MicroHS support

Cabal/Cabal.cabal

@@ -163,6 +163,7 @@ library
     Distribution.Simple.Test.LibV09
     Distribution.Simple.Test.Log
     Distribution.Simple.UHC
+    Distribution.Simple.MHS
     Distribution.Simple.UserHooks
     Distribution.Simple.SetupHooks.Errors
     Distribution.Simple.SetupHooks.Internal

Cabal/src/Distribution/Simple/Build.hs

@@ -73,6 +73,7 @@ import Distribution.Package
 import qualified Distribution.Simple.GHC as GHC
 import qualified Distribution.Simple.GHCJS as GHCJS
 import qualified Distribution.Simple.PackageIndex as Index
+import qualified Distribution.Simple.MHS as MHS
 import qualified Distribution.Simple.UHC as UHC
 
 import Distribution.Simple.Build.Macros (generateCabalMacrosHeader)
@@ -316,6 +317,7 @@ dumpBuildInfo verbosity distPref dumpBuildInfoFlag pkg_descr lbi flags = do
     flavorToProgram GHCJS = Just ghcjsProgram
     flavorToProgram UHC = Just uhcProgram
     flavorToProgram JHC = Just jhcProgram
+    flavorToProgram MHS = Just mhsProgram
     flavorToProgram _ = Nothing
 
 repl
@@ -974,6 +976,7 @@ buildLib flags numJobs pkg_descr lbi lib clbi =
         GHC -> GHC.buildLib flags numJobs pkg_descr lbi lib clbi
         GHCJS -> GHCJS.buildLib verbosity numJobs pkg_descr lbi lib clbi
         UHC -> UHC.buildLib verbosity pkg_descr lbi lib clbi
+        MHS -> MHS.buildLib verbosity pkg_descr lbi lib clbi
         _ -> dieWithException verbosity BuildingNotSupportedWithCompiler
 
 -- | Build a foreign library
@@ -1006,6 +1009,7 @@ buildExe verbosity numJobs pkg_descr lbi exe clbi =
     GHC -> GHC.buildExe verbosity numJobs pkg_descr lbi exe clbi
     GHCJS -> GHCJS.buildExe verbosity numJobs pkg_descr lbi exe clbi
     UHC -> UHC.buildExe verbosity pkg_descr lbi exe clbi
+    MHS -> MHS.buildExe verbosity pkg_descr lbi exe clbi
     _ -> dieWithException verbosity BuildingNotSupportedWithCompiler
 
 replLib

Cabal/src/Distribution/Simple/Configure.hs

@@ -123,6 +123,7 @@ import Distribution.Version
 import qualified Distribution.Simple.GHC as GHC
 import qualified Distribution.Simple.GHCJS as GHCJS
 import qualified Distribution.Simple.UHC as UHC
+import qualified Distribution.Simple.MHS as MHS
 
 import Control.Exception
   ( try
@@ -2071,6 +2072,7 @@ getInstalledPackages verbosity comp mbWorkDir packageDBs progdb = do
     GHC -> GHC.getInstalledPackages verbosity comp mbWorkDir packageDBs' progdb
     GHCJS -> GHCJS.getInstalledPackages verbosity mbWorkDir packageDBs' progdb
     UHC -> UHC.getInstalledPackages verbosity comp mbWorkDir packageDBs' progdb
+    MHS -> MHS.getInstalledPackages verbosity comp mbWorkDir packageDBs' progdb
     flv ->
       dieWithException verbosity $ HowToFindInstalledPackages flv
   where
@@ -2504,6 +2506,7 @@ configCompilerEx (Just hcFlavor) hcPath hcPkg progdb verbosity = do
     GHC -> GHC.configure verbosity hcPath hcPkg progdb
     GHCJS -> GHCJS.configure verbosity hcPath hcPkg progdb
     UHC -> UHC.configure verbosity hcPath progdb
+    MHS -> MHS.configure verbosity hcPath progdb
     _ -> dieWithException verbosity UnknownCompilerException
   return (comp, fromMaybe buildPlatform maybePlatform, programDb)
 
@@ -2524,6 +2527,7 @@ configCompiler mbFlavor hcPath progdb verbosity = do
           GHC -> GHC.configureCompiler verbosity hcPath progdb
           GHCJS -> GHCJS.configureCompiler verbosity hcPath progdb
           UHC -> UHC.configure verbosity hcPath progdb
+          MHS -> MHS.configure verbosity hcPath progdb
           _ -> dieWithException verbosity UnknownCompilerException
   return (comp, fromMaybe buildPlatform maybePlatform, programDb)
 

