2.1. Version 9.14.1

The significant changes to the various parts of the compiler are listed in the following sections. See the migration guide on the GHC Wiki for specific guidance on migrating programs to this release.

2.1.1. Language

  • GHC proposal 493: allow expressions in SPECIALISE pragmas has been implemented. SPECIALISE pragmas now allow arbitrary expressions such as:

    {-# SPECIALISE f @Int False :: Int -> Char #-}

    The ability to specify multiple specialisations in a single SPECIALISE pragma, with syntax of the form (note the comma between the type signatures):

    {-# SPECIALISE g : Int -> Int, Float -> Float #-}

    has been deprecated, and is scheduled to be removed in GHC 9.18. This deprecation is controlled by the newly introduced -Wdeprecated-pragmas flag in -Wdefault.

  • Visible GADT syntax can now be used in GADT data constructors (#25127)

    data KindVal a where
    K :: forall k.
      forall (a::k) ->   -- now allowed!
      k ->
      KindVal a

  • -Wincomplete-record-selectors is now part of -Wall, as specified by GHC Proposal 516: add warning for incomplete record selectors. Hence, if a library is compiled with -Werror, compilation may now fail. Solution: fix the library. Workaround: add -Werror=no-incomplete-record-selectors.

    Note that this warning is at least as serious as a warning about missing patterns from a function definition, perhaps even more so, since it is invisible in the source program.

  • The combination of ScopedTypeVariables and TypeApplications no longer enables type applications in patterns, which now always requires TypeAbstractions. The warning flag``deprecated-type-abstractions`` has also been removed from the compiler.

  • OverloadedRecordUpdate now passes the arguments to a setField function in the flipped order, as specified by GHC Proposal 583: HasField redesign.

    Previously GHC expected setField to have this type:

    setField :: forall (fld :: Symbol) a r. r -> a -> r

    And that’s what GHC expects now:

    setField :: forall (fld :: Symbol) a r. a -> r -> r

    That will break the combination of OverloadedRecordUpdate with RebindableSyntax.

  • Multiline strings are now accepted in foreign imports. (#25157)

  • GHC now does a better job at inferring types in calls to coerce: instead of complaining about ambiguous type variables, GHC will consider that such type variables are determined by the Coercible constraints they appear in.

  • With LinearTypes record fields can now be non-linear. This means that the following record declaration is now valid:

    data Record = Rec { x %'Many :: Int, y :: Char }

    This causes the constructor to have type Rec :: Int %'Many -> Char %1 -> Record.

  • The ExplicitNamespaces extension now allows the data namespace specifier in import and export lists.

  • The -Wdata-kinds-tc warning has been deprecated, and the use of promoted data types in kinds is now an error (rather than a warning) unless the DataKinds extension is enabled. For example, the following code will be rejected unless DataKinds is on:

    import Data.Kind (Type)
import GHC.TypeNats (Nat)

-- Nat shouldn't be allowed here without DataKinds
data Vec :: Nat -> Type -> Type

    (The -Wdata-kinds-tc warning was introduced in GHC 9.10 as part of a fix for an accidental oversight in which programs like the one above were mistakenly accepted without the use of DataKinds.)

  • The MonadComprehensions extension now implies ParallelListComp as was originally intended (see Monad Comprehensions).

  • In accordance with GHC Proposal #281, section 4.7 “Data constructors”, the RequiredTypeArguments extension now allows visible forall in types of data constructors (#25127). The following declaration is now accepted by GHC:

    data T a where
  Typed :: forall a -> a -> T a

    See Visible forall in GADTs for details.

  • Explicit level import support, allowing import declarations to explicitly state which compilation stages they are are visible to.

2.1.2. Compiler

  • An improved error message is introduced to refer users to the heap-controlling flags of the RTS when there is a heap overflow during compilation. (#25198)

  • The kind checker now does a better job of finding type family instances for use in the kinds of other declarations in the same module. This fixes a number of tickets: #12088, #12239, #14668, #15561, #16410, #16448, #16693, #19611, #20875, #21172, #22257, #25238, #25834.

  • The compiler no longer accepts invalid type namespace specifiers in subordinate import lists (#22581).

