Can cabal-install drive MicroHS?

[ulysses4ever]

This is a spinoff from #11341

Has anyone experimented with this? @augustss maybe?

[augustss]

I didn't really try it. I just looked a bit at the code. And then decided to write my own.

[ulysses4ever]

I got you. I understand bootstrapping is high on the list of goals for MicroHS. But independently, I'm curious if we have cabal-install built with GHC (as usual), can it drive mhs. It'd be interesting to try it I think...

[augustss]

I think that would be very interesting to try.

[ulysses4ever]

My first experiment was to cabal init -nm --lib and try to "just build" it, and it exploded quickly:

[nix-shell:~/dev]$ cabal build -w mhs
Warning: this is a debug build of cabal-install with assertions enabled.
Error: [Cabal-8038]
The program 'ghc' version >=7.0.1 is required but the version found at /nix/store/3sg4dss3j9i8ikpzvm1gmrp0vncsm2x3-microhs-0.14.21.0/bin/mhs is version 0.14.21.0

[geekosaur]

I'm surprised it got even that far; there's a fair amount of support that needs to be added to actually drive a compiler.

[ulysses4ever]

after debugging this s bit: although the MHS compiler flavor was added recently, the flavor detection mechanism identifies GHC flavor for MHS and proceeds accordingly. We need to correctly detect the flavor at least.

[geekosaur]

My recollection is that the only thing that was added was recognition in a cabal file. There's no way to detect it, no way to run it, and in particular no way to know what would go into the store or what hashes to use and what modifies those.

[ulysses4ever]

There's no logic to detect a flavor, rather, ghc is the default, and if you don't like it, you're supposed to set it manually with the compiler flag. Funny enough, the docs say that it should be possible to do it via CLI, but I'm getting:

[nix-shell:~/dev]$ cabal build -w mhs --compiler=mhs -v3
Error: cabal: unrecognized 'build' option `--compiler=mhs'

(and same with MHS, all caps). A project file works though. It get's me a bit further but not by much:

[nix-shell:~/dev]$ cabal build -w mhs  -v3
Warning: this is a debug build of cabal-install with assertions enabled.
Searching for curl in path.
Found curl at /run/current-system/sw/bin/curl
Searching for powershell in path.
Cannot find powershell on the path
Searching for wget in path.
Found wget at /run/current-system/sw/bin/wget
Selected http transport implementation: curl
File monitor 'config' changed: file /home/artem/Dev/cabal.project
Project settings changed, reconfiguring...
Reading project file using the fallback parser
creating /home/artem/Dev/dist-newstyle
creating /home/artem/Dev/dist-newstyle/cache
Configuration is affected by cabal.project at '/home/artem/Dev'.
File monitor 'improved-plan' changed: first run
File monitor 'elaborated-plan' changed: first run
File monitor 'compiler' changed: first run
Compiler settings changed, reconfiguring...
Error: [Cabal-3022]
Unknown compiler
CallStack (from HasCallStack):
  dieWithException, called at src/Distribution/Simple/Configure.hs:2528:16 in Cabal-3.17.0.0-inplace:Distribution.Simple.Configure

Configure.hs:2528 is:

-- | Configure the compiler ONLY.
configCompiler
  :: Maybe CompilerFlavor
  -> Maybe FilePath
  -- ^ user-specified @hc@ path (optional)
  -> ProgramDb
  -> Verbosity
  -> IO (Compiler, Platform, ProgramDb)
configCompiler mbFlavor hcPath progdb verbosity = do
  print mbFlavor
  (comp, maybePlatform, programDb) <-
    case mbFlavor of
      Nothing -> dieWithException verbosity UnknownCompilerException
      Just hcFlavor ->
        case hcFlavor of
          GHC -> GHC.configureCompiler verbosity hcPath progdb
          GHCJS -> GHCJS.configureCompiler verbosity hcPath progdb
          UHC -> UHC.configure verbosity hcPath progdb
          _ -> dieWithException verbosity UnknownCompilerException
  return (comp, fromMaybe buildPlatform maybePlatform, programDb)

the next step would be to see if we can implement configure for MHS. Maybe it can be trivial. E.g. for UHC it reads:

-- Cabal/src/Distribution/Simple/UHC.hs:59
configure
  :: Verbosity
  -> Maybe FilePath
  -> ProgramDb
  -> IO (Compiler, Maybe Platform, ProgramDb)
configure verbosity hcPath progdb = do
  (_uhcProg, uhcVersion, progdb') <-
    requireProgramVersion
      verbosity
      uhcProgram
      (orLaterVersion (mkVersion [1, 0, 2]))
      (userMaybeSpecifyPath "uhc" hcPath progdb)

  let comp =
        Compiler
          { compilerId = CompilerId UHC uhcVersion
          , compilerAbiTag = NoAbiTag
          , compilerCompat = []
          , compilerLanguages = uhcLanguages
          , compilerExtensions = uhcLanguageExtensions
          , compilerProperties = Map.empty
          , compilerWiredInUnitIds = Nothing
          }
      compPlatform = Nothing
  return (comp, compPlatform, progdb')

[geekosaur]

Right, that's part of what I was getting at. But after that we have a possibly silent problem where mhs-options may contain things that need a different build hash in the store. Also I think objects aren't OS objects but combinator bytecode, and final link tacks a small bytecode interpreter on the front.

[augustss]

MicroHs has no concept of object files. There are two kinds of build products: 1, a .pkg file which is the result of compiling an entire library from a package. 2, an executable which is the result of compiling a runnable program from a package.
The format of the .pkg file is private and should not concern cabal.

It's true that you can get other outputs, like a combinator file, but that's more for debugging and testing.

Telling mhs to install a package will populate a directory with various files, again, the contents of these is private.

[ulysses4ever]

@augustss where is this documented? do I switch between the two modes explicitly or MHS just figures it out? I've started some experimenting here: #11392

[augustss]

To create an executable mhs MDL -o FILE.
To create a package mhs -PPKG MDL MDL ... -o FILE.
Where MDL are module names, FILE are file names, and PKG a package name.

To install a package mhs -Q FILE, where FILE is produced by mhs -P.

You can run mcabal -v -v to see what it's doing.