cabal-install doesn't consider build-tools as dependencies

[guest]

On #haskell, nelhage mentioned that one of the bugs he ran into while trying to install Yi-0.3 through cabal-install was an error which looked like this:

<pre> Installing: /home/gwern/.cabal/lib/regex-posix-0.72.0.2/ghc-6.8.2/regex-posix-0.72.0.2/ghc-6.8.2 Registering regex-posix-0.72.0.2... Reading package info from "dist/installed-pkg-config" ... done. Saving old package config file... done. Writing new package config file... done. 'yi-0.3' is cached. [1 of 1] Compiling Main ( Setup.hs, dist/setup/Main.o ) Linking dist/setup/setup ... Configuring yi-0.3... setup: alex version >=2.0.1&&<3 is required but it could not be found. </pre>

That is, the dependency on Alex (a version which satisfied >=2.0.1&&<3) was not met. Perfectly sensible as he did not have Alex installed. Puzzled, he did a 'cabal install alex', which worked, and then re-started. (I guess he had his GHC installed locally?).

He and I were puzzled, because cabal-install is supposed/usually does track down dependencies. Is build-tools excluded for a reason, or was it just accidentally omitted from dependency-tracking? If the latter, I think this is a bit of a bug.

(I use Cabal-1.2.3.0, btw, and the error message above was my reproduction of Nelhage's problem. I had to uninstall Alex from my system, but that did the trick, and reinstalling bypassed the issue just fine - which is why I hadn't seen it before.)

-- gwern

[duncan]

One problem is that not all build-tools correspond to haskell packages. Some do some don't. We have a hard coded list of them at the moment (which can be extended in Setup.hs files) so we could extend that with what haskell package if any the tools correspond to. Any better suggestions to make it a tad more generic?

[duncan]

This limitation clearly confuses people.

•  http://www.haskell.org/pipermail/cabal-devel/2008-June/003220.html
•  http://www.haskell.org/pipermail/haskell-cafe/2008-June/044222.html

[darrint]

Seems like what you need is to be able to determine which packages provide which executables. Alex, for instance, declares that it provides an executable by that name. For the next release of cabal-install, maybe change the package repository metadata format to include more (all?) the cabal info instead of a file list. That will allow you to solve more situations and do a full install plan up front.

Would that work?

[duncan]

See also #342

[mcandre]

I have the same problem with ncurses and c2hs. I'd really like dependencies to be handled automatically whenever possible.