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.. _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.
Highlights
----------
The highlights, since the 8.2.1 release, are:
- 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.
Compiler
~~~~~~~~
- 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 ``Functor``, ``Foldable``, ``Traversable``, ``Generic``, and
``Generic1`` instances now have better, and generally better-documented,
behaviors for types with no constructors. In particular, ::
fmap _ x = case x of
foldMap _ _ = mempty
traverse _ x = pure (case x of)
to x = case x of
to1 x = case x of
from x = case x of
from1 x = case x of
The new behavior generally leads to more useful error messages than the
old did, and lazier semantics for ``foldMap`` and ``traverse``.
- 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.
- Derived instances for types with no constructors now have appropriate
arities: they take all their arguments before producing errors. This may not
be terribly important in practice, but it seems like the right thing to do.
Previously, we generated ::
(==) = error ...
Now we generate ::
_ == _ = error ...
- `-fsplit-sections` is now supported on x86_64 Windows and is on by default.
See :ghc-ticket:`12913`.
- Configure on Windows now supports ``--enable-distro-toolchain`` which can be
used to build a GHC using compilers on your ``PATH`` instead of using the
bundled bindist. See :ghc-ticket:`13792`
- 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.
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.
Template Haskell
~~~~~~~~~~~~~~~~
``ghc`` library
~~~~~~~~~~~~~~~
- hsSyn Abstract Syntax Tree (AST) is now extensible via the mechanism described in `Trees that Grow <http://www.jucs.org/jucs_23_1/trees_that_grow/jucs_23_01_0042_0062_najd.pdf>`_
The main change for users of the GHC API is that the AST is no longer indexed
by the type used as the identifier, but by a specific index type, ::
type GhcPs = GhcPass 'Parsed -- Old 'RdrName' type param
type GhcRn = GhcPass 'Renamed -- Old 'Name' type param
type GhcTc = GhcPass 'Typechecked -- Old 'Id' type para,
type GhcTcId = GhcTc -- Old 'TcId' type param
The simplest way to support the current GHC as well as earlier ones is to define ::
#if MIN_VERSION_ghc(8,3,0)
type ParseI = GhcPs
type RenameI = GhcRn
type TypecheckI = GhcTc
#else
type ParseI = RdrName
type RenameI = Name
type TypecheckI = Var
#endif
and then replace all hardcoded index types accordingly. For polymorphic types,
the constraint ::
#if MIN_VERSION_ghc(8,3,0)
-- |bundle up the constraints required for a trees that grow pass
type IsPass pass = (DataId pass, OutputableBndrId pass, SourceTextX pass)
else
type IsPass pass = (DataId pass, OutputableBndrId pass)
#endif
can be used.
``base`` library
~~~~~~~~~~~~~~~~
- Blank strings can now be used as values for environment variables using the
System.Environment.Blank module. See :ghc-ticket:`12494`
|
