.. _release-9-8-1: Version 9.8.1 ============= The significant changes to the various parts of the compiler are listed in the following sections. See the `migration guide `_ on the GHC Wiki for specific guidance on migrating programs to this release. The :ghc-flag:`LLVM backend <-fllvm>` of this release is to be used with LLVM 11, 12, 13, 14 or 15. Breaking changes ~~~~~~~~~~~~~~~~ - In accordance with GHC proposal `#425 `_ GHC no longer implicitly quantifies over type variables that appear only in the RHS of type and data family instances. This code will no longer work: :: type family F1 a :: k type instance F1 Int = Any :: j -> j Instead you should write:: type instance F1 @(j -> j) Int = Any :: j -> j Or:: type instance forall j . F1 Int = Any :: j -> j - GHC proposal `#475 `_ has been partially implemented. Namely, tuple data types, which were previously represented using a brackets-with-commas syntax form ((), (,), (,,), and so on) have been renamed to common names of the form ``Unit``, ``Tuple2``, ``Tuple3``, and so on, where the number after ``Tuple`` indicates its arity: :: data Unit = () data Tuple2 a b = (a,b) data Tuple3 a b c = (a, b, c) -- and so on, up to Tuple64 For consistency, we also introduce type aliases: :: type Tuple0 = Unit type Tuple1 = Solo The renamed tuple data types and the new type aliases can be found in the ``GHC.Tuple`` module. This renaming does not break existing code that directly uses tuple data types, but it does affect tools and libraries that have access to the data type names, such as ``Generic`` and Template Haskell. - Data types with ``deriving`` clauses now reject inferred instance contexts that mention ``TypeError`` constraints (see :ref:`custom-errors`), such as this one: :: newtype Foo = Foo Int class Bar a where bar :: a instance (TypeError (Text "Boo")) => Bar Foo where bar = undefined newtype Baz = Baz Foo deriving Bar Here, the derived ``Bar`` instance for ``Baz`` would look like this: :: instance TypeError (Text "Boo") => Bar Baz While GHC would accept this before, GHC 9.8 now rejects it, emitting "``Boo``" in the resulting error message. If you really want to derive this instance and defer the error to sites where the instance is used, you must do so manually with :extension:`StandaloneDeriving`, e.g. :: deriving instance TypeError (Text "Boo") => Bar Baz Language ~~~~~~~~ - There is a new extension :extension:`ExtendedLiterals`, which enables sized primitive numeric literals, e.g. ``123#Int8`` is a literal of type ``Int8#``. See the GHC proposal `#451 `_. Derived ``Show`` instances for datatypes containing sized literals (``Int8#``, ``Word8#``, ``Int16#`` etc.) now use the extended literal syntax, per GHC proposal `#596 `_. Furthermore, it is now possible to derive ``Show`` for datatypes containing fields of types ``Int64#`` and ``Word64#``. - GHC Proposal `#425 `_ has been partially implemented. Namely, the ``@k``-binders in type declarations are now permitted:: type T :: forall k. k -> forall j. j -> Type data T @k (a :: k) @(j :: Type) (b :: j) This feature is guarded behind :extension:`TypeAbstractions`. Compiler ~~~~~~~~ - Added a new warning :ghc-flag:`-Wterm-variable-capture` that helps to make code compatible with the future extension ``RequiredTypeArguments``. - Rewrite rules now support a limited form of higher order matching when a pattern variable is applied to distinct locally bound variables, as proposed in `GHC Proposal #555 `_. For example: :: forall f. foo (\x -> f x) Now matches: :: foo (\x -> x*2 + x) - GHC Proposal `#496 `_ has been implemented, allowing ``{..