  • A new flag, -Wuseless-specialisations, controls warnings emitted when GHC determines that a SPECIALISE pragma would have no effect.

  • A new flag, -Wrule-lhs-equalities, controls warnings emitted for RULES whose left-hand side attempts to quantify over equality constraints that previous GHC versions accepted quantifying over. GHC will now drop such RULES, emitting a warning message controlled by this flag.

    This warning is intended to give visibility to the fact that the RULES that previous GHC versions generated in such circumstances could never fire.

  • A new flag, -Wunusable-unpack-pragmas, controls warnings emitted when GHC is unable to unpack a data constructor field annotated by the {-# UNPACK #-} pragma.

    Previous GHC versions issued this warning unconditionally. Now it is possible to disable it with -Wno-unusable-unpack-pragmas or turn it into an error with -Werror=unusable-unpack-pragmas.

  • Introduce a new warning -Wpattern-namespace-specifier to detect uses of the now deprecated pattern namespace specifier in import/export lists. See GHC Proposal #581, section 2.3.

  • Code coverage (-fhpc) now treats uses of record fields via RecordWildCards or NamedFieldPuns as if the fields were accessed using the generated record selector functions, marking the fields as covered in coverage reports (#17834).

  • SIMD support in the x86 native code generator has been extended with 128-bit integer operations. Also, shuffleFloatX4# and shuffleDoubleX2# no longer require -mavx.

  • Initial native code generator support for the LoongArch CPU architecture.

2.1.3. GHCi

  • Support for multiple home units in GHCi (#20889)

  • :info now outputs type declarations with @-binders that are considered semantically significant. See the documentation for :info itself for a more detailed explanation.

  • GHCi errors and warnings now have their own numeric error codes that are displayed alongside the error.

  • Many performance and correctness improvements in the bytecode interpreter.

  • Numerous improvements in the GHCi debugger including:

    • Significantly improved performance of the :steplocal command (#25779)

    • Every do statement is now a breakpoint, resulting in more predictable and intuitive :break behavior

    • A :stepout command is now available, allowing stepping out from functions and bindings when stopped at a breakpoint. Note that this feature is made available as a technology preview and we expect further refinement in future releases.

    • Improvements in the GHCi debugger API, setting the groundwork for integration with Debug Adapter Protocol (DAP) and other interactive debugging clients

    • Internal refactorings towards making the debugger multi-thread aware (#26064)

2.1.4. WebAssembly backend

The WebAssembly backend now supports evaluation via the interpreter, allowing both interactive usage via GHCi and TemplateHaskell evaluation. This includes usage of foreign import javascript, allowing interactive usage within the browser including interaction with the DOM (modulo various constraints imposed by WebAsm implementations).

See the blog post on Tweag’s blog for more information.

2.1.5. Runtime system

  • Add new runtime flag --optimistic-linking which instructs the runtime linker to continue in the presence of unknown symbols. By default this flag is not passed, preserving previous behavior.

2.1.6. base library

2.1.7. ghc-prim library

ghc-prim is now a legacy interface providing access to primitive operations and types which are now also exposed via the ghc-experimental package.

2.1.8. ghc library

  • The UnknownDiagnostic constructor now takes an additional type argument for the type of hints corresponding to the diagnostic, and an additional value-level argument used for existential wrapping of the hints of the inner diagnostic.

  • Changes to the HPT and HUG interface:

    • addToHpt and addListToHPT were moved from GHC.Unit.Home.ModInfo to GHC.Unit.Home.PackageTable and deprecated in favour of addHomeModInfoToHpt and addHomeModInfosToHpt.

    • UnitEnvGraph and operations unitEnv_lookup_maybe, unitEnv_foldWithKey, ``unitEnv_singleton, unitEnv_adjust, unitEnv_insert, unitEnv_new were moved from GHC.Unit.Env to GHC.Unit.Home.Graph.

    • The HomePackageTable (HPT) is now exported from GHC.Unit.Home.PackageTable, and is now backed by an IORef to avoid by construction very bad memory leaks. This means the API to the HPT now is for the most part in IO. For instance, emptyHomePackageTable and addHomeModInfoToHpt are now in IO.

