2.1. Version 10.0.1¶

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¶

The default language edition has been bumped to GHC2024, as per the accepted GHC proposal 632 (#26039).
Fix a bug introduced in GHC 9.10 where GHC would erroneously accept infix uses of promoted data constructors without enabling DataKinds. As a result, you may need to enable DataKinds in code that did not previously require it.
Type and Constraint are now (at last) completely distinct types, just as much as Int and Bool. For example, you can now write:
```
type family F a

type instance F Type = Int
type instance F Constraint = Bool
```
which was previously rejected with “Conflicting family instance declarations”.
The extension ExplicitNamespaces now allows namespace-specified wildcards type .. and data .. in import and export lists.
Implicit parameters and ImpredicativeTypes. GHC now knows that if ?foo::S is coecible to ?foo::T only if S is coercible to T. Example (from #26737):
```
{-# LANGUAGE ImplicitParams, ImpredicativeTypes #-}
newtype N = MkN Int
test :: ((?foo::N) => Bool) -> ((?foo::Int) => Bool)
test = coerce
```
This is achieved by arranging that ?foo :: T has a representational role for T.
Implement -XQualifiedStrings (proposal)
The experimental Modifiers extension now allows %foo syntax to attach types to various places in the syntax tree.
Some previously-valid uses of LinearTypes are now rejected, since its syntax has been generalized for use with Modifiers. In particular, some kind annotations must be added:
```
f :: Int %m -> Int -- no longer accepted
f :: forall (m :: Multiplicity). Int %m -> Int -- still accepted
```
Alternatively, most previously-valid code can still be accepted by setting -XLinearTypes -XNoModifiers. But this doesn’t reliably work with -XStrict, and sometimes parentheses must be added:
```
let %1 x:xs = ... in ... -- previously accepted with -XLinearTypes -XStrict
let %1 (x:xs) = ... in ... -- still accepeted

let %1 Just x = ... in ... -- previously accepted with -XLinearTypes -XStrict
let %1 (Just x) = ... in ... -- still accepeted
```
-XLinearTypes -XNoModifiers doesn’t accept the old versions, because modifiers now bind tighter than constructors. The first now parses as let (%1 x):xs = ... in ..., and the second no longer parses at all.
The treatment of static forms has been simplified, implementing GHC proposal 732 (#26556). There is a new, simple rule for (static e), namely that the free term variables of e must be bound at top level. This also fixes #26545, #24464, #24773, #16981, #26466 and #27664.

Allow kinds of the form k -> * and * -> k to occur in expression syntax, i.e. to be used as required type arguments (#26587, #26967). For example:

{-# LANGUAGE RequiredTypeArguments, StarIsType #-}
x1 = f (* -> * -> *)
x2 = f (forall k. k -> *)
x3 = f ((* -> *) -> Constraint)

Infix holes (t1 `_` t2) are now permitted in types, following the precedent set by term-level expressions (#11107). Error messages for illegal promotion ticks are now reported at more precise source locations.
List comprehensions are now considered to be completely non-linear under LinearTypes (#25081).

2.1.2. Compiler¶

-fpolymorphic-specialisation is now switched on by default (#23559).
Code coverage’s (-fhpc) treatment of record fields now extends beyond record fields accessed via RecordWildCards and NamedFieldPuns, and also handles access to nested record fields. That is, in a pattern such as Foo{bar = Bar{baz = b}} both bar and baz will now be marked as covered if b is evaluated. Note that this currently only works when record fields (or values contained within them) are bound to variables. The very similar pattern Foo{bar = Bar{baz = 42}} will will not yet mark bar or baz as covered.
Pattern synonyms can now be suggested as valid hole fits (except, of course, if they are unidirectional). Valid hole fits also now use deep subsumption for data constructors, matching up multiplicities (#26338).
GHC uses the information from the definition of a closed type family to generate some extra functional dependencies for type equalities involving that type family. As a consequence:
- typechecking will succeed a bit more often (see #23162)
- pattern-match incompleteness checking is a bit smarter, giving fewer false warnings (see #22652)
When multiple -msse* flags are given, the maximum version takes effect. For example, -msse4.2 -msse2 is now equivalent to -msse4.2. Previously, only the last flag took effect.
Some x86 architecture flags now imply other flags. For example, -mavx now implies -msse4.2, and -mavx512f now implies -mfma in addition to -mavx2. Refer to the users’ guide for more details about each individual flag.

