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 8.0 branch.
The highlights since the 8.0 release include:
GHC will now use ld.gold or ld.lld instead of the system’s default ld, if available. Linker availability will be evaluated at configure time. The user can manually override which linker to use by passing the LD variable to configure. You can revert to the old behavior of using the system’s default ld by passing the --disable-ld-override flag to configure.
GHC now uses section splitting (i.e. -split-sections) instead of object splitting (i.e. -split-objs) as the default mechanism for linker-based dead code removal. While the effect is the same, split sections tends to produce significantly smaller objects than split objects and more closely mirrors the approach used by other compilers. Split objects will be deprecated and eventually removed in a future GHC release.
Note that some versions of the ubiquitous BFD linker exhibit performance trouble with large libraries with section splitting enabled (see Trac #13739). It is recommended that you use either the gold or lld linker if you observe this. This will require that you install one of these compilers, rerun configure, and reinstall GHC.
Split sections is enabled by default in the official binary distributions for platforms that support it.
Old profiling flags -auto-all, -auto, and -caf-all are deprecated and their usage provokes a compile-time warning.
Support for adding cost centres to declarations is added. The same SCC syntax can be used, in addition to a new form for specifying the cost centre name. See Inserting cost centres by hand for examples.
GHC is now much more particular about -XDefaultSignatures. The type signature for a default method of a type class must now be the same as the corresponding main method’s type signature modulo differences in the signatures’ contexts. Otherwise, the typechecker will reject that class’s definition. See Default method signatures for further details.
-XDeriveAnyClass is no longer limited to type classes whose argument is of kind * or * -> *.
The means by which -XDeriveAnyClass infers instance contexts has been completely overhauled. The instance context is now inferred using the type signatures (and default type signatures) of the derived class’s methods instead of using the datatype’s definition, which often led to over-constrained instances or instances that didn’t typecheck (or worse, triggered GHC panics). See the section on DeriveAnyClass for more details.
GHC now allows standalone deriving using -XDeriveAnyClass on any data type, even if its data constructors are not in scope. This is consistent with the fact that this code (in the presence of -XDeriveAnyClass):
deriving instance C T
is exactly equivalent to:
instance C T
and the latter code has no restrictions about whether the data constructors of T are in scope.
-XGeneralizedNewtypeDeriving now supports deriving type classes with associated type families. See the section on GeneralizedNewtypeDeriving and associated type families.
-XGeneralizedNewtypeDeriving will no longer infer constraints when deriving a class with no methods. That is, this code:
class Throws e
newtype Id a = MkId a
deriving Throws
will now generate this instance:
instance Throws (Id a)
instead of this instance:
instance Throws a => Throws (Id a)
This change was motivated by the fact that the latter code has a strictly redundant Throws a constraint, so it would emit a warning when compiled with -Wredundant-constraints. The latter instance could still be derived if so desired using -XStandaloneDeriving:
deriving instance Throws a => Throws (Id a)
Add warning flag -Wcpp-undef which passes -Wundef to the C pre-processor causing the pre-processor to warn on uses of the #if directive on undefined identifiers.
GHC will no longer automatically infer the kind of higher-rank type synonyms; you must explicitly explicitly annotate the synonym with a kind signature. For example, given:
data T :: (forall k. k -> Type) -> Type
to define a synonym of T, you must write:
type TSyn = (T :: (forall k. k -> Type) -> Type)
The Mingw-w64 toolchain for the Windows version of GHC has been updated. GHC now uses GCC 6.2.0 and binutils 2.27.
Previously, -Wmissing-methods would not warn whenever a type class method beginning with an underscore was not implemented in an instance. For instance, this code would compile without any warnings:
class Foo a where
_Bar :: a -> Int
instance Foo Int
-Wmissing-methods will now warn that _Bar is not implemented in the Foo Int instance.
A new flag -ddump-json has been added. This flag dumps compiler output as JSON documents. It is experimental and will be refined depending on feedback from tooling authors for the next release.
GHC is now able to better optimize polymorphic expressions by using known superclass dictionaries where possible. Some examples:
-- uses of `Monad IO` or `Applicative IO` here are improved
foo :: MonadBaseControl IO m => ...
-- uses of `Monoid MyMonoid` here are improved
bar :: MonadWriter MyMonoid m => ...
GHC now derives the definition of <$ when using DeriveFunctor rather than using the default definition. This prevents unnecessary allocation and a potential space leak when deriving Functor for a recursive type.
The -XExtendedDefaultRules extension now defaults multi-parameter typeclasses. See Trac #12923.
GHC now ignores RULES for data constructors (Trac #13290). Previously, it accepted:
{-# RULES "NotAllowed" forall x. Just x = e #-}
That rule will no longer take effect, and a warning will be issued. RULES may still mention data constructors, but not in the outermost position:
{-# RULES "StillWorks" forall x. f (Just x) = e #-}
Type synonyms can no longer appear in the class position of an instance. This means something like this is no longer allowed:
type ReadShow a = (Read a, Show a)
instance Read Foo
instance Show Foo
instance ReadShow Foo -- illegal
See Trac #13267.
Validity checking for associated type family instances has tightened somewhat. Before, this would be accepted:
class Foo a where
type Bar a
instance Foo (Either a b) where
type Bar (Either c d) = d -> c
This is now disallowed, as the type variables used in the Bar instance do not match those in the instance head. This instance can be fixed by changing it to:
instance Foo (Either a b) where
type Bar (Either a b) = b -> a
See the section on associated type family instances <assoc-data-inst> for more information.
A bug involving the interaction between -XMonoLocalBinds and -XPolyKinds has been fixed. This can cause some programs to fail to typecheck in case explicit kind signatures are not provided. See Kind generalisation for an example.
Reifying types that contain unboxed tuples now works correctly. (Previously, Template Haskell reified unboxed tuples as boxed tuples with twice their appropriate arity.)
Splicing singleton unboxed tuple types (e.g., (# Int #)) now works correctly. Previously, Template Haskell would implicitly remove the parentheses when splicing, which would turn (# Int #) into Int.
Add support for type signatures in patterns. (Trac #12164)
Make quoting and reification return the same types. (Trac #11629)
More kind annotations appear in the left-hand sides of reified closed type family equations, in order to disambiguate types that would otherwise be ambiguous in the presence of -XPolyKinds. (Trac #12646)
Quoted type signatures are more accurate with respect to implicitly quantified type variables. Before, if you quoted this:
[d| id :: a -> a
id x = x
|]
then the code that Template Haskell would give back to you would actually be this instead:
id :: forall a. a -> a
id x = x
That is, quoting would explicitly quantify all type variables, even ones that were implicitly quantified in the source. This could be especially harmful if a kind variable was implicitly quantified. For example, if you took this quoted declaration:
[d| idProxy :: forall proxy (b :: k). proxy b -> proxy b
idProxy x = x
|]
and tried to splice it back in, you’d get this instead:
idProxy :: forall k proxy (b :: k). proxy b -> proxy b
idProxy x = x
Now k is explicitly quantified, and that requires turning on -XTypeInType, whereas the original declaration did not!
Template Haskell quoting now respects implicit quantification in type signatures, so the quoted declarations above now correctly leave the type variables a and k as implicitly quantified. (Trac #13018 and Trac #13123)
Looking up type constructors with symbol names (e.g., +) now works as expected (Trac #11046)
See changelog.md in the base package for full release notes.
The ghc-compact library provides an experimental API for placing immutable data structures into a contiguous memory region. Data in these regions is not traced during garbage collection and can be serialized to disk or over the network.