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.. _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 #17823.
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 avaiable 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 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`.
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. (#18027)
Runtime system
~~~~~~~~~~~~~~
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
~~~~~~~~~~~~
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
|