The flag -fhide-source-paths is enabled by default at verbosity level 1 and below by default. This improves the readability of the compiler output, as we transition from:

[  1 of 139] Compiling Distribution.Compat.Binary ( src/Distribution/Compat/Binary.hs, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Binary.o, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Binary.dyn_o )
[  2 of 139] Compiling Distribution.Compat.Exception ( src/Distribution/Compat/Exception.hs, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Exception.o, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Exception.dyn_o )
[  3 of 139] Compiling Distribution.Compat.Newtype ( src/Distribution/Compat/Newtype.hs, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Newtype.o, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Newtype.dyn_o )
[  4 of 139] Compiling Distribution.Compat.Semigroup ( src/Distribution/Compat/Semigroup.hs, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Semigroup.o, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Compat/Semigroup.dyn_o )
[  5 of 139] Compiling Distribution.PackageDescription.Utils ( src/Distribution/PackageDescription/Utils.hs, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/PackageDescription/Utils.o, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/PackageDescription/Utils.dyn_o )
[  6 of 139] Compiling Distribution.Utils.Base62 ( src/Distribution/Utils/Base62.hs, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Utils/Base62.o, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Utils/Base62.dyn_o )
[  7 of 139] Compiling Distribution.Utils.MD5 ( src/Distribution/Utils/MD5.hs, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Utils/MD5.o, /home/tchoutri/Code/cabal/dist-newstyle/build/x86_64-linux/ghc-9.12.2/Cabal-syntax-3.17.0.0/build/Distribution/Utils/MD5.dyn_o )

to

[  1 of 139] Compiling Distribution.Compat.Binary
[  2 of 139] Compiling Distribution.Compat.Exception
[  3 of 139] Compiling Distribution.Compat.Newtype
[  4 of 139] Compiling Distribution.Compat.Semigroup
[  5 of 139] Compiling Distribution.PackageDescription.Utils
[  6 of 139] Compiling Distribution.Utils.Base62
[  7 of 139] Compiling Distribution.Utils.MD5

See #25345.

Add several options for x86 extensions: -mavx512bw, -mavx512dq, -mavx512vl, and -mgfni.
Improved treatment of floating-point in constant folding. GHC will now preserve the sign of zero and NaN payloads during constant folding, fixing #9811 and #21227.

As a side effect, x + 0 will no longer be constant-folded to x, because -0 + 0 = 0, where -0 is the IEEE-754 negative zero floating point constant. Instead, it is x + -0 that is constant-folded to x.
SIMD support has been added to the AArch64 Native Code Generator. Currently, 128-bit wide vectors are supported via ASIMD (also known as NEON).
Added the -fwrite-byte-code option which makes GHC emit a .gbc file which contains a serialised representation of bytecode.

The bytecode can be loaded by the compiler to avoid having to reinterpret a module when using the bytecode interpreter (for example, in GHCi).

There are also the new options:
- -gbcdir ⟨dir⟩: Specify the directory to place the gbc files
- -gbcsuf ⟨suffix⟩: Specify the suffix for gbc files
The option -fbyte-code-and-object-code now implies -fwrite-byte-code.
Added support for building bytecode libraries. A bytecode library is a collection of bytecode files (.gbc) and a library which combines together additional object files. A bytecode library is created by invoking GHC with the -bytecodelib flag.

