4. Release notes for version 8.10.1¶
The significant changes to the various parts of the compiler are listed in the following sections.
4.1. Highlights¶
- The
UnliftedNewtypes
extension.
4.2. Full details¶
4.2.1. Language¶
Kind variables are no longer implicitly quantified when an explicit
forall
is used, see GHC proposal #24.-Wimplicit-kind-vars
is now obsolete.Kind variables are no longer implicitly quantified in constructor declarations:
data T a = T1 (S (a :: k)) | forall (b::k). T2 (S b) -- no longer accepted data T (a :: k) = T1 (S (a :: k)) | forall (b::k). T2 (S b) -- still accepted
Implicitly quantified kind variables are no longer put in front of other variables:
f :: Proxy (a :: k) -> Proxy (b :: j) ghci> :t +v f -- old order: f :: forall k j (a :: k) (b :: j). Proxy a -> Proxy b ghci> :t +v f -- new order: f :: forall k (a :: k) j (b :: j). Proxy a -> Proxy b
This is a breaking change for users of
TypeApplications
.In type synonyms and type family equations, free variables on the RHS are no longer implicitly quantified unless used in an outermost kind annotation:
type T = Just (Nothing :: Maybe a) -- no longer accepted type T = Just Nothing :: Maybe (Maybe a) -- still accepted
A new extension
StandaloneKindSignatures
allows one to explicitly specify the kind of a type constructor, as proposed in GHC proposal #54:type TypeRep :: forall k. k -> Type data TypeRep a where TyInt :: TypeRep Int TyMaybe :: TypeRep Maybe TyApp :: TypeRep a -> TypeRep b -> TypeRep (a b)
Analogous to function type signatures, a standalone kind signature enables polymorphic recursion. This feature is a replacement for
CUSKs
.GHC now parses visible, dependent quantifiers (as proposed in GHC proposal 35), such as the following:
data Proxy :: forall k -> k -> Type
See the section on explicit kind quantification for more details.
Type variables in associated type family default declarations can now be explicitly bound with a
forall
whenExplicitForAll
is enabled, as in the following example:class C a where type T a b type forall a b. T a b = Either a b
This has a couple of knock-on consequences:
Wildcard patterns are now permitted on the left-hand sides of default declarations, whereas they were rejected by previous versions of GHC.
It used to be the case that default declarations supported occurrences of left-hand side arguments with higher-rank kinds, such as in the following example:
class C a where type T a (f :: forall k. k -> Type) type T a (f :: forall k. k -> Type) = f Int
This will no longer work unless
f
is explicitly quantified with aforall
, like so:class C a where type T a (f :: forall k. k -> Type) type forall a (f :: forall k. k -> Type). T a f = f Int
A new extension
UnliftedNewtypes
that relaxes restrictions around what kinds of types can appear inside of the data constructor for anewtype
. This was proposed in GHC proposal #13.A new extension
ImportQualifiedPost
allows the syntaximport M qualified
, that is, to annotate a module as qualified by writingqualified
after the module name. This was proposed in GHC proposal #49.New flag
-Wderiving-defaults
that controls a warning message when bothDeriveAnyClass
andGeneralizedNewtypeDeriving
are enabled and no explicit deriving strategy is in use. The warning is enabled by default and has been present in earlier GHC versions but without the option of disabling it. For example, this code would trigger the warning:class C a newtype T a = MkT a deriving C
GHC now performs more validity checks on inferred type signatures. One consequence of this change is that some programs that used to be accepted will no longer compile without enabling the required language extensions. For example, in these two modules:
{-# LANGUAGE RankNTypes #-} module A where foo :: (forall a. a -> a) -> b -> b foo f x = f x module B where import A bar = foo
Notice that
A
enables-XRankNTypes
, butB
does not. Previous versions of GHC would allowbar
to typecheck, even though its inferred type is higher-rank. GHC 8.10 will now reject this, as one must now enable-XRankNTypes
inB
to accept the inferred type signature.Type family dependencies (also known as injective type families) sometimes now need
-XUndecidableInstances
in order to be accepted. Here is an example:type family F1 a = r | r -> a type family F2 a = r | r -> a type instance F2 [a] = Maybe (F1 a)
Because GHC needs to look under a type family to see that
a
is determined by the right-hand side ofF2
‘s equation, this now needs-XUndecidableInstances
. The problem is very much akin to its need to detect some functional dependencies.
4.2.2. Compiler¶
- Add new flags
-Wunused-record-wildcards
and-Wredundant-record-wildcards
which warn users when they have redundant or unused uses of a record wildcard match. - Calls to
memset
andmemcpy
are now unrolled more aggressively and the produced code is more efficient on x86-64 with added support for 64-bitMOV
s. In particular,setByteArray#
andcopyByteArray#
calls that were not optimized before, now will be. See #16052. - GHC’s runtime linker no longer uses global state. This allows programs that use the GHC API to safely use multiple GHC sessions in a single process, as long as there are no native dependencies that rely on global state.
- When loading modules that use
UnboxedTuples
orUnboxedSums
into GHCi, it will now automatically enable-fobject-code
for these modules and all modules they depend on. Before this change, attempting to load these modules into the interpreter would just fail, and the only convenient workaround was to enable-fobject-code
for all modules. See the GHCi FAQ for further details. - The eventlog now contains events for biographical and retainer profiling. The biographical profiling events all appear at the end of the eventlog but the sample start event contains a timestamp of when the census occurred. The retainer profiling events are emitted using the standard events.
