.. _release-8-4-1:
Release notes for version 8.4.1
===============================
The significant changes to the various parts of the compiler are listed in the
following sections. There have also been numerous bug fixes and performance
improvements over the 8.2.1 release.
.. note::
This compiling this release requires GCC 4.7 or newer due to
:ghc-ticket:`14244`.
Highlights
----------
The highlights, since the 8.2.1 release, are:
- GHC is now capable of deriving more instances
- More refinement of the :ghc-flag:`-XTypeInType` story and improvements in type
error messages.
- Further improvements in code generation
- Incorporation of library changes including the Phase 2 of the Semigroup-Monoid
Proposal
- A variety of Windows compatibility improvements
- Many, many bug fixes.
Full details
------------
Language
~~~~~~~~
- Data families have been generalised a bit: a data family declaration can now
end with a kind variable ``k`` instead of ``Type``. Additionally, data/newtype
instance no longer need to list all the patterns of the family if they don't
wish to; this is quite like how regular datatypes with a kind signature can omit
some type variables.
- There are now fewer restrictions regarding whether kind variables can appear
on the right-hand sides of type and data family instances. Before, there was
a strict requirements that all kind variables on the RHS had to be explicitly
bound by type patterns on the LHS. Now, kind variables can be *implicitly*
bound, which allows constructions like these: ::
data family Nat :: k -> k -> *
-- k is implicitly bound by an invisible kind pattern
newtype instance Nat :: (k -> *) -> (k -> *) -> * where
Nat :: (forall xx. f xx -> g xx) -> Nat f g
class Funct f where
type Codomain f :: *
instance Funct ('KProxy :: KProxy o) where
-- o is implicitly bound by the kind signature
-- of the LHS type pattern ('KProxy)
type Codomain 'KProxy = NatTr (Proxy :: o -> *)
- Implicitly bidirectional pattern synonyms no longer allow bang patterns
(``!``) or irrefutable patterns (``~``) on the right-hand side. Previously,
this was allowed, although the bang patterns and irrefutable patterns would
be silently ignored when used in an expression context. This is now a proper
error, and explicitly bidirectional pattern synonyms should be used in their
stead. That is, instead of using this (which is an error): ::
data StrictJust a = Just !a
Use this: ::
data StrictJust a <- Just !a where
StrictJust !a = Just a
- GADTs with kind-polymorphic type arguments now require :ghc-flag:`-XTypeInType`.
For instance, consider the following, ::
data G :: k -> * where
GInt :: G Int
GMaybe :: G Maybe
In previous releases this would compile with :ghc-flag:`-XPolyKinds` alone due
to bug :ghc-ticket:`13391`. As of GHC 8.4, however, this requires
:ghc-flag:`-XTypeInType`. Note that since GADT kind signatures aren't generalized,
this will also require that you provide a :ref:`CUSK
<complete-kind-signatures>` by explicitly quantifying over the kind argument,
``k``, ::
data G :: forall k. k -> * where
GInt :: G Int
GMaybe :: G Maybe
- The order in which type variables are quantified in GADT constructor type
signatures has changed. Before, if you had ``MkT`` as below: ::
data T a where
MkT :: forall b a. b -> T a
Then the type of ``MkT`` would (counterintuitively) be
``forall a b. b -> T a``! Now, GHC quantifies the type variables in the
order that the users writes them, so the type of ``MkT`` is now
``forall b a. b -> T a`` (this matters for :ghc-flag:`-XTypeApplications`).
- The new :ghc-flag:`-XEmptyDataDeriving` extension allows deriving ``Eq``,
``Ord``, ``Read``, and ``Show`` instances directly for empty data types, as
in ``data Empty deriving Eq``. (Previously, this would require the use of
:ghc-flag:`-XStandaloneDeriving` to accomplish.)
One can also now derive ``Data`` instances directly for empty data types (as
in ``data Empty deriving Data``) without needing to use
:ghc-flag:`-XStandaloneDeriving`. However, since already requires a GHC
extension (:ghc-flag:`-XDeriveDataTypeable`), one does not need to enable
:ghc-flag:`-XEmptyDataDeriving` to do so. This also goes for other classes
which require extensions to derive, such as :ghc-flag:`-XDeriveFunctor`.
- Hexadecimal floating point literals (e.g. ``0x0.1p4``), enabled with
:ghc-flag:`-XHexFloatLiterals`. See
:ref:`Hexadecimal floating point literals <hex-float-literals>`
for the full details.
Compiler
~~~~~~~~
- LLVM code generator (e.g. :ghc-flag:`-fllvm`) compatible with LLVM releases in
the |llvm-version| series.
- Add warning flag :ghc-flag:`-Wmissing-export-lists` which causes the type
checker to warn when a module does not include an explicit export list.
