2.8. Version 9.4.1
The significant changes to the various parts of the compiler are listed in the following sections.
The LLVM backend of this release is to be used with LLVM
10, 11, 12, or 13.
2.8.1. Breaking Changes
In this section we list changes that may require changes in user programs while upgrading to GHC 9.4:
- There were previously cases around functional dependencies and injective type families where the result of type inference would depend on the order of constraints, as written in a source file. These cases are fundamentally ambiguous. While GHC previously made an arbitrary decision, it now notices the ambiguity and rejects the program. This means that some previously accepted programs are now rejected. The solution is to add a type annotation or type application to resolve the ambiguity. - This is the fix for #18851. 
- Unboxed sums now require the - UnboxedSumsextension to be enabled.
- ArrayArray#and its operations are no longer exported from- GHC.Primand are deprecated, having been superceded by the now levity-polymorphic- Array#type.
- The type equality operator, - (~), is now considered to be a type operator (exported from- Prelude) and therefore requires the enabling of the- TypeOperatorsextension rather than- GADTsor- TypeFamiliesas was sufficient previously.
- GHC is now a bit better at detecting redundant - hs-bootfiles and may report warnings where it previously did not. In such cases the correct solution is generally to remove the- hs-bootfile in question.
- The boxed - Word64and- Int64types are now internally represented by the unboxed- Word64#and- Int64#primitive types, in contrast previous releases where they were represented by- Word#and- Int#on 64-bit platforms.
- Due to various changes in the typechecker’s constraint solver, some programs may need additional constraints to be explicitly provided. See the Migration guide for details. 
- When - LambdaCaseis enabled, GHC will now parse the sequence- \ casesas the herald of a multi-pattern lambda-case expression. Consequently, programs of the form- \ cases -> ...will not parse; the solution is likely to rename the- casesbinder.
2.8.2. Language
- GHC Proposal #511 has been implemented, introducing a new language extension, - DeepSubsumption. This extension allows the user to opt-in to the deep type subsumption-checking behavior implemented by GHC 8.10 and earlier.
- A small change has been made to the way GHC infers types for definitions with no type signature: GHC will no longer generalize a function over a type variable determined by a functional dependency. For example: - class C a b | a -> b where op :: a -> b -> () f x = op True x - Previously, GHC inferred - f :: C Bool b => b -> (). However, the functional dependency says that only one type could ever be used for- b: this function is hardly valid “for all”- bs. With the change, GHC will reject, looking for the (non-existent) instance for- C Bool b.- If you want to retain the old behavior, add a (backward-compatible) type signature, explicitly requesting this unusual quantification. 
- GHC Proposal #371 has been implemented. This means: - The use of equality constraints no longer requires - -XGADTsor- -XTypeFamilies.
- The use of equality constraint syntax - a ~ brequires- -XTypeOperators, otherwise results in a warning (- -Wtype-equality-requires-operators).
- (~)is now a legal name for a user-defined type operator:- class a ~ b where ... - This used to be rejected with “Illegal binding of built-in syntax”. 
- The built-in type equality is now exported from - Data.Type.Equalityand re-exported from- Prelude. When- (~)is not in scope, its use results in a warning (- -Wtype-equality-out-of-scope).
 
- GHC Proposal #302 has been implemented. This means under - -XLambdaCase, a new expression heralded by- \casesis available, which works like- \casebut can match on multiple patterns. This means constructor patterns with arguments have to be parenthesized here, just like in lambda expressions.
- The parsing of implicit parameters is slightly more permissive, as GHC now allows - foo :: (?ip :: forall a. a -> a) - without requiring parentheses around - forall a. a -> a. Note that implicit parameters with such kinds are unlikely to be very useful, due to #18759.
- Changes to the treatment of - UnboxedSums:- GHC can now parse unboxed sum type constructors - (# | #),- (# | | #),- (# | | | #), etc. Partial applications need to be written in prefix form, e.g.- (# | #) Int#.
- Unboxed sums now require the - UnboxedSumsextension to be enabled.
- The - UnboxedTuplesextension now implies- UnboxedSums. This means that code using unboxed sums that enabled the- UnboxedTuplesextension but didn’t explicitly enable- UnboxedSumswill continue to work without changes.
 
2.8.3. Compiler
- The compiler now accepts arguments via GNU-style response files (#16476). 
- New - -Wredundant-strictness-flagsthat checks for strictness flags (- !) applied to unlifted types, which are always strict.
