3. バージョン 8.4.1 のリリースノート¶

コンパイラのさまざまな部分での重要な変更は以降の節で一覧表にしてあります． 8.2.1 リリースに対して大量のバグ修正と性能改善を行いました．

注釈

このバージョンのリリースをコンパイルするには GCC 4.7 以降が必要です． Trac #14244 参照

3.1. ハイライト¶

8.2.1 リリースからの変更のハイライトは以下のとおりです．

GHC はより多くのインスタンスを導出できるようなりました．
-XTypeInType の手順を改良し型エラーのメッセージを改善しました．.
コード生成をさらに改善しました．
Semigroup-Monoid提案のフェーズ2を含むライブラリ変更を行いました．
さまざまな Windows 互換性を改善しました．
大量のバグを修正しました．

3.2. 詳細¶

3.2.1. 言語¶

データ族は少し一般化されました．データ族宣言では， Type ではなくカインド変数 k で終端できます．さらに，data/newtype のインスタンスは，不要であれば，その族のすべてのパターンを並べなくてもよくなりました．これは，カインドシグネチャを持つ通常のデータ型ではいくつかの型変数を省略できるのと同じことです．

カインド変数がデータ族あるいは型族のインスタンスの右辺に現れるかどうかに関する制限はほとんどなくなりました．以前は，右辺にあるすべてのカインド変数が左辺の型パターンによって明示的に束縛されていなければならないという厳しい要件がありましたが，カインド変数を暗黙に束縛できるようになりました．これにより，次のような構成が可能になります。
```
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 -> *)
```

暗黙に双方向のパターンシノニムは，右辺では，バンパターン(!)あるいは反駁可能なパターン(~)を使えなくなりました．以前は使えていましたが，式コンテキストで使用すると，バンパターンと不可反駁パターンは暗黙のうちに無視されていました．これは今や適切にエラーになります．代わりに明示的に双方向のパターンシノニムを使うべきです．すなわち，エラーになるような
```
data StrictJust a = Just !a
```
は使わず．以下のように
```
data StrictJust a <- Just !a where
  StrictJust !a = Just a
```
を使うべきです．

カインド多相の型引数をもつ GADT を扱うには -XTypeInType が有効でなければなりません．たとえいば，以下の
```
data G :: k -> * where
  GInt   :: G Int
  GMaybe :: G Maybe
```
の宣言を考えましょう．一つ前のリリースでは，-XPolyKinds だけを有効にすれば，コンパイルできましたが，これはバグ Trac #13391 でした．しかしながら，GHC 8.4 では， -XTypeInType が必要です． GADT のカインドシグネチャは一般化されていませんので，カインド引数 k を明示的に限量化して， CUSK を提供する必要があります．
```
data G :: forall k. k -> * where
  GInt   :: G Int
  GMaybe :: G Maybe
```

GADT 構成子の型シグネチャで型変数が限量化される順番が変更になりました．以前は，以下のように MkT を書いたとすれば
```
data T a where
  MkT :: forall b a. b -> T a
```
MkT の型は直感に反して forall a b. b -> T a でした． GHC は型変数をユーザが書いた順序で限量化するようになったので， MkT の型は， forall b a. b -> T a です(:ghc-flag:`-XTypeApplications`の場合)．

新たに追加された -XEmptyDataDeriving 拡張により，空のデータ型に対する Eq ， Ord ， Read ， Show のインスタンスを data Empty deriving Eq のように直接導出できるようになりました． (以前は -XStandaloneDeriving を有効にしなければなりませんでした．)

また -XStandaloneDeriving を使わなくても，空のデータ型に対して Data インスタンスを (data Empty deriving Data のように)直接導出できるようになりました．とはいえ，この導出には -XDeriveDataTypeable という GHC拡張が必要であり，それが有効になっていれば，あらためて -XEmptyDataDeriving を有効しなくてもすみます．同じことが -XDeriveFunctor などの拡張を必要する他の型クラスについてもいえます．

-XHexFloatLiterals を有効にすると16進浮動小数点数リテラル(たとえば 0x0.1p4)表記が可能になります．詳細については Hexadecimal floating point literals を参照してください．

3.2.2. Compiler¶

LLVM code generator (e.g. -fllvm) compatible with LLVM releases in the 5.0 series.
Add warning 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 Deriving Functor instances, Deriving Foldable instances, and Deriving Traversable instances.

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 Trac #13792
GHC now enables -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 Trac #13747
Lots of other bugs. See Trac for a complete list.
New flags -fignore-optim-changes and -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 --interactive. See Trac #13604

3.2.3. 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 Trac #13606, Trac #12499, Trac #12498
The GHCi runtime linker on Windows now supports the big-obj file format.
The runtime system's native stack backtrace 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 --install-seh-handlers= Can be used on Wndows to completely disable the runtime's handling of exceptions. See Trac #13911, Trac #12110.
The GHC runtime on Windows can now generate crash dumps on unhandled exceptions using the 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>.

3.2.4. 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.

3.2.5. `base` library¶

Blank strings can now be used as values for environment variables using the System.Environment.Blank module. See Trac #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.

3.2.6. 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 Trac #5987).

3.3. 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.

Package	Version	Reason for inclusion
ghc	8.4.3	The compiler itself
Cabal	2.2.0.1	Dependency of `ghc-pkg` utility
Win32	2.6.1.0	Dependency of `ghc` library
array	0.5.2.0	Dependency of `ghc` library
base	4.11.1.0	Core library
binary	0.8.5.1	Dependency of `ghc` library
bytestring	0.10.8.2	Deppendency of `ghc` library
containers	0.5.11.0	Dependency of `ghc` library
deepseq	1.4.3.0	Dependency of `ghc` library
directory	1.3.1.5	Dependency of `ghc` library
filepath	1.4.2	Dependency of `ghc` library
ghc-boot	8.4.3	Internal compiler library
ghc-compact	0.1.0.0	Core library
ghc-prim	0.5.2.0	Core library
ghci	8.4.3	The REPL interface
haskeline	0.7.4.2	Dependency of `ghci` executable
hpc	0.6.0.3	Dependency of `hpc` executable
integer-gmp	1.0.2.0	Core library
mtl	2.2.2	Dependency of `Cabal` library
parsec	3.1.13.0	Dependency of `Cabal` library
process	1.6.3.0	Dependency of `ghc` library
template-haskell	2.13.0.0	Core library
text	1.2.3.0	Dependency of `Cabal` library
time	1.8.0.2	Dependency of `ghc` library
transformers	0.5.5.0	Dependency of `ghc` library
unix	2.7.2.2	Dependency of `ghc` library
xhtml	3000.2.2.1	Dependency of `haddock` executable

3.3.1. Win32¶

注意

This release is a backwards incompatible release which corrects the type of certain APIs. See issue #24.

Glasgow Haskell Compiler User's Guide

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3. バージョン 8.4.1 のリリースノート¶

3.1. ハイライト¶

3.2. 詳細¶

3.2.1. 言語¶

3.2.2. Compiler¶

3.2.3. Runtime system¶

3.2.4. Template Haskell¶

3.2.5. `base` library¶

3.2.6. Build system¶

3.3. Included libraries¶

3.3.1. Win32¶

Glasgow Haskell Compiler User's Guide

3. バージョン 8.4.1 のリリースノート¶

3.1. ハイライト¶

3.2. 詳細¶

3.2.1. 言語¶

3.2.2. Compiler¶

3.2.3. Runtime system¶

3.2.4. Template Haskell¶

3.2.5. base library¶

3.2.6. Build system¶

3.3. Included libraries¶

3.3.1. Win32¶

3.2.5. `base` library¶