A bytecode library can be used to satisfy a package dependency when using the interpreter. If a user enables -fprefer-byte-code, then if a package provides a bytecode library, that will be loaded and used to satisfy the dependency (#26298).
Support statically linking executables properly (#26434). This does a number of things:
- Makes GHC aware of extra-libraries-static (this changes the package database format).
- Adds a switch -static-external that will honour extra-libraries-static to link external system dependencies statically. See also -exclude-static-external ⟨lib1,lib2,...⟩.
- Adds a new field to settings/targets: “ld supports verbatim namespace”.
- Adds a switch -fully-static that is meant as a high-level interface for e.g. cabal. This also honours extra-libraries-static.
Warn when -dynamic is mixed with -staticlib.
GHC now builds the external interpreter program on demand when it is missing (#24731). iserv has been removed from the tree completely: Hadrian no longer builds or distributes iserv, and the GHC driver uses the on-demand external interpreter by default when invoked with -fexternal-interpreter, without needing to specify -pgmi "". The on-demand external interpreter program is linked with the threaded RTS if it is available in the target RTS ways.
Support larger unboxed sums: the known constructor encoding for sums in interfaces now uses 11 bits for both the arity and the alternative (up from 8 and 6, respectively).
Changed the way data constructors are typechecked, in particular how multiplicities are made to line up (#26072, #26311).
The LoongArch64 native code generator now supports finer-grained DBAR hints on LA664 and newer microarchitectures, including for atomic writes, implements MO_BSwap and MO_BRev with bit-manipulation instructions, and adds cmpxchg and xchg primops. The new -mla664 flag enables the LA664 instructions. LA464 remains the default.
Fixed implicit lifting to use a precise level check (#26088).
-fdiagnostics-as-json output now includes the rendered diagnostic message, in the same format GHC would produce without the flag (#26173).
-fdiagnostics-as-json is now respected for error messages from pre-processors (#25480), for Core diagnostics and for driver diagnostics (see #24113).
Changed the error message of ghcExit from <no location info>: error: Compilation had errors to Compilation had errors.
Handle non-fractional CmmFloat literals (infinity and NaN) in Cmm’s common block eliminator (#26229).
Allow defining HasField instances for naughty fields (#26295).
Don’t warn about unused imports when a generated import is used (#21730).
Serialize wired-in names as external names when creating HIE files (#26238).
Fixed the simplifier firing rules or inlinings that are not active throughout the whole activation range of a RULE or stable unfolding when simplifying its right-hand side (#26323).
Non-pointer fields of data constructors are now sorted by size, always storing the largest field first, which reduces wasted padding/alignment space in closures with differently sized fields. Unboxed sums with a small enough number of constructors now use Word8, Word16 or Word32 for the tag field.
Typechecker plugins (and defaulting plugins) are now run during pattern-match checking, which significantly improves pattern-match warnings for programs that rely on typechecking plugins in order to typecheck (e.g. dealing with GADTs indexed by natural numbers and using natural number arithmetic) (#26395).
Core plugins can now access unoptimized Core: the first simple optimization pass after desugaring is now a real CoreToDo pass, which allows CorePlugins to decide whether they want to be executed before or after this pass (#23337).
Added the -dno-builtin-rules and -dno-bignum-rules debug flags to disable built-in rules and bignum rules respectively (#20298).
Fixed LLVM linking of Intel BMI intrinsics pdep{8,16} and pext{8,16} (#26065).
Fixed a regression that caused overloaded functions to no longer be specialised as effectively as in previous releases, hurting runtime performance (#26831).
Warn when -finfo-table-map is used with -fllvm, as these are currently not supported together (#26435).
The driver now recognises .dyn_o files as object files when passed on the command line, fixing plugins compiled with this suffix (#24486).
Fixed -dsuppress-uniques to also suppress the unique of free variables shown in demand signatures (#27106).
Improved the detection of custom type errors in equality constraints, which improves detection of redundant pattern match warnings (#26400).
Introduced -fno-distinct-constructor-tables and -fdistinct-constructor-tables-only=⟨cs⟩, to only generate distinct constructor tables for specific constructors (#23703).
Fixed typechecking of pattern synonym declarations without a type signature that contain unfilled metavariables (#26465).
Added hints and explanations for unsolved HasField constraints, such as similar name suggestions and import suggestions (#18776, #22382, #26480).
Improved compiler performance by making OccAnal stricter, reducing residency and improving compile times.
Fixed an oversight that caused explicit namespace specifiers of subordinate export items to be ignored; module M (T (type A)) is now rejected (#12488).
Added an HsWrapper optimiser, fixing a regression where SPECIALIZE pragmas with higher-rank quantifiers were rejected with “RULE left-hand side too complicated to desugar” (#26349).
Fixed a shadowing bug with implicit parameters (#26451).
Fixed a bug in the Cmm register-conflict analysis where an assignment’s left-hand side register was not accounted for, which could generate incorrect code (#26550).
Fixed a register allocator bug where the format of a spilled register was not updated after reassignment, which could corrupt vector register contents (#26542).
Fixed the order of spill/reload instructions emitted by the AArch64 native code generator (#26537).
Pass the +evex512 attribute to LLVM 18+ when -mavx512f is set (#26410).
Properly handle errors during the link step in the driver (#26496).
User-written kinds of data declarations are now preserved instead of being expanded during kind checking. This affects the kinds stored in interface files and thus the documentation generated by Haddock.
Units with no exposed modules are now excluded from the -Wunused-packages check (#24120).
Fixed a bug in defaulting, which was doing some unification but then failing to iterate (#26582).
ghc -M now reports all missing modules at once, as opposed to only reporting a single missing module (#26551).
Improved the performance of pattern-match overlap checking when there is a very large set of patterns (#26514).
Fixed PIC jump tables on Windows by avoiding overflows in jump tables (#24016).
Fixed an incorrect optimisation that folded multiplication, division, or remainder by an out-of-range power-of-2 literal into a shift (#25664).
Reinstated the Template Haskell level check when reporting out-of-scope identifiers, fixing misleading errors caused by staging errors (#26099).
Fixed a compiler panic when computing reachability to a module graph node that does not exist (#26568).
Reduced memory usage of the package database by using OsPath instead of FilePath in PkgDbRef and UnitDatabase.
Reduced allocations during recompilation checking by using OsPath when checking file modification times.
Fixed a number of register allocation bugs relating to register formats, some of which could corrupt vector register contents when spilling and reloading (#26411, #26526, #26668).
Enabled a Cmm comparison-folding optimisation on all architectures, previously restricted to x86/x86-64 (#26664).
Bumped the maximum supported LLVM version to 23 (#26813, #27764).
Fixed the inferred quantification order of function arrows with an explicit multiplicity, a %m -> b, to be [a, m, b] (#23764).
Fixed data family instance type class instance changes not triggering recompilation (#26705).
Fixed associated type family changes not triggering recompilation in dependent modules (#26183).
Fixed a missing InVar->OutVar lookup in SetLevels (#26681).
Fixed LLVM backend pdep/pext handling for i386 targets (#26450).
Account for “stupid theta” in the demand signature of data constructor wrappers (#26748).
Fixed split sections on Windows (#26696, #26494).
Fixed scoping errors in the type-class specialiser (#26682, #27629).
