haskell - How to avoid recompiling in this cabal file?

Question

I've been working on this Haskell project, and I have a cabal file for it. Now, my project is structured as a library that implements a simple interpreter. I also have a very short Main file which needs to be build into an executable to call the library. What I want to do is:

1) compile the library and expose some of the modules

2) compile the executable

I have a cabal file that works and seems to do this. The problem is that when it compiles the executable it recompiles all the modules which have already been compiled in step (1). I don't quite understand why it does this - is there any way to stop it, short of creating two separate cabal files?

I don't really want to create two separate cabal files, because cabal doesn't seem to like having both the cabal files in the same directory, and I don't really want to set up a separate project directory for the second step, since it basically just amounts to compiling a single file.

cabal-version:      >= 1.6
build-type:         Simple
name:               HaSC
version:            0.2.3
license:            OtherLicense
category:           Language
author:             Chris B
maintainer:         Chris B
copyright:          Chris B 2010 - 2011
synopsis:           (HA)skell (S)ound (C)hange applier (HaSC) library
description:        HaSC implements a little language for applying sound changes to words
homepage:           http://www.chrisdb.me.uk/redmine/projects/haskell-sound-change
stability:          Alpha
data-files:         doc/HaSCDoc.pdf
license-file:       LICENSE

library
    build-depends:
        base >= 4.3,
        containers >= 0.3,
        parsec >= 3,
        parallel >= 3.1,
        deepseq >= 1.1,
        mtl >= 1.1, 
        transformers >= 0.2,
        text >= 0.10,
        text-icu >= 0.6.3,
        pretty >= 1,
        directory >= 1.1,
        filepath >= 1.2
    hs-source-dirs:  src
    exposed-modules: HaSC.IO.Disk,
                     HaSC.IO.Memory,
                     HaSC.Exec
    other-modules:   HaSC.AST,
                     HaSC.IO,
                     HaSC.IdentMap,
                     HaSC.Parse,
                     HaSC.Regex,
                     HaSC.Representation,                     
                     HaSC.Transformations,
                     HaSC.Search,
                     HaSC.State

executable HaSC
    GHC-Options: -rtsopts
    hs-source-dirs:  src
    main-is:         Main.hs    

Answer 1

In your executable section, add the library in Build-Depends so that the executable depends on the library.

There's a small gotcha, though: You also have to move the Main.hs of the executable (and any other source files specific to the executable) to a different subdirectory and specify a different Hs-Source-Dirs so that it doesn't pick up the library modules by being in the same folder.

executable HaSC
    Build-Depends: HaSC
    Main-Is: Main.hs
    Hs-Source-Dirs: foo -- Directory you moved Main.hs to

For this to work, you will need to specify Cabal-Version >= 1.8. See Cabal ticket #89 for the details.