- New - -Wforall-identifier(enabled by default) that warns against using the name- forallas an identifer on the term level.
- New - -fprof-latethat adds automatic CCS annotations to all top level functions after core optimisation have been run.
- New - -fprof-manualwhich allows surpression of profiling cost centre annotations. It can be disabled as well which can be useful to surpress cost centres originating in library code.
- Typechecking plugins now support type-family rewriting. The - TcPlugindatatype now contains an extra field,- tcPluginRewrite, which allows typechecking plugin authors to specify which type families should be rewritten by the plugin, returning for each type family application a- TcPluginRewriteResult. In addition, typechecking plugins now have the ability to emit new constraints at the same time as contradictions. To account for these changes, the- TcPluginResultdatatype has been renamed to- TcPluginSolveResult, which bundles pattern synonyms- TcPluginOkand- TcPluginContradictionto recover the old interface. Typechecking plugins now have access to irreducible Given constraints, e.g., to enable the plugins to reduce the constraints.
- A new type of plugin: defaulting plugins. These plugins can propose defaults for ambiguous variables that would otherwise cause errors just like the built-in defaulting mechanism. 
- GHC.Plugins.parsedResultActionnow takes and returns a value of type- ParsedResult, containing the- HsParsedModuleas well as- PsMessages, which contains warnings and errors encountered by the parser before they’re shown to the user, as long as none of the errors prevented the AST from being built. This means plugins can remove, modify, or add any of these, or simply pass them through unchanged.
- The way GHC checks for representation polymorphism has been overhauled: all the checks are now done during typechecking. The error messages now contain more detailed information about the specific check that was performed. 
- A new pragma, - OPAQUE, that ensures that every call of a named function annotated with an- OPAQUEpragma remains a call of that named function, not some name-mangled variant. This implements GHC Proposal #415.
- Constructed Product Result analysis (c.f. - -fcpr-anal) has been overhauled and will now unbox nestedly, if termination properties of the function permit. This allows unboxing of constructed results returned by- IOactions. E.g.:- sumIO :: [Int] -> IO Int sumIO [] = return 0 sumIO (x:xs) = do r <- sumIO xs return $! x + r - Note the use of - $!: Without it, GHC would be unable to see that evaluation of- rand- xterminates (and rapidly, at that). An alternative would be to evaluate both with a bang pattern or a- seq, but the- return $! <res>idiom should work more reliably and needs less thinking.
- Demand analysis (cf. - -fstrictness) now integrates a Boxity Analysis that tracks whether a function needs a parameter boxed. If that is the case, the worker/wrapper transformation (cf.- -fworker-wrapper) will not unbox that parameter, leading to less reboxing in many cases.- For reasons of backwards-compatible performance, you may find that the new mechanism is too aggressive in a few cases (e.g., still unboxing a parameter that is used boxed in a hot path). Do post a bug report with your example! Then wrap the uses of the parameter in - GHC.Exts.lazyfor a short-term fix.
- Tag inference has been implemented. - It’s a new backend optimization pass aimed at avoiding redundant evaluatedness checks. The basic pass is always enabled and not optional. When using - -fworker-wrapper-cbvit additionally will generate workers for functions with strict arguments, pushing the evaluation+tagging of the arguments into the wrapper and allowing the worker to simply assume all arguments are fully evaluated and properly tagged. Usually the wrapper will then inline, and if the argument is known to be properly tagged at the call site the wrapper will become a no-op. Giving us a more efficient worker without adding any overhead. If the argument isn’t known to be evaluated we perform the same amount of work, but do it at call sites instead of inside the called function.- In general - -fworker-wrapper-cbvis very beneficial and can be safely enabled. However sadly there are two exceptions. It can break rules for code which made assumptions about which functions get a W/W split which now no longer hold. See #20364 for the details. For this reason it isn’t enabled by default. For code which has the proper- INLINABLE(INLINABLE pragma) and- INLINE(INLINE pragma) or that doesn’t define any rule-relevant functions this shouldn’t happen. The longterm fix here is to apply the proper pragmas. There is also a known issue where a function taking multiple unlifted arguments can cause excessive spilling (#20334). This seems to be an edge case. But if you think you are hitting this case please comment on the ticket so that we can prioritize it accordingly.
- Support for Sun SPARC architecture has been dropped (#16883). 