}`` syntax for constructors without fields, for consistency. This is convenient for TH code generation, as you can now uniformly use record wildcards regardless of number of fields. - Specialisation of incoherent instance applications can now be disabled with :ghc-flag:`-fno-specialise-incoherents`. This is necessary as the current specialisation implementation can result in in nondeterministic instance resolution in certain cases, breaking the specification described in the documentation of the :pragma:`INCOHERENT` pragma. See :ghc-ticket:`22448` for further details. - Fix a bug in TemplateHaskell evaluation causing excessive calls to ``setNumCapabilities`` when :ghc-flag:`-j[⟨n⟩]` is greater than :rts-flag:`-N`. See :ghc-ticket:`23049`. - The ``-Wno-⟨wflag⟩``, ``-Werror=⟨wflag⟩`` and ``-Wwarn=⟨wflag⟩`` options are now defined systematically for all warning groups (for example, ``-Wno-default``, ``-Werror=unused-binds`` and ``-Wwarn=all`` are now accepted). See :ref:`options-sanity`. - ``WARNING`` pragmas may now be annotated with a category, following `GHC proposal #541 `_, in which case they are controlled with new ``-Wx-⟨category⟩`` flags rather than :ghc-flag:`-Wdeprecations`. A new warning group :ghc-flag:`-Wextended-warnings` includes all such warnings regardless of category. See :ref:`warning-deprecated-pragma`. - GHC is now better at disambiguating record updates in the presence of duplicate record fields. The following program is now accepted :: {-# LANGUAGE DuplicateRecordFields #-} data R = MkR1 { foo :: Int } | MkR2 { bar :: Int } data S = MkS { foo :: Int, bar :: Int } blah x = x { foo = 5, bar = 6 } The point is that only the type S has a constructor with both fields ``foo`` and ``bar``, so this record update is unambiguous. - GHC Proposal `#540 `_ has been implemented. This adds the :ghc-flag:`-jsem` flag, which instructs GHC to act as a jobserver client. This enables multiple GHC processes running at once to share system resources with each other, communicating via the system semaphore specified by the flag argument. Complementary support for this feature in ``cabal-install`` will come soon. - GHC Proposal `#433 `_ has been implemented. This adds the class ``Unsatisfiable :: ErrorMessage -> Constraint`` to the :base-ref:`GHC.TypeError.` module. Constraints of the form ``Unsatisfiable msg`` provide a mechanism for custom type errors that reports the errors in a more predictable behaviour than ``TypeError``, as these constraints are handled purely during constraint solving. For example: :: instance Unsatisfiable (Text "There is no Eq instance for functions") => Eq (a -> b) where (==) = unsatisfiable This allows errors to be reported when users use the instance, even when type errors are being deferred. - GHC now deals with "insoluble Givens" in a consistent way. For example: :: k :: (Int ~ Bool) => Int -> Bool k x = x GHC used to accept the contradictory ``Int~Bool`` in the type signature, but reject the ``Int~Bool`` constraint that arises from typechecking the definition itself. Now it accepts both. More details in :ghc-ticket:`23413`, which gives examples of the previous inconsistency. GHC now implements the "PermissivePlan" described in that ticket. - The :ghc-flag:`-ddump-spec` flag has been split into :ghc-flag:`-ddump-spec` and :ghc-flag:`-ddump-spec-constr`, allowing only output from the typeclass specialiser or data-constructor specialiser to be dumped if desired. - The compiler may now be configured to compress the debugging information included in :ghc-flag:`-finfo-table-map` enabled binaries. To do so, one must build GHC from source (see `here `_ for directions) and supply the ``--enable-ipe-data-compression`` flag to the ``configure`` script. **Note**: This feature requires that the machine building GHC has `libzstd `_ version 1.4.0 or greater installed. The compression library `libzstd` may optionally be statically linked in the resulting compiler (on non-darwin machines) using the ``--enable-static-libzstd`` configure flag. In a test compiling GHC itself, the size of the :ghc-flag:`-finfo-table-map` enabled