--only-deps flag to `stack build`

[mboes]

For CI, it's useful to distinguish build dependencies from building project packages. Granted, in principle with Stackage the dependency building part of the build should not fail, but there might be combinations of flags for which it could, and it's useful to have the build time for the project counted separately from the build time for the rest.

Also, Travis CI now supports caching. With --only-deps, we could in theory builds the deps once, then have Travis cache that, then in subsequent runs it won't be necessary to rebuild the deps.

[snoyberg]

Huh, interesting. I'm excited to check out the caching support.

[mboes]

As noted somewhere at the bottom of that page, we need to be careful not to touch the folder that includes the dependencies artifacts when we build the project packages, in order to avoid invalidating the cache. This may or may not be an issue - depends if we're already doing that!

[snoyberg]

Oh, it's the same caching I saw last time, which won't work on non-container builds (where container builds don't allow you to run sudo). That's disappointing.

Given that, I don't see any urgency on this feature, so deferring to later milestone.

[mboes]

well, but do we need sudo? If we build stack from source, then build deps from source, and cache that, then this could significantly speed up the build process for most people.

Alternatively install the stack binary in a non-global location.

[snoyberg]

I'm thinking about system libraries like gmp and zlib. I suppose we could put them in a user location and then LD_LIBRARY_PATH to make them available... that feels like a bad thing to do on a CI system though.

[radix]

gmp and zlib should already be installed on the travis nodes, so it shouldn't be necessary, right?

[snoyberg]

I thought they weren't, but I haven't tested. I guess that's what we really need: a test project to try this out on. Someone wanna take a stab at it?

[mboes]

Building stack from scratch on the Travis nodes works fine. I tried it this weekend: https://travis-ci.org/haskell-distributed/cloud-haskell/builds/66650203#L270

[snoyberg]

--only-snapshot flag added. @mboes confirmed?