    • mkHomeUnitEnv was moved to GHC.Unit.Home.PackageTable, and now takes two extra explicit arguments. To restore previous behaviour, pass emptyUnitState and Nothing as the first two arguments additionally.

    • hugElts was removed. Users should prefer allUnits to get the keys of the HUG (the typical use case), or traverse or unitEnv_foldWithKey in other cases.

  • Changes to Language.Haskell.Syntax.Expr

    • The ParStmtBlock list argument of the ParStmt constructor of StmtLR is now NonEmpty.

  • As part of the implementation of GHC proposal 493 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0493-specialise-expressions.rst>``_, the ``SpecSig constructor of Sig has been deprecated. It is replaced by the constructor SpecSigE which supports expressions at the head, rather than a lone variable.

2.1.9. ghc-heap library

  • The functions getClosureInfoTbl_maybe, getClosureInfoTbl, getClosurePtrArgs and getClosurePtrArgs_maybe have been added to allow reading of the relevant Closure attributes without reliance on incomplete selectors.

2.1.10. ghc-experimental library

  • ghc-experimental now exposes GHC.RTS.Flags and GHC.Stats as GHC.RTS.Flags.Experimental and GHC.Stats.Experimental. These are also exposed in base, however the base versions will be deprecated as part of the split base project. See CLC proposal 289. Downstream consumers of these flags are encouraged to migrate to the ghc-experimental versions.

2.1.11. template-haskell library

  • As part of the implementation of GHC proposal 493, the SpecialiseP constructor of the Template Haskell Pragma type, as well as the helpers pragSpecD and pragSpecInlD, have been deprecated.

    They are replaced, respectively, by SpecialiseEP, pragSpecED and pragSpecInlED.

2.1.12. 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

 9.14.0.20251028

The compiler itself

Cabal-syntax

 3.16.0.0

Dependency of ghc-pkg utility

Cabal

 3.16.0.0

Dependency of ghc-pkg utility

Win32

 2.14.2.1

Dependency of ghc library

array

 0.5.8.0

Dependency of ghc library

base

 4.22.0.0

Core library

binary

 0.8.9.3

Dependency of ghc library

bytestring

 0.12.2.0

Dependency of ghc library

containers

 0.8

Dependency of ghc library

deepseq

 1.5.1.0

Dependency of ghc library

directory

 1.3.10.0

Dependency of ghc library

exceptions

 0.10.9

Dependency of ghc and haskeline library

file-io

 0.1.5

Dependency of directory library

filepath

 1.5.4.0

Dependency of ghc library

ghc-bignum

 1.4

Core library.

ghc-boot-th

 9.14.0.20251028

Internal compiler library

ghc-boot

 9.14.0.20251028

Internal compiler library

ghc-compact

 0.1.0.0

Core library

ghc-experimental

 9.1400.0

Core library.

ghc-heap

 9.14.0.20251028

Core library.

ghc-internal

 9.1400.0

Internal implementation.

ghc-platform

 0.1.0.0

Internal compiler library.

ghc-prim

 0.13.1

Core library

ghci

 9.14.0.20251028

The REPL interface

haskeline

 0.8.3.0

Dependency of ghci executable

hpc

 0.7.0.2

Dependency of hpc executable

integer-gmp

 1.1

Core library

mtl

 2.3.1

Dependency of Cabal library

os-string

 2.0.7

Dependency of filepath library

parsec

 3.1.18.0

Dependency of Cabal library

pretty

 1.1.3.6

Dependency of ghc library

process

 1.6.26.1

Dependency of ghc library

semaphore-compat

 1.0.0

Dependency of ghc library

stm

 2.5.3.1

Dependency of haskeline library

template-haskell

 2.24.0.0

Core library

terminfo

 0.4.1.7

Dependency of haskeline library

text

 2.1.3

Dependency of Cabal library

time

 1.15

Dependency of ghc library

transformers

 0.6.1.2

Dependency of ghc library

unix

 2.8.8.0

Dependency of ghc library

xhtml

 3000.2.2.1

Dependency of haddock executable

ghc-toolchain

 0.1.0.0

Internal compiler library.

haddock-api

 2.33.0

Dependency of haddock executable

haddock-library

 1.11.0

Dependency of haddock executable