Cabal/src/Distribution/Simple/MHS.hs

@@ -0,0 +1,156 @@
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE DataKinds #-}
+
+-----------------------------------------------------------------------------
+
+-- |
+-- Module      :  Distribution.Simple.MHS
+-- Copyright   :  Cabal Devs 2026
+-- License     :  BSD3
+--
+-- Maintainer  :  cabal-devel@haskell.org
+-- Portability :  portable
+--
+-- This module contains most of the MHS-specific code for configuring, building
+-- and installing packages.
+
+module Distribution.Simple.MHS
+  ( configure
+  , getInstalledPackages
+  , buildLib
+  , buildExe
+  , installLib
+  , registerPackage
+  ) where
+
+import Distribution.Compat.Prelude
+import Prelude ()
+
+import Distribution.InstalledPackageInfo
+import Distribution.PackageDescription
+import Distribution.Pretty
+import Distribution.Simple.Compiler
+import Distribution.Simple.LocalBuildInfo
+import Distribution.Simple.Program
+import Distribution.Simple.Utils
+import Distribution.Utils.Path
+import Distribution.Verbosity
+import qualified Data.Map as Map (empty)
+import Distribution.System (Platform)
+import Distribution.Version (mkVersion, orLaterVersion)
+import Language.Haskell.Extension
+import Distribution.Simple.PackageIndex (InstalledPackageIndex)
+import qualified Distribution.Simple.PackageIndex as PackageIndex
+
+-- -----------------------------------------------------------------------------
+-- Configuring
+
+configure
+  :: Verbosity
+  -> Maybe FilePath
+  -> ProgramDb
+  -> IO (Compiler, Maybe Platform, ProgramDb)
+configure verbosity hcPath progdb = do
+  (_mhsProg, mhsVersion, progdb') <-
+    requireProgramVersion
+      verbosity
+      mhsProgram
+      (orLaterVersion (mkVersion [0, 10, 0]))
+      (userMaybeSpecifyPath "mhs" hcPath progdb)
+
+  let comp =
+        Compiler
+          { compilerId = CompilerId MHS mhsVersion
+          , compilerAbiTag = NoAbiTag
+          , compilerCompat = []
+          , compilerLanguages = mhsLanguages
+          , compilerExtensions = mhsLanguageExtensions
+          , compilerProperties = Map.empty
+          , compilerWiredInUnitIds = Nothing
+          }
+      compPlatform = Nothing
+  info verbosity $ "MHS configured with languages: " ++ show (map fst mhsLanguages)
+  return (comp, compPlatform, progdb')
+
+mhsLanguages :: [(Language, CompilerFlag)]
+mhsLanguages = [(Haskell2010, ""), (Haskell98, "")] -- MHS docs say it supports 2010 but 98 should be fine too
+
+mhsLanguageExtensions :: [(Extension, Maybe CompilerFlag)]
+mhsLanguageExtensions =
+  [
+  -- TODO: use the list from the MHS documentation
+  ]
+
+getInstalledPackages
+  :: Verbosity
+  -> Compiler
+  -> Maybe (SymbolicPath CWD (Dir from))
+  -> PackageDBStackX (SymbolicPath from (Dir PkgDB))
+  -> ProgramDb
+  -> IO InstalledPackageIndex
+getInstalledPackages _verbosity _comp _mbWorkDir _packagedbs _progdb =
+  return (PackageIndex.fromList [])
+
+-- -----------------------------------------------------------------------------
+-- Building
+
+buildLib
+  :: Verbosity
+  -> PackageDescription
+  -> LocalBuildInfo
+  -> Library
+  -> ComponentLocalBuildInfo
+  -> IO ()
+buildLib _verbosity _pkg_descr _lbi _lib _clbi = return ()
+
+buildExe
+  :: Verbosity
+  -> PackageDescription
+  -> LocalBuildInfo
+  -> Executable
+  -> ComponentLocalBuildInfo
+  -> IO ()
+buildExe verbosity _pkg_descr lbi exe _clbi = do
+  let runMhsProg = runDbProgramCwd verbosity (mbWorkDirLBI lbi) mhsProgram (withPrograms lbi)
+
+  let mbWorkDir = mbWorkDirLBI lbi
+  srcMainPath <- findFileCwd verbosity mbWorkDir (hsSourceDirs $ buildInfo exe) (modulePath exe)
+
+  let odir = buildDir lbi
+      u = interpretSymbolicPathCWD
+
+      mhsArgs = 
+           ["-i" ++ u d | d <- hsSourceDirs (buildInfo exe)]
+        ++ ["-o", u $ odir </> makeRelativePathEx (prettyShow (exeName exe))]
+        ++ [u srcMainPath]
+
+  runMhsProg mhsArgs
+
+-- -----------------------------------------------------------------------------
+-- Installation
+
+installLib
+  :: Verbosity
+  -> LocalBuildInfo
+  -> FilePath
+  -> FilePath
+  -> FilePath
+  -> PackageDescription
+  -> Library
+  -> ComponentLocalBuildInfo
+  -> IO ()
+installLib _verbosity _lbi _targetDir _dynlibTargetDir _builtDir _pkg _library _clbi = return ()
+
+-- -----------------------------------------------------------------------------
+-- Registering
+
+registerPackage
+  :: Verbosity
+  -> Maybe (SymbolicPath CWD (Dir from))
+  -> Compiler
+  -> ProgramDb
+  -> PackageDBStackS from
+  -> InstalledPackageInfo
+  -> IO ()
+registerPackage _verbosity _mbWorkDir _comp _progdb _packageDbs _installedPkgInfo = return ()

Cabal/src/Distribution/Simple/Program.hs

@@ -113,6 +113,7 @@ module Distribution.Simple.Program
   , ghcjsPkgProgram
   , jhcProgram
   , uhcProgram
+  , mhsProgram
   , gccProgram
   , gppProgram
   , arProgram

Cabal/src/Distribution/Simple/Program/Builtin.hs

@@ -22,6 +22,7 @@ module Distribution.Simple.Program.Builtin
   , ghcjsPkgProgram
   , jhcProgram
   , uhcProgram
+  , mhsProgram
   , gccProgram
   , gppProgram
   , arProgram
@@ -76,6 +77,7 @@ builtinPrograms =
   , ghcjsPkgProgram
   , jhcProgram
   , uhcProgram
+  , mhsProgram
   , hpcProgram
   , -- preprocessors
     hscolourProgram
@@ -196,6 +198,12 @@ uhcProgram =
     { programFindVersion = findProgramVersion "--version-dotted" id
     }
 
+mhsProgram :: Program
+mhsProgram =
+  (simpleProgram "mhs")
+    { programFindVersion = findProgramVersion "--numeric-version" id
+    }
+
 hpcProgram :: Program
 hpcProgram =
   (simpleProgram "hpc")