- The eventlog now records the cost centre stack on each profiler sample. This
enables the
.prof
file to be partially reconstructed from the eventlog. - Add new flag
-fkeep-going
which makes the compiler continue as far as it can despite errors. - Deprecated flag
-fwarn-hi-shadowing
because it was not implemented correctly, and appears to be largely unused. This flag will be removed in a later version of GHC. - Windows bindist has been updated to GCC 9.2 and binutils 2.32. These binaries have
been patched to no longer have have the
MAX_PATH
limit. Windows users should no longer have any issues with long path names. - Introduce
DynFlags
plugins, that allow users to modidy theDynFlags
that GHC is going to use when processing a set of files, from plugins. They can be used for applying tiny configuration changes, registering hooks and much more. See the user guide for more details as well as an example.
4.2.3. GHCi¶
- Added a command
:instances
to show the class instances available for a type. - Added new debugger commands
:disable
and:enable
to disable and re-enable breakpoints. - Improved command name resolution with option
!
. For example,:k!
resolves to:kind!
.
4.2.4. Runtime system¶
4.2.5. Template Haskell¶
The
Lift
typeclass is now levity-polymorphic and has aliftTyped
method. Previously disallowed instances for unboxed tuples, unboxed sums, an primitive unboxed types have also been added. Finally, the code generated byDeriveLift
has been simplified to take advantage of expression quotations.Using
TupleT 1
,TupE [exp]
, orTupP [pat]
will now produce unary tuples (i.e., involving theUnit
type fromGHC.Tuple
) instead of silently dropping the parentheses. This brings Template Haskell’s treatment of boxed tuples in line with that of unboxed tuples, asUnboxedTupleT`, ``UnboxedTupE
, andUnboxedTupP
also produce unary unboxed tuples (i.e.,Unit#
) when applied to only one argument.GHC’s constraint solver now solves constraints in each top-level group sooner. This has practical consequences for Template Haskell, as TH splices necessarily separate top-level groups. For example, the following program would compile in previous versions of GHC, but not in GHC 8.10:
data T = MkT tStr :: String tStr = show MkT $(return []) instance Show T where show MkT = "MkT"
This is because each top-level group’s constraints are solved before moving on to the next, and since the top-level group for
tStr
appears before the top-level group that defines aShow T
instance, GHC 8.10 will throw an error about a missingShow T
instance in the expressionshow MkT
. The issue can be fixed by rearranging the order of declarations. For instance, the following will compile:data T = MkT instance Show T where show MkT = "MkT" $(return []) tStr :: String tStr = show MkT
4.2.6. ghc-prim
library¶
- Add new
bitReverse#
primops that, for aWord
of 8, 16, 32 or 64 bits, reverse the order of its bits e.g.0b110001
becomes0b100011
. These primitives use optimized machine instructions when available.
4.2.7. ghc
library¶
4.2.8. base
library¶
4.2.9. Build system¶
- Countless bug fixes in the new Hadrian build system
- Hadrian now supports a simple key-value configuration language, eliminating
the need for users to use Haskell to define build configuration. This should
simplify life for packagers and users alike. See #16769 and the
documentation in
hadrian/doc/user-settings.md
.
4.3. 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 | 8.10.0.20191210 | The compiler itself |
Cabal | 3.1.0.0 | Dependency of ghc-pkg utility |
Win32 | 2.6.1.0 | Dependency of ghc library |
array | 0.5.4.0 | Dependency of ghc library |
base | 4.14.0.0 | Core library |
binary | 0.8.7.0 | Dependency of ghc library |
bytestring | 0.10.9.0 | Dependency of ghc library |
containers | 0.6.2.1 | Dependency of ghc library |
deepseq | 1.4.4.0 | Dependency of ghc library |
directory | 1.3.4.0 | Dependency of ghc library |
filepath | 1.4.2.1 | Dependency of ghc library |
ghc-boot-th | 8.10.0.20191210 | Internal compiler library |
ghc-boot | 8.10.0.20191210 | Internal compiler library |
ghc-compact | 0.1.0.0 | Core library |
ghc-heap | 8.10.0.20191210 | GHC heap-walking library |
ghc-prim | 0.6.1 | Core library |
ghci | 8.10.0.20191210 | The REPL interface |
haskeline | 0.8.0.0 | Dependency of ghci executable |
hpc | 0.6.0.3 | Dependency of hpc executable |
integer-gmp | 1.0.2.0 | Core library |
libiserv | 8.10.0.20191210 | Internal compiler library |
mtl | 2.2.2 | Dependency of Cabal library |
parsec | 3.1.14.0 | Dependency of Cabal library |
pretty | 1.1.3.6 | Dependency of ghc library |
process | 1.6.6.0 | Dependency of ghc library |
stm | 2.5.0.0 | Dependency of haskeline library |
template-haskell | 2.16.0.0 | Core library |
terminfo | 0.4.1.4 | Dependency of haskeline library |
text | 1.2.3.1 | Dependency of Cabal library |
time | 1.9.3 | Dependency of ghc library |
transformers | 0.5.6.2 | Dependency of ghc library |
unix | 2.7.2.2 | Dependency of ghc library |
xhtml | 3000.2.2.1 | Dependency of haddock executable |