Disable library building, only build other components

[snoyberg]

(I may be missing something obvious, in which case, please let me know how this should be handled.)

When you specify components, you can get fine-grained control over which executables are built. However, there seems to be no way to control the library component. This has a number of repercussions:

• No way to build only the library: cabal build will build all components
• No way to build just an executable/test suite/benchmark without building the library
• Due to the fact that --enable-tests or --enable-benchmarks can modify the cabal_macros.h file, building your library and then rebuilding with test suites enabled can force the entire library to be recompiled, even though that's not necessary (this may actually be worth opening as a separate bug report).

Backwards compatibility will be a problem for any changes we want to make here. Ideally, I'd have said that lib is its own component, and a command cabal build lib would build only the library, whereas cabal build exe:foo would not build the library. However, the latter would be a breaking change. So perhaps we can add a --no-build-library flag?

(And I've referred to cabal the executable here, but I'd really like to see this in Cabal-the-library.)

[snoyberg]

After working through the codebase, it seems like one part of this is in fact supported: cabal build packagename will build only the library, with none of the other components. Can someone confirm that that's intended behavior?

This addresses some of my need, but I do still need a way to disable building the library.

[snoyberg]

And after a little more experimentation: this is specifically in the case where the executable/test suite/benchmark depends on the local library. Nonetheless, for that case, I'd like the ability to turn off rebuilding of the local library and use the already built copy. Figuring out a fix for the cabal_macros.h issue would go a long way to making such a change unnecessary.

[hvr]

Can someone confirm that that's intended behavior?

Afaik yes, but you can be more explicit by saying cabal build lib:<packagename (you may have seen there's a bit of machinery in Distribution.Simple.BuildTarget). It was planned to have cabal build also be able to build single modules of components at some point.

/cc @dcoutts

[ezyang]

The macros issue is #1893, there's a patch in the works.

[ezyang]

The macro issue has been fixed in HEAD.

If you still want to build an executable against an external copy of the library, I feel that #3286 should solve this case.