan Laforge wrote:
Thanks for bringing this up. I've been having the same problem and was
thinking there was something wrong with my system. I have 3 main files
and one util that they all import. Every build recompiles every file,
even if there were no changes.

On Feb 15, 2012 8:39 PM, "Conrad Parker" <con...@metadecks.org
<mailto:con...@metadecks.org>> wrote:

    Hi,

    We have a project with around 200 Haskell source files, and around 20
    executables which import these (as well as importing third-party
    libraries). We first build all the non-main files using a makefile
    generated with ghc -M. We then build the executables using a separate
    invocation of "ghc --make --main-is foo.hs", for each foo.hs. This
    worked fine with ghc-7.2.1; each final "ghc --make" would simply link
    the pre-built object files -- but with ghc-7.4.1 all these files are
    recompiled for each target executable. As a result our full build
    takes around 10-20x longer with ghc-7.4.1 than ghc-7.2.1.

    Looking at compiler/iface/FlagChecker.hs, it seems that the --main-is
    flag is used as an input to the recompilation checker. It would make
    sense that --main-is should force recompilation of the particular file
    that exports 'main', but it also forces recompilation of its
    dependencies. Is this a bug or is there a good reason to recompile
    everything?

    Conrad.

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