Improved case merging by floating join points out of DEFAULT alternatives (#26709).
The driver now uses -O3 as the LLVM optimization level when compiling with -O2, trading compilation time for faster generated code.
Improved newtype unwrapping in the constraint solver (#26746).
Add evals for strict data constructor arguments in worker functions (#26722).
Fixed PPC NCG shift right operations at smaller than word sizes (#26519).
Fixed split sections for the LLVM backend (#26770).
PPC NCG: generate the clear-right instruction at architecture width for all MachOp widths (#24145).
Fixed two bugs in short-cut constraint solving, one of which stopped the constraints package from compiling (#26805).
Fixed a compiler loop caused by not taking the current simplifier phase into account when computing the phase range used to simplify the right-hand side of a RULE or stable unfolding (#26826).
Fixed a subtle bug in GHC.Core.Utils.mkTick that could generate type-incorrect code (#26772).
Improved error messages for unsolved representational equalities (#15850, #20289, #20468, #23731, #25949, #26137).
Improved defaulting of representational equalities, which can now handle situations involving functional dependencies and type-family injectivity annotations.
Fixed non-determinism in TyLitMap by using deterministic maps for strings (#26846).
Under ExplicitLevelImports, imported types are now subject to the same level (staging) checks as values (#26098).
Fix the interaction of ExplicitLevelImports with qualified imports (#26616, #27385).
PPC NCG: Use libcall for 64-bit cmpxchg on 32-bit PowerPC (#23969).
NCG for PPC: add pattern for CmmRegOff to iselExpr64 (#26828).
Fixed cast worker/wrapper incorrectly firing on INLINE functions (#26903).
Fixed a long-standing interaction between ticks and casts in Eliminate Identity Case.
Fixed non-determinism in WithHsDocIdentifiers binary instance by using a stable sort (#26858).
Make the order of usages in interface files deterministic (#26877).
Fixed a bug where mergeCaseAlts could move a tick in between a join point and its jump, producing invalid Core (#26642, #26693, #26929).
Fixed negative type literals bypassing the renamer check via RequiredTypeArguments, causing the compiler to hang (#26861).
Fixed -fcheck-prim-bounds for non-constant arguments; previously bounds were only checked for constant (literal) arguments (#26958).
Fixed linking against libm on toolchains that enable --as-needed by default, by adding -lm after the object files on the link line.
Improved error messages for unsupported type literals, such as unboxed or fractional literals (#26862, #25121).
Vector literals on AArch64 without the LLVM backend now report “SIMD operations on AArch64 currently require the LLVM backend” instead of panicking.
NOINLINE pragmas are now applied to generated Typeable bindings, reducing the number of exported top level names and unfoldings, which reduces interface file sizes and the number of global/dynamic linker symbols.
Fixed the Haddock documentation for GHC.Prim (#26954).
Fixed non-determinism in the order of packages passed to the linker, which resulted in non-reproducible builds (#26838).
Fixed demand analysis giving an absent demand to an argument that was still used by the function’s stable unfolding, which could cause a run-time crash (#26416, #27626).
Constant-fold numeric literal casts when building unboxed sums during unarisation, so that top-level closures built from unboxed-sum literals are statically allocated instead of being allocated as thunks (#25650).
Fixed spurious incomplete record selector warnings for bare record field projections such as .fld (#26686).
Fixed capi wrappers failing to compile with newer GCC/Clang by collapsing chains of void pointers to a single void* (#26852).
Unpacking of enumeration types now goes to Word8#/Word16#/Word# directly instead of through an intermediate unboxed sum, allowing a branchless conversion (#26970).
Fixed an infinite loop in the type checker triggered by deep subsumption (#26823).
AArch64: fixed a register allocation bug where MOVK’s destination register was not marked as both read and written, which could clobber live values (#26980).
Implemented basic value range analysis to determine at compile time the result of some comparison primops, and to filter unreachable alternatives in case expressions (#25718).
Word-to-float conversions on x86 now emit direct assembly instead of a C function call (#22252).
Fixed a segfault caused by refining the DEFAULT alternative for dictionaries of unary typeclasses (#27071).
Fix a bug where an absent constraint argument could be replaced by an error thunk, which GHC then evaluated, crashing the program (#27627).
Fix a bug where the specialiser could drop an argument that the function’s stable unfolding still used, resulting in a runtime crash (#27703).
Fix a bug where a class declared abstractly in an hs-boot file was assumed not to be unary, so GHC speculatively evaluated a dictionary that could be bottom, crashing the program (#27704).
Fix a bug where -fspec-eval-dictfun could speculatively evaluate a looping dictionary whose recursion went through an hs-boot import, hanging the program (#27717).
Cache unit databases and use a global UnitIndex to deduplicate UnitInfos across multiple home units, reducing memory usage (#26423, #27500, #27748).
AArch64 code generation: use SXTW instead of SXTH for W32 sign extension (#26978).
On AArch64, use a logical instead of an arithmetic right shift for the unsigned right shift (MO_U_Shr) at 8/16 bit word size (#26979).
Avoid AArch64 register clobbering bug in MUL2 (#27046).
Fix incorrect overflow bit for MUL2 on AArch64 for sub-W64 operands (#27047).
-finter-module-far-jumps is now enabled by default for profiled ways on AArch64 Linux, working around jump offset overflow errors (“relocation truncated to fit: R_AARCH64_JUMP26”) from some binutils and gcc versions when linking large profiled libraries (#26994).
A number of fixes to the ARM64 ncg, fixing a number of bugs, including incorrect runtime results when using subword operations (#27430, #27539, #27538, #27537, #27550, #27533).
Fixed mkTick to avoid attaching profiling ticks to coercions, preventing a compiler panic (#27121).
Fix two crashes that could happen in a multithreaded setting when profiling (#27123).
Fix “failed to detect OverLit” panic in the pattern-match checker (#25926, #27124).
Fix getStgArgFromTrivialArg panic in CoreToStg (#27182).
Another fix for another getStgArgFromTrivialArg panic in CoreToStg (#27386).
Don’t drop cost centres around variables of type IO () (#27225).
Fix spurious -Wincomplete-uni-patterns warning under -finfo-table-map (#27314).
Recognise considerAccessible under ticks (-g, -finfo-table-map, -fhpc etc) (#27360).
Fix a panic on @ty in a pattern synonym RHS (#27440).
Fix spurious out-of-scope errors from type ty in a pattern synonym RHS (#27583).
Fix a panic on a required type argument in a pattern synonym RHS (#27586).
Stop representation-polymorphism checks from producing a coercion that fails Core Lint (#27639).
Fixed an issue that caused the specializer to sometimes loop on recursive dictionary superclasses (#27705).
Fix a CorePrep miscompilation that could project a field out of an absent dictionary, resulting in a segfault (#25924).
Fix exponential-time desugaring of nested case expressions. The scrutinee is no longer desugared a second time when recording long-distance information for the pattern-match checker (#27383, #20251).
Fix invalid cmm basic block output when proc-point splitting is enabled (wasm/llvm/unregisterised) (#27447).
Fix a token leak in the -jsem jobserver shutdown path (#27253).
Update to semaphore-compat 2.0.1 (-jsem protocol v2) (#25087).
Introduce a cache of home module name providers (#27055).
Reference the correct package in error messages when trying to import a reexported module from a hidden package (#27417).
Eliminate redundant thunks introduced by tag inference (#27005).
Rename ZonkAny to UnusedType and add pretty printing logic for it (#27390).
Fix module finalizers on multiple platforms (#27072).
Fix redundant AP thunk codegen when not using -ticky-ap-thunk (#27502).
When generating IPE stack frames, GHC now insists on using a source location that is local to the current module (#27749).
-fobject-determinism sorts the list of object files to be linked for reproducible builds (#27612).

