change to signature of withTempFile breaks custom Setup.hs files

[mzero]

Change [c3a5b96] changed the signature of withTempFile by adding an additional Bool argument. This causes custom Setup.hs files that use that function to break. See, for example, the package GLFW.

What's more, I can't see any way to cleanly handling this situation in a Setup.hs file - and still support building in environments both 1.16 and earlier, and post- 1.17 (which seems slated to include this change.)

I understand the desire for the functionality, but a new function should be added, perhaps withTempFile' or withTemporaryFile, or withTempFilePossiblyDeleting, and have the old API call it.

[23Skidoo]

Implemented this proposal. In general, though, we should make it possible to conditionally compile parts of Setup.hs based on the Cabal lib version since there are no stability guarantees for the internal Cabal API.

[mzero]

I think we should change our stance here. Distribution.Simple now has a significant body of clients, and it would extremely problematic if we adhered to Everything under Distribution.Simple.* has no stability guarantee.

Introducing conditional compilation in Setup.hs should be seen as a means to support older Cabal. It should be possible to write a Setup.hs file today and have reasonable expectation that it will work with Cabal versions into the future (until, say, some apocalyptic 2.x series). As such, it isn't clear to me how useful it would be, since any conditional compilation feature wouldn't be implemented in Cabals before it was introduced.

[dcoutts]

This has been a problem since the very beginning. The original Cabal spec makes the Setup.hs the initial point of entry, so first thing the user has to do is get that to compile. That basically means that it's impossible to use cpp and impossible to have the Setup depend on any interesting packages, or specify a version of the Cabal lib.

Since the introduction of the cabal-install tool we have gradually moved most users over to this other user interface. The remaining users of Setup.hs are distributions and tools like cabal-install and others.

Strictly speaking we'd need to have a spec change to say that instead of compiling and running Setup.hs, users/tools first have to consult the .cabal file, look at a field that gives dependencies for the Setup.hs and then compile the Setup.hs with those dependencies available (and perhaps with some cpp flags to give the versions of all these dependencies). "Ordinary" users would not notice, they continue to use cabal-install. Distro people (and people doing interesting builds that use the Setup.hs directly) would have to adjust their build scripts.

My thought on this before is that we would add a new .cabal file field to specify dependencies of the Setup.hs. This would become required for build-type other than Simple (once they opt-in to the more recent Cabal spec via the cabal-version field). Specifying dependencies would then allow depending on packages beyond base and Cabal, and would also allow constraints on the version.

[23Skidoo]

@dcoutts Should I merge #1388 in the meantime?