- The ``configure`` script now no longer accepts ``--with-TOOL`` flags (e.g.
``--with-nm``, ``--with-ld``, etc.). Instead, these are taken from environment
variables, as is typical in ``autoconf`` scripts. For instance,
``./configure --with-nm=/usr/local/bin/nm`` turns into
``./configure NM=/usr/local/bin/nm``.
- Derived ``Functor``, ``Foldable``, and ``Traversable`` instances are now
optimized when their last type parameters have phantom roles.
Specifically, ::
fmap _ = coerce
traverse _ x = pure (coerce x)
foldMap _ _ = mempty
These definitions of ``foldMap`` and ``traverse`` are lazier than the ones we
would otherwise derive, as they may produce results without inspecting their
arguments at all.
See also :ref:`deriving-functor`, :ref:`deriving-foldable`, and
:ref:`deriving-traversable`.
- Derived instances for empty data types are now substantially different
than before. Here is an overview of what has changed. These examples will
use a running example of ``data Empty a`` to describe what happens when an
instance is derived for ``Empty``:
- Derived ``Eq`` and ``Ord`` instances would previously emit code that used
``error``: ::
instance Eq (Empty a) where
(==) = error "Void =="
instance Ord (Empty a) where
compare = error "Void compare"
Now, they emit code that uses maximally defined, lazier semantics: ::
instance Eq (Empty a) where
_ == _ = True
instance Ord (Empty a) where
compare _ _ = EQ
- Derived ``Read`` instances would previous emit code that used
``parens``: ::
instance Read (Empty a) where
readPrec = parens pfail
But ``parens`` forces parts of the parsed string that it doesn't need to.
Now, the derived instance will not use ``parens`` (that it, parsing
``Empty`` will always fail, without reading *any* input): ::
instance Read (Empty a) where
readPrec = pfail
- Derived ``Show`` instances would previously emit code that used
``error``: ::
instance Show (Empty a) where
showsPrec = error "Void showsPrec"
Now, they emit code that inspects the argument. That is, if the argument
diverges, then showing it will also diverge: ::
instance Show (Empty a) where
showsPrec _ x = case x of {}
- Derived ``Functor``, ``Foldable``, ``Traversable``, ``Generic``,
``Generic1``, ``Lift``, and ``Data`` instances previously emitted code that
used ``error``: ::
instance Functor Empty where
fmap = error "Void fmap"
instance Foldable Empty where
foldMap = error "Void foldMap"
instance Traversable Empty where
traverse = error "Void traverse"
instance Generic (Empty a) where
from = M1 (error "No generic representation for empty datatype Empty")
to (M1 _) = error "No values for empty datatype Empty"
-- Similarly for Generic1
instance Lift (Empty a) where
lift _ = error "Can't lift value of empty datatype Empty"
instance Data a => Data (Empty a) where
gfoldl _ _ _ = error "Void gfoldl"
toConstr _ = error "Void toConstr"
...
Now, derived ``Functor``, ``Traversable, ``Generic``, ``Generic1``,
``Lift``, and ``Data`` instances emit code which inspects their
arguments: ::
instance Functor Empty where
fmap _ x = case x of {}
instance Traversable Empty where
traverse _ x = pure (case x of {})
instance Generic (Empty a) where
from x = M1 (case x of {})
to (M1 x) = case x of {}
-- Similarly for Generic1
instance Lift (Empty a) where
lift x = pure (case x of {})
instance Data a => Data (Empty a) where
gfoldl _ x = case x of {}
toConstr x = case x of {}
...
Derived ``Foldable`` instances now are maximally lazy: ::
instance Foldable Empty where
foldMap _ _ = mempty
- Derived ``Foldable`` instances now derive custom definitions for ``null``
instead of using the default one. This leads to asymptotically better
performance for recursive types not shaped like cons-lists, and allows ``null``
to terminate for more (but not all) infinitely large structures.
- Configure on Windows now supports the ``--enable-distro-toolchain``
``configure`` flag, which can be used to build a GHC using compilers on your
``PATH`` instead of using the bundled bindist. See :ghc-ticket:`13792`
- GHC now enables :ghc-flag:`-fllvm-pass-vectors-in-regs` by default. This means
that GHC will now use native vector registers to pass vector arguments across
function calls.
- The optional ``instance`` keyword is now usable in type family instance
declarations. See :ghc-ticket:`13747`
- Lots of other bugs. See `Trac <https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.4.1&col=id&col=summary&col=status&col=type&col=priority&col=milestone&col=component&order=priority>`_
for a complete list.
- New flags :ghc-flag:`-fignore-optim-changes` and
:ghc-flag:`-fignore-hpc-changes` allow GHC to reuse previously compiled
modules even if they were compiled with different optimisation or HPC
flags. These options are enabled by default by :ghc-flag:`--interactive`.