2.1.3. GHCi¶

Added the :version command. This displays the current GHC version.
Improved bytecode loading performance by caching MallocStrings requests for repeated breakpoint strings, avoiding redundant remote heap allocation (#26995).
Added the :shell command, which works similarly to :!, except it guarantees to run the command via sh -c. On POSIX hosts the behavior is identical to :!, but on Windows it uses the msys2 shell instead of the system cmd.exe shell.
The GHCi startup banner now includes the active language edition, plus an indication of whether this was the default (#26037).
Added the -fimport-loaded-targets flag, which automatically imports all loaded targets into the GHCi session (#26866).
Added support for custom external interpreter commands, allowing GHC API clients to extend the external interpreter with their own message handlers (#26652).
Removed the size limit on unboxed tuples supported by the bytecode interpreter on 64-bit platforms (#26946).
External interpreter trace messages are now printed to stderr instead of stdout (#26807).
Initialize plugins for :set +c in GHCi (#23110).
Fixed the order in which :info lists instances, which previously depended on the order in which interface files happened to be loaded (#27532).
Fix regression to allow loading modules into the GHCi after startup (#27202, #27640).
Allocate static constructors for bytecode, fixing segfaults when certain programs are loaded by the bytecode interpreter (#25636).

2.1.4. JavaScript backend¶

The JavaScript backend now exports the HEAP8 and HEAPU8 symbols, which newer Emscripten versions require (#26290).
The JavaScript backend now supports more than 128 registers, fixing runtime ReferenceError failures for functions taking very many arguments (#26558).
Fixed JavaScript backend linking of units exposed in the unit database but not explicitly passed on the command line (#24886).
Fixed Enum serialisation in the JavaScript backend, which went through Word16 (#24593).
Fixed recompilation avoidance for the JavaScript backend (#23013).
Replaced the BigInt-based implementations of 32-bit and 64-bit quot/rem in the JavaScript backend with pure Number arithmetic, avoiding the overhead of BigInt promotion (#23597).

2.1.5. WebAssembly backend¶

The internal-interpreter flag is now enabled for the ghc library in wasm stage1, making it possible to launch a GHC API session that makes use of the internal interpreter (#26431, #25400).
The wasm dyld script can now be used to load and run wasm shared libraries fully client-side in the browser without needing a wasm32-wasi-ghci backend.
Fixed handling of ByteArray#/MutableByteArray# arguments in JSFFI imports on the wasm backend.
Use import.meta.main for proper distinction of Node.js main modules (#26916).
Added an /assets endpoint to the wasm dyld HTTP server and the -fghci-browser-assets-dir flag to specify the assets root directory, so assets can be fetched from the same host without a separate HTTP server (#26951).
Fixed an Illegal foreign declaration error when wasm GHCi loads modules with JSFFI exports (#26998).
Ensure post-linker output is synchronous ESM and fix loading in ServiceWorker (#27257).

