Language

The language is an extended subset of Haskell 2010.

Differences:

• Kind variables need an explicit forall.
• Extensions are always enabled, with the only exception being CPP.
• Data type contexts are not supported.
• main in the top module given to mhs serves at the program entry point.
• Many things that should be an error (but which are mostly harmless) are not reported.
• Text file I/O always uses UTF8, as does the source code.
• The BangPatterns extension is parsed, but only effective at the a top level let/where.
• More differences that I don't remember right now.

Mutually recursive modules are allowed the same way as with GHC, using .hs-boot files.

Extensions

The following extensions are supported:

• AllowAmbiguousTypes
• BangPatterns
• BinaryLiterals
• BlockArguments
• CApiFFI
• ConstrainedClassMethods
• ConstraintKinds
• CPP
• DefaultSignatures
• DeriveAnyClass
• DeriveDataTypeable
• DeriveFoldable
• DeriveFunctor
• DeriveTraversable
• DerivingStrategies
• DerivingVia
• DisambiguateRecordFields
• DoAndIfThenElse
• DuplicateRecordFields
• EmptyCase
• EmptyDataDecls
• EmptyDataDeriving
• ExistentialQuantification
• ExplicitForAll
• ExplicitNamespaces
• ExtendedDefaultRules
• ExtraInstanceBindings
• FlexibleContexts
• FlexibleInstances
• FunctionalDependencies
• GADTs
• GADTSyntax
• GeneralisedNewtypeDeriving
• ImportQualifiedPost
• ImportShadowing
• IncoherentInstances
• InstanceSigs
• KindSignatures
• LambdaCase
• LiberalTypeSynonyms
• MagicHash
• MultilineStrings
• MultiParamTypeClasses
• MultiWayIf
• NamedDefaults
• NamedFieldPuns
• NegativeLiterals
• NoDatatypeContexts
• NondecreasingIndentation
• NoStarIsType
• NumericUnderscores
• OrPatterns
• OverlappingInstances
• OverloadedRecordDot
• OverloadedRecordUpdate
• OverloadedStrings
• PatternSynonyms
• PolyKinds
• PolymorphicComponents
• PostfixOperators
• QualifiedDo
• RankNTypes
• RecordWildCards
• RecursiveDo (without optimal mdo desugaring)
• RequiredTypeArguments
• ScopedTypeVariables
• StandaloneDeriving
• StandaloneKindSignatures
• TupleSections (only pairs right now)
• TypeApplications
• TypeLits
• TypeOperators
• TypeSynonymInstances
• UndecidableInstances
• UndecidableSuperClasses
• UnicodeSyntax
• ViewPatterns

Standard library modules

This table provides an overview of which standard library modules are report compliant.

Module	Report compliant	Comment
Prelude	yes1	missing catch (as in GHC)
Control.Monad	yes1	fail not in Monad (as in GHC)
Data.Array	yes	part of array-mhs
Data.Bits	yes
Data.Char	no	missing lexLitChar, readLitChar (see commit b7370759bfb726d997ac238123de5ff6d38c4aa5)
Data.Complex	yes1	no datatype context, instance (RealFloat a) => Show a
Data.Int	yes
Data.Ix	yes
Data.List	yes
Data.Maybe	yes
Data.Ratio	yes
Data.Word	yes
Foreign	yes
Foreign.C	yes
Foreign.C.Error	yes
Foreign.C.String	no	missing functions (in particular CWString related)
Foreign.C.Types	no	missing/wrong instances
Foreign.ForeignPtr	no	missing FinalizerEnvPtr, newForeignPtrEnv, addForeignPtrFinalizerEnv. finalizeForeignPtr, unsafeForeignPtrToPtr
Foreign.Marshal	yes
Foreign.Marshal.Alloc	yes
Foreign.Marshal.Array	yes
Foreign.Marshal.Error	yes1	void :: Functor f => f a -> f ()
Foreign.Marshal.Utils	yes
Foreign.Ptr	yes	freeHaskellFunPtr does nothing
Foreign.StablePtr	yes
Foreign.Storable	yes
Numeric	yes
System.Environment	yes
System.Exit	yes
System.IO	no	missing functions2, missing types3, missing IOMode instances
System.IO.Error	yes1	renamed catch, try to catchIOError, tryIOError (as in GHC)

1. not quite, but close enough, see comment

2. hFileSize, hSetFileSize, hIsEOF, hGetPosn, hSetPosn, hSeek, hTell, hIsOpen, hIsClosed, hIsReadable, hIsWritable, hIsSeekable, hSetEcho, hGetEcho, hShow, hWaitForInput, hReady, hPrint

3. HandlePosn, SeekMode