build results was reduced by over 20% when compression was enabled. - GHC Proposal `#134 `_ has been implemented. This makes it possible to deprecate certain names exported from a module, without deprecating the name itself. You can check the full specification of the feature at :ref:`warning-deprecated-pragma`. For example :: module X ( {-# DEPRECATE D(D1) "D1 will not be exposed in a version 0.2 and later" #-} D(D1, D2) ) where data D = D1 | D2 This allows for changing the structure of a library without immediately breaking user code, but instead being able to warn the user that a change in the library interface will occur in the future. - Guard polymorphic specialisation behind the flag :ghc-flag:`-fpolymorphic-specialisation`. This optimisation has led to a number of incorrect runtime result bugs, so we are disabling it by default for now whilst we consider more carefully an appropiate fix. (See :ghc-ticket:`23469`, :ghc-ticket:`23109`, :ghc-ticket:`21229`, :ghc-ticket:`23445`) - The warning about incompatible command line flags can now be controlled with the :ghc-flag:`-Winconsistent-flags`. In particular this allows you to silence a warning when using optimisation flags with :ghc-flag:`--interactive` mode. GHCi ~~~~ - The deprecated ``:ctags`` and ``:etags`` GHCi commands have been removed. See this `wiki page `_ if you want to add a macro to recover similar functionality. Runtime system ~~~~~~~~~~~~~~ - On POSIX systems that support timerfd, RTS shutdown no longer has to wait for the next RTS 'tick' to occur before continuing the shutdown process. See :ghc-ticket:`22692`. ``base`` library ~~~~~~~~~~~~~~~~ Note that this is not an exhaustive list of changes in ``base``. See the ``base`` changelog for full details. - Added ``{-# WARNING in "x-partial" #-}`` to ``Data.List.{head,tail}``. - :base-ref:`Data.Tuple` now exports ``getSolo :: Solo a -> a``. - Updated to `Unicode 15.1.0 `_. - Fixed exponent overflow/underflow bugs in the ``Read`` instances for ``Float`` and ``Double`` (`CLC proposal #192 `_) ``ghc-prim`` library ~~~~~~~~~~~~~~~~~~~~ - Primitive pointer comparison functions are now levity-polymorphic, e.g. :: sameArray# :: forall {l} (a :: TYPE (BoxedRep l)). Array# a -> Array# a -> Int# This change affects the following functions: - ``sameArray#``, ``sameMutableArray#``, - ``sameSmallArray#``, ``sameSmallMutableArray#``, - ``sameMutVar#``, ``sameTVar#``, ``sameMVar#`` - ``sameIOPort#``, ``eqStableName#``. - New primops for fused multiply-add operations. These primops combine a multiplication and an addition, compiling to a single instruction when the :ghc-flag:`-mfma` flag is enabled and the architecture supports it. The new primops are ``fmaddFloat#, fmsubFloat#, fnmaddFloat#, fnmsubFloat# :: Float# -> Float# -> Float# -> Float#`` and ``fmaddDouble#, fmsubDouble#, fnmaddDouble#, fnmsubDouble# :: Double# -> Double# -> Double# -> Double#``. These implement the following operations, while performing one single rounding at the end, leading to a more accurate result: - ``fmaddFloat# x y z``, ``fmaddDouble# x y z`` compute ``x * y + z``. - ``fmsubFloat# x y z``, ``fmsubDouble# x y z`` compute ``x * y - z``. - ``fnmaddFloat# x y z``, ``fnmaddDouble# x y z`` compute ``- x * y + z``. - ``fnmsubFloat# x y z``, ``fnmsubDouble# x y z`` compute ``- x * y - z``. Warning: on unsupported architectures, the software emulation provided by the fallback to the C standard library is not guaranteed to be IEEE-compliant. ``ghc`` library ~~~~~~~~~~~~~~~ - The ``RecordUpd`` constructor of ``HsExpr`` now takes an ``HsRecUpdFields`` instead of ``Either [LHsRecUpdField p] [LHsRecUpdProj p]``. Instead of ``Left ..``, use the constructor ``RegularRecUpdFields``, and instead of ``Right ..