Please declare dependency on 'cpphs' in the build tools stanza of your cabal file ...

[peti]

... because builds fail if that tool isn't available.

[liyang]

@peti I haven't been able to reproduce this. cpphs is only required on recent OS X due to Clang's CPP breaking the -traditional way of doing token-pasting (see #23). I have confirmed that everything builds fine without cpphs on Ubuntu at least…

Could you show me the build error on NixOS? Thanks.

[peti]

We've had the error compiling on Darwin. (Note that Nix is cross-platform; we build on Linux, Darwin, BSD, and even Cygwin.) In https://github.com/NixOS/nixpkgs/commit/0e588f35fc4ab1a94437653f1ad0ad5dc3b961dd @jwiegley added cpphs to fix the issue. Unfortunately, I cannot provide a build log because I don't have access to Darwin, but maybe @jwiegley can help out?

[jwiegley]

I don't quite remember what the issue was. If someone can point me to a log, I'd be happy to track it down.

[liyang]

Ah, I hadn't realised NixOS was being built on anything other than the GNU toolchain. I think this is a Clang issue (see #23), and Cabal can't really tell the native CPP is coming from so I guess it'll be easier just to switch to cpphs for all platforms.

[liyang]

Thinking about it a little more, I already have the following in thyme.cabal:

    if os(darwin)
        build-tools: cpphs
        ghc-options: -pgmP cpphs -optP--cpp

Could you confirm that Cabal isn't detecting os(darwin) correctly in your case? What's $OSTYPE for NixOS/Darwin? (I think this is the same as System.Info.os.)

[peti]

Oh, that is an interesting problem. Nix controls the exact set of packages that the build can see, i.e. only those paths that have been declared as a dependency will be visible in the chroot environment. The build instructions, https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/haskell/thyme/default.nix, don't mention cpphs, however, because we generate those expressions by running Cabal's finalizePackageDescription assuming our target platform is Linux/i686: https://github.com/NixOS/cabal2nix/blob/master/src/Cabal2Nix/Generate.hs#L58. In theory, we should probably parse the Cabal description separately for every target architecture/platform supported by Nix and then merge the result into one Nix expression that combines all that know-how, but this would be very hard to implement.

What we've do instead to remedy the situation is to hard-code that know-how about cpphs into our cabal2nix tool: https://github.com/NixOS/cabal2nix/blob/master/src/Cabal2Nix/PostProcess.hs#L106.

So, arguably, this bug report is invalid. Thyme does the right thing, it's just our particular build environment that fails to take advantage of the information properly.

[liyang]

Ah, I see! I guess most packages would be checking if os(windows) or something which doesn't affect Nix. I made an attempt to look for other affected packages that's already in Nix, and came across fsnotify: NixOS/cabal2nix#99 —I guess that's basically the same underlying issue…

[peti]

Yeah, exactly. We had to duplicate the selection logic in Nix to get that package to build on something other than Linux.

[LeviSchuck]

I just ran into this today, please fix it already.

I use http://ghcformacosx.github.io GHC For Mac OS X, which is a very minimal installation.