2.1.6. Runtime system¶

Add a new poll I/O manager, based on the classic unix poll() API. It is the default I/O manager in the single-threaded RTS on all posix platforms except macOS (where the select I/O manager remains the default due to macOS platform limitations). The I/O manager can be selected via the runtime flag --io-manager=(name).

Compared to the select I/O manager, this one has slightly less severe restrictions on the number of sockets/pipes that can be waited on, though it still scales poorly for waiting on socket/pipe readiness. It does however scale much better for thread timers (such as threadDelay), and improves timer precision on 32bit Linux and 32bit FreeBSD from milliseconds to microseconds.

This I/O manager introduces new infrastructure for I/O managers that is intended to be used in future for a new generation of in-RTS I/O managers, using more scalable platform-specific APIs (such as epoll, io_uring and kqueue). This first one, based on poll(), is merely intended to be portable.
--eventlog-flush-interval=⟨seconds⟩ is now disabled when compiled against the non-threaded RTS, where using it led to eventlog corruption (#26222).
IPE entries now have a stable identifier instead of being indexed by their address, so profiles from different runs can be compared, and the IPE metadata is placed in a specific .ipe section so it can be stripped from the final binary if desired (#21766).
Added the -hT ⟨type⟩ and -hi ⟨addr⟩ heap profile filtering options, which are available in non-profiled builds (#26361). Also fixed the brace syntax of -h filter options, which had stopped working, and a bug where combining -he⟨era⟩ and -hr⟨retainer⟩ would ignore whether the retainer matches.
Removed the signal-based ticker implementations. All platforms now use the pthreads and nanosleep based ticker (#27073).
Removed the unnecessary libm, libdl and need-pthread Cabal flags from the RTS package, performing those checks via autoconf macros instead.
Used computed goto for instruction dispatch in the bytecode interpreter, improving interpreter performance (#12953).
Fixed a deadlock with eventlog flush interval and RTS shutdown (#26573).
Handle 16-bit overflow of ELF section header string table (#26603).
Fixed object file format detection in loadArchive (#26630).
Fixed a number of instances of undefined behaviour in the bytecode interpreter, including a zero-length variable length array, an unaligned read, and signed integer overflow in subword arithmetic.
Fixed an ABI mismatch in calls to the variadic barf function, both in compiler-generated code and hand-written Cmm (#22882).
shrinkMutableByteArray# now opportunistically reclaims slop space, reducing heap fragmentation, and resizeMutableByteArray# now grows the MutableByteArray# in-place if possible.
Use INFO_TABLE_CONSTR for stg_dummy_ret_closure (#26745).
Switch prim to use modern atomic compiler builtins (#26729).
The runtime linker now supports COMMON symbols (#6107).
Fixed a potential crash when decoding stack snapshots, caused by an invalid pointer left in a GC-visible slot (#27009).
Fixed a missing profiling header in the origin_thunk stack frame info table, which could cause corruption when the frame was copied by the garbage collector (#27007).
Fixed the SLIDE bytecode instruction crashing when sliding off the end of a stack chunk (#27001).
Fixed a stack alignment bug on x86 that could cause segfaults or corrupted registers when using AVX/AVX-512 vector code (#26595, #26822).
Fixed a race condition where a cloned-stack request could be mishandled after its target thread migrated to another capability (#27008).
STM no longer creates a transaction for the right-hand side of catchRetry#, avoiding quadratic behaviour for nested orElse (#26028).
Windows: fixed crashes and memory leaks in the legacy I/O manager’s handling of asynchronous I/O results (#26341).
Fixed a race condition between flushEventLog and startEventLogging/endEventLogging that could corrupt the eventlog (#27082).
Fix a possible use-after-free bug with TSOs (#26716, #26717).
Add rts Message to set/unset TSO flags (#27131).
Fix several black hole handling bugs that could lead to deadlocks or crashes in multithreaded programs. These could show up as the program hanging or “END_TSO_QUEUE object entered” errors (#26922, #26936).
Fix “unknown/strange object 24 crash” in compacting GC (#27434).
Fix a parallel GC race on weakly-ordered architectures (AArch64) that could crash or silently corrupt the heap (#27477).
Fix “unknown/strange object 1” crash in the compacting GC when collecting large data constructor closures (#27649).
Fix a crash when capturing or resuming a delimited continuation that adjusts the async exception masking state (#27651).
Fix a segfault that could occur when querying the label of an unlabeled thread (#27618).
Rethrow exceptions in overlapped IO when using the WinIO IO manager (#27283).
The RTS API now exposes the RUNTIME_TRACE_FLAG type and the getTraceFlag and setTraceFlag functions that can be used to change the trace flags at runtime (#27186).
The RTS -l flag now accepts the new event class I, which controls whether or not IPE events are emitted (#27239).

2.1.7. Cmm¶

Info tables now use half-word literals, so they are mapped to the same assembler code on big-endian and little-endian platforms (#26579).
Fix miscompiled %load_relaxed primop, add missing %store_relaxed (#27483).