``, use the constructor ``OverloadedRecUpdFields``. - The ``loadWithCache`` function now takes an extra argument which allows API users to embed GHC diagnostics in their own diagnostic type before they are printed. This allows how messages are rendered and explained to users to be modified. We use this functionality in GHCi to modify how some messages are displayed. - The extensions fields of constructors of ``IE`` now take ``Maybe (WarningTxt p)`` in ``GhcPs`` and ``GhcRn`` variants of the Syntax Tree. This represents the warning assigned to a certain export item, which is used for :pragma:`deprecated exports `. ``template-haskell`` library ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Record fields now belong to separate ``NameSpace``\ s, keyed by the parent of the record field. This is the name of the first constructor of the parent type, even if this constructor does not have the field in question. This change enables :extension:`TemplateHaskell` support for :extension:`DuplicateRecordFields`. ``text`` library ~~~~~~~~~~~~~~~~ The version of the ``text`` library included changes ``Data.Text.Array.Array`` to be a type synonym of ``Data.Array.Byte.ByteArray``. While its former data constructor, ``ByteArray``, has been replaced with a pattern synonym, it cannot be imported as bundled with the type constructor. Consequently, imports like: :: import Data.Text.Array (Array(..)) will need to avoid using a bundled import (e.g. by qualification): :: import Data.Text.Array as A Included libraries ~~~~~~~~~~~~~~~~~~ The package database provided with this distribution also contains a number of packages other than GHC itself. See the changelogs provided with these packages for further change information. .. ghc-package-list:: libraries/array/array.cabal: Dependency of ``ghc`` library libraries/base/base.cabal: Core library libraries/binary/binary.cabal: Dependency of ``ghc`` library libraries/bytestring/bytestring.cabal: Dependency of ``ghc`` library libraries/Cabal/Cabal/Cabal.cabal: Dependency of ``ghc-pkg`` utility libraries/Cabal/Cabal-syntax/Cabal-syntax.cabal: Dependency of ``ghc-pkg`` utility libraries/containers/containers/containers.cabal: Dependency of ``ghc`` library libraries/deepseq/deepseq.cabal: Dependency of ``ghc`` library libraries/directory/directory.cabal: Dependency of ``ghc`` library libraries/exceptions/exceptions.cabal: Dependency of ``ghc`` and ``haskeline`` library libraries/filepath/filepath.cabal: Dependency of ``ghc`` library compiler/ghc.cabal: The compiler itself libraries/ghci/ghci.cabal: The REPL interface libraries/ghc-boot/ghc-boot.cabal: Internal compiler library libraries/ghc-boot-th/ghc-boot-th.cabal: Internal compiler library libraries/ghc-compact/ghc-compact.cabal: Core library libraries/ghc-heap/ghc-heap.cabal: GHC heap-walking library libraries/ghc-prim/ghc-prim.cabal: Core library libraries/haskeline/haskeline.cabal: Dependency of ``ghci`` executable libraries/hpc/hpc.cabal: Dependency of ``hpc`` executable libraries/integer-gmp/integer-gmp.cabal: Core library libraries/mtl/mtl.cabal: Dependency of ``Cabal`` library libraries/parsec/parsec.cabal: Dependency of ``Cabal`` library libraries/pretty/pretty.cabal: Dependency of ``ghc`` library libraries/process/process.cabal: Dependency of ``ghc`` library libraries/semaphore-compat/semaphore-compat.cabal: Dependency of ``ghc`` library libraries/stm/stm.cabal: Dependency of ``haskeline`` library libraries/template-haskell/template-haskell.cabal: Core library libraries/terminfo/terminfo.cabal: Dependency of ``haskeline`` library libraries/text/text.cabal: Dependency of ``Cabal`` library libraries/time/time.cabal: Dependency of ``ghc`` library libraries/transformers/transformers.cabal: Dependency of ``ghc`` library libraries/unix/unix.cabal: Dependency of ``ghc`` library libraries/Win32/Win32.cabal: Dependency of ``ghc`` library libraries/xhtml/xhtml.cabal: Dependency of ``haddock`` executable