3. 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 Trac #14244.
3.1. Highlights¶
The highlights, since the 8.2.1 release, are:
- GHC is now capable of deriving more instances
- More refinement of the
-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.
3.2. Full details¶
3.2.1. Language¶
Data families have been generalised a bit: a data family declaration can now end with a kind variable
k
instead ofType
. 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
-XTypeInType
. For instance, consider the following,data G :: k -> * where GInt :: G Int GMaybe :: G Maybe
In previous releases this would compile with
-XPolyKinds
alone due to bug Trac #13391. As of GHC 8.4, however, this requires-XTypeInType
. Note that since GADT kind signatures aren’t generalized, this will also require that you provide a CUSK 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) beforall a b. b -> T a
! Now, GHC quantifies the type variables in the order that the users writes them, so the type ofMkT
is nowforall b a. b -> T a
(this matters for-XTypeApplications
).The new
-XEmptyDataDeriving
extension allows derivingEq
,Ord
,Read
, andShow
instances directly for empty data types, as indata Empty deriving Eq
. (Previously, this would require the use of-XStandaloneDeriving
to accomplish.)One can also now derive
Data
instances directly for empty data types (as indata Empty deriving Data
) without needing to use-XStandaloneDeriving
. However, since already requires a GHC extension (-XDeriveDataTypeable
), one does not need to enable-XEmptyDataDeriving
to do so. This also goes for other classes which require extensions to derive, such as-XDeriveFunctor
.Hexadecimal floating point literals (e.g.
0x0.1p4
), enabled with-XHexFloatLiterals
. See Hexadecimal floating point literals for the full details.
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 inautoconf
scripts. For instance,./configure --with-nm=/usr/local/bin/nm
turns into./configure NM=/usr/local/bin/nm
.Derived
Functor
,Foldable
, andTraversable
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
andtraverse
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 forEmpty
:Derived
Eq
andOrd
instances would previously emit code that usederror
: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 usedparens
: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 useparens
(that it, parsingEmpty
will always fail, without reading any input):instance Read (Empty a) where readPrec = pfail
Derived
Show
instances would previously emit code that usederror
: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
, andData
instances previously emitted code that usederror
: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
, andData
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 fornull
instead of using the default one. This leads to asymptotically better performance for recursive types not shaped like cons-lists, and allowsnull
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 yourPATH
instead of using the bundled bindist. See Trac #13792GHC 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 #13747Lots 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 tohs_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 ofSIGUSR2
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 anddata 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 byForallC [PlainTV a, PlainTV b]
. Now, reifyingMkT
will give a type headed byForallC [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 utilitygen-dll
. This utility can transparently split and compile any DLLs that require this. Note that therts
andbase
can not be split at this point because of the mutual recursion betweenbase
andrts
. There is currently no explicit dependency between the two in the build system and such there is no way to notifybase
that therts
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 | 8.4.2 | 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 | 8.4.2 | Internal compiler library |
ghc-compact | 0.1.0.0 | Core library |
ghc-prim | 0.5.2.0 | Core library |
ghci | 8.4.2 | 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 |