2.1.8. `base` library¶

GHC.Num.{BigNat, Integer, Natural} are no longer exposed. Users should import them from ghc-bignum instead (CLC proposal #359).
GHC internals in GHC.Num have been deprecated and will be removed after one major release (CLC proposal #360).
Removed GHC.Desugar, which was deprecated and should have been removed in GHC 9.14.
Removed GHC.JS.Prim.Internal.Build (CLC proposal #329).
Modified the implementation of Data.List.sortOn to use (>) instead of compare (CLC proposal #332).
Removed extra laziness from Data.Bifunctor.Bifunctor instances for all tuples to have the same laziness as their Data.Functor.Functor counterparts, i.e. they became more strict than before (CLC proposal #339).
Adjusted the strictness of Data.List.iterate' to be more reasonable: every element of the output list is forced to WHNF when the (:) containing it is forced (CLC proposal #335).
Changed hIsReadable and hIsWritable such that they always throw a respective exception when encountering a closed or semi-closed handle, not just in the case of a file handle (CLC proposal #371).
Added {-# WARNING in "x-partial" #-} to Data.List.{init,last}. Use {-# OPTIONS_GHC -Wno-x-partial #-} to disable it (CLC proposal #292).
Added Data.List.NonEmpty.mapMaybe (CLC proposal #337).
Added thenA and thenM (CLC proposal #351).
Added nubOrd and nubOrdBy to Data.List and Data.List.NonEmpty (CLC proposal #336).
Added Semigroup and Monoid instances for Control.Monad.ST.Lazy (CLC proposal #374).
Generalized deleteBy and deleteFirstsBy (CLC proposal #372).
Added System.IO.hGetNewlineMode (CLC proposal #370).
Added a new module System.IO.OS with operations for obtaining operating-system handles (file descriptors, Windows handles) (CLC proposal #369).
Exported labelThread from Control.Concurrent (CLC proposal #376).
Fixed issues with toRational for types capable to represent infinite and not-a-number values (CLC proposal #338).
Ensured that rationalToFloat and rationalToDouble always inline in the end (CLC proposal #356).
Improved the error message for Data.Char.chr (CLC proposal #384).
GHC.Conc.throwSTM and GHC.Conc.Sync.throwSTM now carry a HasCallStack constraint and attach a Backtrace annotation to the thrown exception (#25365).
GHC.Conc.catchSTM and GHC.Conc.Sync.catchSTM now attach a WhileHandling annotation to exceptions thrown from the handler (#25365).
Backtraces for error exceptions are now evaluated at the moment they are thrown (CLC proposal #383, #26751).
Implementation details are now hidden when throwing exceptions in throw and throwSTM (CLC proposal #387).
The onException continuation is now annotated with WhileHandling (CLC proposal #397, #26759).
Added @since annotation to System.Info.fullCompilerVersion (#26973).
Don’t drop ExceptionContext in SomeException(toException) (#27455).
Fix retry and async exception delivery inside a catchSTM handler (#27657).
Show ExceptionContext in the displayExceptionAnnotation implementation of WhileHandling (#27456).

2.1.9. `ghc-prim` library¶

2.1.10. `ghc` library¶

Fixed the Data instance for ModuleName to give a non-bottom toConstr implementation (#27129).
SourceError now stores the context needed to print its diagnostics, via the new SourceErrorContext datatype (#26387).
HsTyLit has been dropped in favor of HsLit, with new HsNatural and HsDouble constructors (#26862, #25121).
thNameToGhcName has been generalised via the new HasHscEnv class, modelled on HasDynFlags. It is the recommended way of looking up names in GHC plugins.
Added an Outputable instance for Natural, as well as a natural :: Natural -> SDoc function that mirrors the existing integer function.
Renamed interpreterBackend to bytecodeBackend. The interpreterBackend binding is left as a deprecated alias.
Removed the Data instance for ClsInst.
Merged HsMultilineString into HsString, so HsLit no longer has a separate constructor for multi-line string literals (#26860).
TopLevelFlag and RuleName moved to Language.Haskell.Syntax.Basic, OverlapMode to the new module Language.Haskell.Syntax.Overlap and OverlapFlag to the new module GHC.Hs.Decls.Overlap. OverlapMode gained a TTG extension point.
Added missing (==) logic for the HsInt{8,16,32} and HsWord{8,16,32} constructors of HsLit.
ForeignCall types that do not need extension points moved to the new Language.Haskell.Syntax.Decls.Foreign module. CCallTarget, CType and Header gained TTG extension points, and their Bool parameters were replaced with descriptive data types.
Removed the backwards compatibility pattern synonym ModLocation (#24932).
Fixed several oversights in GHC.Hs.Syn.Type.hsExprType (#26910).
Drop preloadClosure from UnitState (#27308).
Fix the Data.Data instance of HsCtxt to avoid a crash (#27359).
The records for typechecker plugins and defaulting plugins have been updated to reflect the fact that these plugins may be invoked after the end of typechecking. The fields tcPluginStop and dePluginStop are replaced by tcPluginPostTc/tcPluginShutdown and dePluginPostTc/dePluginShutdown respectively. To migrate a plugin whose stop action was a simple resource release, move that action to tcPluginShutdown and set tcPluginPostTc = const (return ()) (#26839).
typecheckModule, hscTypecheckRename, hscTypecheckRenameWithDiagnostics and hscTypecheckAndGetWarnings all take an additional argument that specifies how to start/stop TcM plugins, and tcRnModule now takes a TcRnModuleOptions record (#26839).
Removed withTcPlugins, withHoleFitPlugins and withDefaultingPlugins in favour of a single withTcMPlugins (#26839).

