path: root/docs/users_guide/8.4.1-notes.rst

.. _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.

- 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:`TypeInType`.
  For instance, consider the following, ::

      data G :: k -> * where
        GInt   :: G Int
        GMaybe :: G Maybe

  In previous releases this would compile with :ghc-flag:`PolyKinds` alone due
  to bug :ghc-ticket:`13391`. As of GHC 8.4, however, this requires
  :ghc-flag:`TypeInType`. 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`).

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.

- 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.

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.

``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`

- ``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`).