- A fix for GHC’s handling of the XDG Base Directory Specification (#6077, #20684, #20669, #20660): - For the package database previously in - ~/.ghc/<arch-ver>, we will continue to use the old path if it exists. For example, if the- ~/.ghc/x86_64-linux-9.4.1directory exists, GHC will use that for its user package database. If this directory does not exist, we will use- $XDG_DATA_HOME/ghc/x86_64-linux-9.4.1. This is in order to give tooling like cabal time to migrate
- For GHCi configuration files previously located in - ~/.ghc/like- ghci.confand- ghci_history, we will first check if they exist in- ~/.ghcand use those if they do. However, we will create new files like- ghci_historyonly in- $XDG_DATA_HOME/ghc. So if you don’t have a previous GHC installation which created- ~/.ghc/ghci_history, the history file will be written to- $XDG_DATA_HOME/ghc. If you already have an older GHC installation which wrote- ~/.ghc/ghci_history, then GHC will continue to write the history to that file.
 
- The - -Wunticked-promoted-constructorswarning is no longer enabled with- -Wall(#20531), as a part of long-term push towards Dependent Haskell.
- In GHCi, the - :typecommand no longer instantiates quantified type variables when given a polymorphic type. (It used to instantiate inferred type variables.)
2.8.4. Packaging
- GHC’s package database now comes with a virtual - system-cxx-std-libpackage which captures the compiler configuration necessary to link aginst the C++ standard library.
2.8.5. Runtime system
- Support for GHC’s eventlog is now enabled in all runtime system configurations, eliminating the need to pass the - -eventlogflag to use the eventlog. This flag has been deprecated (#18948).
- Summary statistics, i.e. the output of - -s [⟨file⟩], now correctly accounts for bytes copied during sequential collections.
2.8.6. base library
- GHC.Genericsnow provides a set of newtypes,- Genericallyand- Generically1, for deriving generic instances via- DerivingVia.- Genericallyinstances include- Semigroupand- Monoid.
- There’s a new special function - withDictin- GHC.Exts:- withDict :: forall {rr :: RuntimeRep} cls meth (r :: TYPE rr). WithDict cls meth => meth -> (cls => r) -> r - where - clsmust be a class containing exactly one method, whose type must be- meth. This requirement is enforced by the constraint- WithDict cls meth.- This function converts - methto a type class dictionary. It removes the need for- unsafeCoercein implementation of reflection libraries. It should be used with care, because it can introduce incoherent instances.
- See the - baselibrary’s- changelog.mdfor a full accounting.
2.8.7. ghc-prim library
- Primitive types and functions which handle boxed values are now levity-polymorphic, meaning that they now also work with unlifted boxed values (i.e. values whose type has kind - TYPE (BoxedRep Unlifted)).- The following type constructors are now levity-polymorphic: - Array#
- SmallArray#
- Weak#
- StablePtr#
- StableName#
- MutableArray#
 - SmallMutableArray#
- MutVar#
- TVar#
- MVar#
- IOPort#
 - For example, - Array#used to have kind:- Type -> UnliftedType - but it now has kind: - forall {l :: Levity}. TYPE (BoxedRep l) -> UnliftedType - Similarly, - MutVar#used to have kind:- Type -> Type -> UnliftedType - but it now has kind: - forall {l :: Levity}. Type -> TYPE (BoxedRep l) -> UnliftedType - This means that in - Array# a,- MutableArray# s a,- MutVar# s a, …, the element type- a, must always be boxed, but it can now either be lifted or unlifted. In particular, arrays and mutable variables can now be used to store other arrays and mutable variables.- All functions which use these updated primitive types are also levity-polymorphic: - all array operations (reading/writing/copying/…), for both arrays and small arrays, mutable and immutable: - newArray#,- readArray#,- writeArray#,- sizeofArray#,- sizeofMutableArray#,- indexArray#,- unsafeFreezeArray#,- unsafeThawArray#,- copyArray#,- copyMutableArray#,- cloneArray#,- cloneMutableArray#,- freezeArray#,- thawArray#,- casArray#,
- newSmallArray#,- shrinkSmallMutableArray#,- readSmallArray#,- writeSmallArray#,- sizeofSmallArray#,- getSizeofSmallMutableArray#,- indexSmallArray#,- unsafeFreezeSmallArray#,- unsafeThawSmallArray#,- copySmallArray#,- copySmallMutableArray#,- cloneSmallArray#,- cloneSmallMutableArray#,- freezeSmallArray#,- thawSmallArray#,- casSmallArray#,
 
- newMutVar#,- readMutVar#,- writeMutVar#,- casMutVar#,
- operations on - MVar#and- TVar#:- newTVar#,- readTVar#,- readTVarIO#,- writeTVar#,
- newMVar#,- takeMVar#,- tryTakeMVar#,- putMVar#,- tryPutMVar#,- readMVar#,- tryReadMVar#,
 
- STMoperations- atomically#,- retry#,- catchRetry#and- catchSTM#.
