path: root/docs/users_guide/8.12.1-notes.rst

.. _release-8-12-1:

Version 8.12.1
==============

The significant changes to the various parts of the compiler are listed in the
following sections.


Highlights
----------

* NCG

  - The linear register allocator saw improvements reducing the number
    of redundant move instructions. Rare edge cases can see double
    digit improvements in runtime for inner loops.

    In the mean this improved runtime by about 0.8%. For details
    see ticket :ghc-ticket:`17823`.

* Pattern-Match Coverage Checking

  - The revamp of the pattern-match coverage checker that started in 8.10 concludes with this release and implements the
    novel `*Lower Your Guards* <https://www.microsoft.com/en-us/research/uploads/prod/2020/03/lyg.pdf>`_ algorithm.
  - Compared to 8.10, end users might notice improvements to "long-distance information": ::

      f True = 1
      f x    = ... case x of { False -> 2; True -> 3 } ...

    GHC is now able to detect the case alt returning 3 as redundant.
  - Some more performance improvements in edge cases.

Full details
------------

Language
~~~~~~~~

* Record field selectors are now given type signatures that preserve the
  user-written order of quantified type variables. Moreover, field selector
  type signatures no longer make inferred type variables available for explicit
  type application. See :ref:`field-selectors-and-type-applications` for more
  details.

  In certain situations, this will constitute a breaking change as this can
  affect :extension:`TypeApplications`. For instance, given the following
  definitions: ::

    {-# LANGUAGE PolyKinds #-}

    newtype P a = MkP { unP :: Proxy a }

    newtype N :: Type -> Type -> Type where
      MkN :: forall b a. { unN :: Either a b } -> N a b

  Previous versions of GHC would give the following types to ``unP`` and
  ``unN``: ::

    unP :: forall k (a :: k). P a -> Proxy a
    unN :: forall a b. N a b -> Either a b

  GHC will now give them the following types instead: ::

    unP :: forall {k} (a :: k). P a -> Proxy a
    unN :: forall b a. N a b -> Either a b

* In obscure scenarios, GHC now rejects programs it previously accepted, but
  with unhelpful types. For example, if (with ``-XPartialTypeSignatures``) you
  were to write ``x :: forall (f :: forall a (b :: a -> Type). b _). f _``, GHC previously
  would have accepted ``x``, but its type would have involved the mysterious ``Any``
  internal type family. Now, GHC rejects, explaining the situation.

* GHC now more faithfully implements the instance-lookup scheme described with
  :extension:`QuantifiedConstraints`. Previous bugs meant that programs like this
  were accepted::

    data T (c :: Type -> Constraint)
    instance (forall h. c h => Functor h) => Functor (T c)
    instance (forall h. c h => Applicative h) => Applicative (T c)

  Note that in the instance declaration for ``Applicative (T c)``, we cannot prove
  ``Functor (T c)``, because the quantified constraint shadows the global instance.
  There is an easy workaround, though: just include ``Functor (T c)`` as an assumption. ::

    instance (forall h. c h => Applicative h, Functor (T c)) => Applicative (T c)

  There is a chance we will tweak the lookup scheme in the future, to make this
  workaround unnecessary.

* GHC now consistently does eager instantiation during type inference.
  As a consequence, visible type application (VTA) now only works when
  the head of the application is:

  * A variable
  * An expression with a type signature

  For example ``(let x = blah in id) @Bool True`` no longer typechecks.
  You should write ``let x = blah in id @Bool True`` instead.

  This change prepares the way for `Quick Look impredicativity
  <https://gitlab.haskell.org/ghc/ghc/issues/18126>`_.

* GHC now implements simplified subsumption, as described in `GHC Proposal #287 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst>`__.
  This change simplifies the type system, and prevents the possibility of GHC
  silently changing the semantics of user programs, but it does mean that some libraries
  may need eta-expansion to typecheck.  More info here: :ref:`simple-subsumption`.

  This change also prepares the way for Quick Look impredicativity.

* GHC now allows users to manually define the specificity of type variable
  binders. By marking a variable with braces ``{tyvar}`` or ``{tyvar :: kind}``,
  it becomes inferred despite appearing in a type signature. This feature
  effectively allows users to choose which variables can or can't be
  instantiated through visible type application. More information can be found
  here: :ref:`Manually-defining-inferred-variables`.

* GADT constructor types now properly adhere to :ref:`forall-or-nothing`. As
  a result, GHC will now reject some GADT constructors that previous versions
  of GHC would accept, such as the following: ::

    data T where
      MkT1 :: (forall a. a -> b -> T)
      MkT2 :: (forall a. a -> T)

  ``MkT1`` and ``MkT2`` are rejected because the lack of an outermost
  ``forall`` triggers implicit quantification, making the explicit ``forall``\ s
  nested. Furthermore, GADT constructors do not permit the use of nested
  ``forall``\ s, as explained in :ref:`formal-gadt-syntax`.

  In addition to rejecting nested ``forall``\ s, GHC is now more stringent about
  rejecting uses of nested *contexts* in GADT constructors. For example, the
  following example, which previous versions of GHC would accept, is now
  rejected: ::

    data U a where
      MkU :: (Show a => U a)

Compiler
~~~~~~~~


