Support specialized GHC bindists

[3noch]

Continuing discussion in #495 and related to #399.

stack's understanding of GHC is a bit primitive as it only knows about version, OS, and architecture. It would help to augment this understanding to other [potentially arbitrary] factors. The current use-case is GHC bindists built with integer-simple instead of integer-gmp. Since this concept is orthogonal to version, OS, and architecture, stack has no good way to represent it. Thus, stack cannot currently

• represent the constraint via the yaml file or command-line arguments
• install specialized GHCs automatically
• check for existing presence

Currently the resolver is responsible for determining which version of GHC to use, and something else determines which OS/architecture distribution to use.

[3noch]

When it comes to integer-simple bindists, I don't know if there's any easy way to "check for existing presence." However, it seems that the Cabal system has some knowledge of integer-simple as a dependency, so perhaps that could be used to detect presence.

[3noch]

That said, checking for presence is not very high on the priority list IMHO. If stack can install an integer-simple build itself, then foregoing support for existing builds is less important for now.

[radix]

is "debugging build" another factor here? I noticed that the blog post at https://www.fpcomplete.com/user/bitonic/perf-for-low-level-profiling mentions that you need the RTS to be built with debug symbols for useful haskell-program debugging. (I kinda think this should be the default... maybe it already is?)

[borsboom]

Specifying a GHC variant on stack.yaml could be tricky, because you may want different variants on different platforms. For example, integer-simple more likely to be wanted on Windows. On Linux, you might want the gmp4 (centos65) variant on some distributions but not on others, and it would be rare to actually specify that rather than autodetect it.

[3noch]

@radix I'm not sure, but I do imagine that there are numerous variants we might want to support.
@manny-fp Certainly it gets complicated, but this takes us closer to that goal, not farther. As it is now, you have to add custom hacks to stack itself to gain these behaviors. If you need something akin to a configure script, you could actually write one that produced a stack.yaml file appropriate for the environment.

[3noch]

@manny-fp Bear in mind we already have the os option, which is not exactly "cross-platform" either. I think the point is that stack.yaml allows you to constrain your build as much as you need it to. Certainly no one is forcing you to use the os field. If you need really complicated constraints on different systems, you could script them, like I said.

[radix]

it seems like this begs for conditionals in the yaml. scary! :)

[3noch]

Hmm...perhaps something like Setup.hs is needed! ;)

[borsboom]

@3nock Agreed, definitely moving in the right direction. So I'd propose something along these lines to start with:

1. Create a newtype CompilerVariant = String. It's a free-form string so that we can add a new variant just by adding an entry to https://github.com/fpco/stackage-content/blob/master/stack/stack-setup.yaml rather than needing to change any code. I suppose we could follow the OS/Arch approach of having a sum type with the known types and an additional OtherCompilerVariant String, which would have the benefit of not needing to hard-code strings for cases like the gmp4 variant which is determined programatically.
2. Create a data PlatformVariant = PlatformVariant Platform (Maybe CompilerVariant). Probably should be a record type, really.
3. Use PlatformVariant most places that Platform is used now.
4. Add a --ghc-variant command-line option and corresponding stack.yaml option.
5. Have Stack.Setup use the CompilerVariant in getOSKey.
6. Move the gmp4 (libgmp.so.3) detection code to where the default for CompilerVariant is set.

[borsboom]

And let's leave conditionals in stack.yaml as a separate issue :)

[3noch]

As far as I can tell (which is not very far!) that plan looks good. I'm not crazy about the name PlatformVariant since it seems that we're not dealing with variants of the platform but only of the compiler. If we must combine the two ideas, I wonder if something like data Compiler = CompilerFor Platform (Maybe CompilerVariant) is best. But recall that I can't tell too far.

Are you thinking of making the programmatic detection be a subset of CompilerVariant? The Maybe begs the question "What's the default?" Perhaps we could avoid that confusion by embedding every case:

data CompilerVariant = AvoidLPGL | IntegerSimpleCompiler | IntegerGmpCompiler
                     | OtherCompilerVariant String

(Of course, you could squash some of these into OtherCompilerVariant or reintroduce Maybe, etc. with similar effect.)

If using AvoidLGPL, we could use logic like this

• If GHC version < 7.12 (hopefully!)
  • On Windows? Use integer-simple.
  • On Linux without libgmp? Use integer-simple.
  • On Linux with libgmp? Use integer-gmp.
• Otherwise use linker flag or make the user do it herself

[3noch]

This actually gave me another idea: stack could check for licensing problems in your dependency tree. Perhaps it's a non issue since virtually all cabal packages are BSD3, but it could be quite useful to know if one of them wasn't.

[borsboom]

stack could check for licensing problems in your dependency tree

That is a good idea (it's something we've done at FP Complete in other projects), and definitely deserves an issue of its own so that it doesn't get lost.