- newIOPort#,- readIOPort#,- writeIOPort#,
- mkWeak#,- mkWeakNoFinalizer#,- addCFinalizerToWeak#,- deRefWeak#,- finalizeWeak#,
- makeStablePtr#,- deRefStablePtr#,- eqStablePtr#,- makeStableName#,- stableNameToInt#,
 - For example, the full type of - newMutVar#is now:- newMutVar# :: forall {l :: Levity} s (a :: TYPE (BoxedRep l)). a -> State# s -> (# State# s, MVar# s a #) - and the full type of - writeSmallArray#is:- writeSmallArray# :: forall {l :: Levity} s (a :: TYPE (BoxedRep l)). SmallMutableArray# s a -> Int# -> a -> State# s -> State# s 
- ArrayArray#and- MutableArrayArray#have been moved from- GHC.Primto- GHC.Exts. They are deprecated, because their functionality is now subsumed by- Array#and- MutableArray#.
- mkWeak#,- mkWeakNoFinalizer#,- touch#and- keepAlive#are now levity-polymorphic instead of representation-polymorphic. For instance:- mkWeakNoFinalizer# :: forall {l :: Levity} {k :: Levity} (a :: TYPE (BoxedRep l)) (b :: TYPE (BoxedRep k)). a -> b -> State# RealWorld -> (# State# RealWorld, Weak# b #) - That is, the type signature now quantifies over the - GHC.Exts.Levityof- ainstead of its- GHC.Exts.RuntimeRep. In addition, this variable is now inferred, instead of specified, meaning that it is no longer eligible for visible type application. Note that- bis now also levity-polymorphic, due to the change outlined in the previous point.
- Primitive functions for throwing and catching exceptions are now more polymorphic than before. For example, - catch#now has type:- catch# :: forall {r :: RuntimeRep} {l :: Levity} (a :: TYPE r) (b :: TYPE (BoxedRep l)). ( State# RealWorld -> (# State# RealWorld, a #) ) -> ( b -> State# RealWorld -> (# State# RealWorld, a #) ) -> State# RealWorld -> (# State# RealWorld, a #) - The following functions have been generalised in this way: - catch#,
- raise#,- raiseIO#,
- maskAsyncExceptions#,- maskUninterruptible#,- unmaskAsyncExceptions#.
 - Note in particular that - raise#is now both representation-polymorphic (with an inferred- RuntimeRepargument) and levity-polymorphic, with type:- raise# :: forall {l :: Levity} {r :: RuntimeRep} (a :: TYPE (BoxedRep l)) (b :: TYPE r). a -> b 
- fork#and- forkOn#are now representation-polymorphic. For example,- fork#now has type:- fork# :: forall {r :: RuntimeRep} (a :: TYPE r). (State# RealWorld -> (# State# RealWorld, a #)) -> (State# RealWorld -> (# State# RealWorld, a #)) 
- GHC.Exts.reallyUnsafePtrEquality#has been made more general, as it is now both levity-polymorphic and heterogeneous:- reallyUnsafePtrEquality# :: forall {l :: Levity} {k :: Levity} (a :: TYPE (BoxedRep l)) (b :: TYPE (BoxedRep k)) . a -> b -> Int# - This means that - GHC.Exts.reallyUnsafePtrEquality#can be used on primitive arrays such as- GHC.Exts.Array#and- GHC.Exts.ByteArray#. It can also be used on values of different types, without needing to call- GHC.Exts.unsafeCoerce#.