GHCi
~~~~

- The ``:script`` command now allows for file names that contain spaces to
  passed as arguments: either by enclosing the file names in double quotes or by
  escaping spaces in file names with a backslash. (:ghc-ticket:`18027`)

Runtime system
~~~~~~~~~~~~~~

- :rts-flag:`-N` without a count now tries to respect the number of processors
  in the process's affinity mask, making GHC's behavior more predictable in
  containerized settings (:ghc-ticket:`14781`).

- Support for Windows Vista has been dropped. GHC-compiled programs now require
  Windows 7 or later.

Template Haskell
~~~~~~~~~~~~~~~~

- Implement the `Overloaded Quotations proposal (#246) <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0246-overloaded-bracket.rst>`_.
  The type of all quotation forms have now been generalised in terms of a
  minimal interface necessary (the ``Quote`` type class) for the
  implementation rather than the overapproximation of the ``Q`` monad.

- Template Haskell quotes now handle fixity declarations in ``let`` and
  ``where`` bindings properly. Previously, such fixity declarations would
  be dropped when quoted due to a Template Haskell bug.

- The ``-XTemplateHaskellQuotes`` extension now allows nested splices as nested
  splices do not lead directly to compile-time evaluation. (Merge request
  `!2288 <https://gitlab.haskell.org/ghc/ghc/-/merge_requests/2288>`_)

Arrow notation
~~~~~~~~~~~~~~

- When combined with :extension:`Arrows`, the :extension:`LambdaCase` extension
  now additionally allows ``\case`` syntax to be used as a command in ``proc``
  notation.

- When combined with :extension:`Arrows`, the effects of the
  :extension:`BlockArguments` extension now also apply to applications of
  arrow control operators in ``(|`` banana brackets ``|)``: ::

    (| untilA (increment -< x + y) do
         within 0.5 -< x
         ... |)

``ghc-prim`` library
~~~~~~~~~~~~~~~~~~~~

- Add a known-key ``cstringLength#`` to ``GHC.CString`` that is eligible
  for constant folding by a built-in rule.

``ghc`` library
~~~~~~~~~~~~~~~

- The type of the ``getAnnotations`` function has changed to better reflect
  the fact that it returns two different kinds of annotations, those on
  names and those on modules: ::

     getAnnotations :: Typeable a
                    => ([Word8] -> a) -> ModGuts
                    -> CoreM (ModuleEnv [a], NameEnv [a])

- The meaning of the ``hs_fixds`` field of ``HsGroup`` has changed slightly.
  It now only contains fixity signatures defined for top-level declarations
  and class methods defined *outside* of the class itself. Previously,
  ``hs_fixds`` would also contain fixity signatures for class methods defined
  *inside* the class, such as the fixity signature for ``m`` in the following
  example: ::

    class C a where
      infixl 4 `m`
      m :: a -> a -> a

  If you wish to attain the previous behavior of ``hs_fixds``, use the new
  ``hsGroupTopLevelFixitySigs`` function, which collects all top-level fixity
  signatures, including those for class methods defined inside classes.

- The ``Exception`` module was boiled down acknowledging the existence of
  the ``exceptions`` dependency. In particular, the ``ExceptionMonad``
  class is not a proper class anymore, but a mere synonym for ``MonadThrow``,
  ``MonadCatch``, ``MonadMask`` (all from ``exceptions``) and ``MonadIO``.
  All of ``g*``-functions from the module (``gtry``, ``gcatch``, etc.) are
  erased, and their ``exceptions``-alternatives are meant to be used in the
  GHC code instead.

``base`` library
~~~~~~~~~~~~~~~~

- ``ForeignPtrContents`` has a new nullary data constructor ``FinalPtr``.
  ``FinalPtr`` is intended for turning a primitive string literal into a
  ``ForeignPtr``.  Unlike ``PlainForeignPtr``, ``FinalPtr`` does not have
  a finalizer. Replacing ``PlainForeignPtr`` that has ``NoFinalizers`` with
  ``FinalPtr`` reduces allocations, reduces the size of compiled binaries,
  and unlocks important Core-to-Core optimizations. ``FinalPtr`` will be used
  in an upcoming ``bytestring`` release to improve the performance of
  ``ByteString`` literals created with ``OverloadedStrings``.

Build system
~~~~~~~~~~~~

Bootstrapping requirements
--------------------------

- GHC now requires a C compiler which supports
  ``__atomic_op_n`` builtins. This raises the requirement for GCC to 4.7.

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/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/libiserv/libiserv.cabal:       Internal compiler 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/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