See :ghc-ticket:`13604`
Runtime system
~~~~~~~~~~~~~~
- Function ``hs_add_root()`` was removed. It was a no-op since GHC-7.2.1
where module initialisation stopped requiring a call to ``hs_add_root()``.
- Proper import library support added to GHC which can handle all of the libraries produced
by ``dlltool``. The limitation of them needing to be named with the suffix
``.dll.a`` is also removed. See :ghc-ticket:`13606`, :ghc-ticket:`12499`,
:ghc-ticket:`12498`
- The GHCi runtime linker on Windows now supports the ``big-obj`` file format.
- The runtime system's :ref:`native stack backtrace <backtrace-signal>` support
on POSIX platforms is now triggered by ``SIGQUIT`` instead of ``SIGUSR2`` as
it was in previous releases. This change is to bring GHC's behavior into
compliance with the model set by the most Java virtual machine
implementations.
- The GHC runtime on Windows now uses Continue handlers instead of Vectorized
handlers to trap exceptions. This change gives other exception handlers a chance
to handle the exception before the runtime does. Furthermore The RTS flag
:rts-flag:`--install-seh-handlers=<yes|no>` Can be used on Wndows to
completely disable the runtime's handling of exceptions. See
:ghc-ticket:`13911`, :ghc-ticket:`12110`.
- The GHC runtime on Windows can now generate crash dumps on unhandled exceptions
using the RTS flag :rts-flag:`--generate-crash-dumps`.
- The GHCi runtime linker now avoid calling GCC to find libraries as much as possible by caching
the list of search directories of GCC and querying the file system directly. This results in
much better performance, especially on Windows.
- The GHC runtime on Windows can now generate stack traces on unhandled exceptions.
When running in GHCi more information is displayed about the symbols if available.
This behavior can be controlled with the RTS flag `--generate-stack-traces=<yes|no>`.
Template Haskell
~~~~~~~~~~~~~~~~
- Template Haskell now reifies data types with GADT syntax accurately.
Previously, TH used heuristics to determine whether a data type
should be reified using GADT syntax, which could lead to incorrect results,
such as ``data T1 a = (a ~ Int) => MkT1`` being reified as a GADT and
``data T2 a where MkT2 :: Show a => T2 a`` *not* being reified as a GADT.
In addition, reified GADT constructors now more accurately track the order in
which users write type variables. Before, if you reified ``MkT`` as below: ::
data T a where
MkT :: forall b a. b -> T a
Then the reified type signature of ``MkT`` would have been headed by
``ForallC [PlainTV a, PlainTV b]``. Now, reifying ``MkT`` will give a type
headed by ``ForallC [PlainTV b, PlainTV a]``, as one would expect.
- ``Language.Haskell.TH.FamFlavour``, which was deprecated in GHC 8.2,
has been removed.
``base`` library
~~~~~~~~~~~~~~~~
- Blank strings can now be used as values for environment variables using the
``System.Environment.Blank`` module. See :ghc-ticket:`12494`
- ``Data.Type.Equality.==`` is now a closed type family. It works for all kinds
out of the box. Any modules that previously declared instances of this family
will need to remove them. Whereas the previous definition was somewhat ad
hoc, the behavior is now completely uniform. As a result, some applications
that used to reduce no longer do, and conversely. Most notably, ``(==)`` no
longer treats the ``*``, ``j -> k``, or ``()`` kinds specially; equality is
tested structurally in all cases.
Build system
~~~~~~~~~~~~
- ``dll-split`` has been removed and replaced with an automatic partitioning utility ``gen-dll``.
This utility can transparently split and compile any DLLs that require this. Note that the ``rts`` and
``base`` can not be split at this point because of the mutual recursion between ``base`` and ``rts``.
There is currently no explicit dependency between the two in the build system and such there is no way
to notify ``base`` that the ``rts`` has been split, or vice versa.
(see :ghc-ticket:`5987`).
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: Deppendency of ``ghc`` library
libraries/Cabal/Cabal/Cabal.cabal: Dependency of ``ghc-pkg`` utility
libraries/containers/containers.cabal: Dependency of ``ghc`` library
libraries/deepseq/deepseq.cabal: Dependency of ``ghc`` library
libraries/directory/directory.cabal: Dependency of ``ghc`` 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-compact/ghc-compact.cabal: Core 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/process/process.cabal: Dependency of ``ghc`` library
libraries/template-haskell/template-haskell.cabal: Core 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
Win32
~~~~~
.. attention::
This release is a backwards incompatible release which corrects the type of
certain APIs. See issue `#24 <https://github.com/haskell/win32/issues/24>`_.