- Added - GHC.Exts.reallyUnsafePtrEqualitywhich recovers the previous behaviour of- GHC.Exts.reallyUnsafePtrEquality#:- reallyUnsafePtrEquality :: forall (a :: Type). a -> a -> Int# 
- Added - GHC.Exts.sameArray#,- GHC.Exts.sameSmallArray#,- GHC.Exts.sameByteArray#and- GHC.Exts.sameArrayArray#:- sameArray# :: Array# a -> Array# a -> Int# sameSmallArray# :: SmallArray# a -> SmallArray# a -> Int# sameByteArray# :: ByteArray# -> ByteArray# -> Int# sameArrayArray# :: ArrayArray# -> ArrayArray# -> Int# 
2.8.8. ghc library
- The - loadfunction no longer automatically caches interface files in memory between calls. If you want to use a cache then you can supply one explicitly using the- loadWithCachefunction, with your own implementation or a simple cache created by- newIfaceCache.
- A new - GHC.Hs.Syn.Typemodule has been introduced which defines functions for computing the- Typeof an- HsExpr GhcTcin a pure fashion. The- hsLitTypeand- hsPatTypefunctions that previously lived in- GHC.Tc.Utils.Zonkhave been moved to this module.
- A - Typeableconstraint has been added to- fromStaticPtrin the class- GHC.StaticPtr.IsStatic. GHC automatically wraps each use of the- statickeyword with- fromStaticPtr. Because- staticrequires its argument to be an instance of- Typeable,- fromStaticPtrcan safely carry this constraint as well.
- The - newWantedfunction exported by- GHC.Tc.Pluginnow passes on the full- CtLocinstead of reconstituting it from the type-checking environment. This makes- newWantedconsistent with- newGiven. For authors of type-checking plugins, this means you don’t need to wrap a call to- newWantedin- setCtLocMto create a new Wanted constraint with the provided- CtLoc.
- GHC no longer carries - Derivedconstraints. Accordingly, several functions in the plugin architecture that previously passed or received three sets of constraints (givens, deriveds, and wanteds) now work with two such sets.
- A new argument has been added to the - HsOpTyconstructor of the- HsTypedatatype, to track the presence of a promotion tick. Plugins which manipulate the Haskell AST will need to take this change into account.
- Removed - lookupOrigIOin favor of- lookupNameCache.
- Added a new - thNameToGhcNameIOfunction that plugins can use outside the- CoreMmonad.
2.8.9. ghc-heap library
- The - linkfield of- GHC.Exts.Heap.WeakClosurehas been replaced with a- weakLinkfield which is- Nothingif and only if- linkwould have been NULL.
2.8.9.1. 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 | 9.4.8 | The compiler itself | 
| Cabal-syntax | 3.8.1.0 | Dependency of  | 
| Cabal | 3.8.1.0 | Dependency of  | 
| Win32 | 2.12.0.1 | Dependency of  | 
| array | 0.5.4.0 | Dependency of  | 
| base | 4.17.2.1 | Core library | 
| binary | 0.8.9.1 | Dependency of  | 
| bytestring | 0.11.5.3 | Dependency of  | 
| containers | 0.6.7 | Dependency of  | 
| deepseq | 1.4.8.0 | Dependency of  | 
| directory | 1.3.7.1 | Dependency of  | 
| exceptions | 0.10.5 | Dependency of  | 
| filepath | 1.4.2.2 | Dependency of  | 
| ghc-boot-th | 9.4.8 | Internal compiler library | 
| ghc-boot | 9.4.8 | Internal compiler library | 
| ghc-compact | 0.1.0.0 | Core library | 
| ghc-heap | 9.4.8 | GHC heap-walking library | 
| ghc-prim | 0.9.1 | Core library | 
| ghci | 9.4.8 | The REPL interface | 
| haskeline | 0.8.2 | Dependency of  | 
| hpc | 0.6.1.0 | Dependency of  | 
| integer-gmp | 1.1 | Core library | 
| libiserv | 9.4.8 | Internal compiler library | 
| mtl | 2.2.2 | Dependency of  | 
| parsec | 3.1.16.1 | Dependency of  | 
| pretty | 1.1.3.6 | Dependency of  | 
| process | 1.6.18.0 | Dependency of  | 
| stm | 2.5.1.0 | Dependency of  | 
| template-haskell | 2.19.0.0 | Core library | 
| terminfo | 0.4.1.5 | Dependency of  | 
| text | 2.0.2 | Dependency of  | 
| time | 1.12.2 | Dependency of  | 
| transformers | 0.5.6.2 | Dependency of  | 
| unix | 2.7.3 | Dependency of  | 
| xhtml | 3000.2.2.1 | Dependency of  |