2.1.11. `ghc-heap` library¶

Fix invalid srtlen field returned by peekItbl when no tables-next-to-code (#27465).

2.1.12. `ghc-internal` library¶

Fixed a crash when decoding a captured stack containing a bytecode object with an empty payload bitmap (#26640).

2.1.13. `ghc-experimental` library¶

New SIMD primops for bitwise logical operations on 128-wide vectors, and new abs/sqrt SIMD primops for absolute value and square root, such as absInt32X4# and sqrtDoubleX4#. These are supported by the LLVM backend and the X86_64 NCG backend (for the latter, only for 128-wide vectors).
Fixed the GHC.Exception.Backtrace.Experimental module, which was not compiled or included in the library.
Added an optional SrcLoc to the StackAnnotation class in GHC.Stack.Annotation.Experimental (#26806).
Added a cumulative gc_sync_elapsed_ns counter to RTSStats (#26944).
Exposed decodeStackWithIpe and related stack-decoding helpers from ghc-experimental (#27065).

2.1.14. `template-haskell` library¶

Introduced namedDefaultQuasiQuoter and defaultQuasiQuoter, which fail with a helpful error when used in an inappropriate context (#24434).
We have added the addDependentDirectory function to match addDependentFile, which adds a directory to the list of dependencies that the recompilation checker will look at to determine if a module needs to be recompiled.

2.1.15. Packaging and Build System¶

The minimal bootstrap GHC version has been bumped to 9.10.
configure no longer probes for the gold linker, which has been dropped from binutils 2.44 (#25716).
configure now checks that python3 is at least version 3.7, as the testsuite driver requires (#23234), and a missing python is now a warning instead of an error, so it is not mandatory when installing a bindist (#26347).
Hadrian now enables terminfo if --with-curses-* flags are given to configure, and cross-builds build terminfo only in upper stages, which re-enables building cross-compilers with terminfo (#26288).
GHC now reads lib/targets/default.target, a serialized ghc-toolchain Target, and the now-redundant entries have been removed from lib/settings (#24212).
CFLAGS, CXXFLAGS and similar flags given to configure apply to building GHC itself and are no longer carried over to the target settings used by GHC at runtime (#25637).
The bindist configure script now checks cc consistently with the source configure script (#26394), and the build, host and target platforms chosen at the initial configure are correctly propagated to it (#21970).
Hadrian builds the in-tree gmp with -fvisibility=hidden, so gmp symbols are no longer exported by the ghc-internal shared library.
ghc-toolchain now detects the PowerPC 64-bit ABI (#26521).
Fixed an -Wincompatible-pointer-types error in the utimbuf FFI wrapper on Windows with recent toolchains (#26337).
Hadrian now places user-supplied arguments after package arguments, making it easier to override default package arguments in UserSettings.hs (#25821).
Added the with_profiled_libs Hadrian flavour transformer, the exact opposite of no_profiled_libs, and fixed the profiled_ghc flavour transformer to include profiled dynamic libraries.
Hadrian now only installs the JavaScript files required by the wasm and JS targets into libdir for those targets, instead of unconditionally on other targets as well.
Removed the --via-asm hsc2hs flag when cross compiling to Windows, which fails with recent llvm-mingw toolchains.
libffi is now built and bundled via the new libffi-clib submodule and Haskell package instead of bespoke Hadrian logic. The ability to link against a system libffi is retained.
Removed a build-system hack that forced -fno-PIC on RTS objects on i386, restoring compatibility with modern toolchains (#26792).
ghc-toolchain now also configures windres on non-Windows platforms, as it may be needed for cross compilation (#24588).
ghc-toolchain and configure now check for a C11-capable C compiler instead of C99 (#26908).
The build system now accepts version 2.2 of the happy parser generator.
Deprecated Hadrian’s --bignum flag in favour of the +native_bignum flavour transformer, which is now enabled automatically for the JS target.
Hadrian now builds profiled dynamic objects with -dynamic-too, improving build parallelism (#27010).
Installed the rts/Types.h header when using the JavaScript backend, fixing use of HsFFI.h (#27033).
Fixed the --target support check in configure to use the stage0 C compiler instead of $CC (#26999).
configure now defaults $LLVMAS to $CC when $CcLlvmBackend is YES, instead of an auto-detected clang from the environment (#26769).
Fixed building the RTS with -Werror on glibc 2.43, which changed the values of _XOPEN_SOURCE and _POSIX_C_SOURCE (#27076).
Debian 9 and 10 and Ubuntu 18.04 and 20.04 are end of life and have been dropped from CI and the release bindists. Debian 13 has been added (#25876).
Fixed the build of getExecutablePath on GNU/Hurd, and GNU/Hurd is now advertised as gnu, like the autotools do.
Bumped the process submodule to 1.6.30.0, which includes a fix for a segfault on macOS 15 with certain command line SDK versions (#27144).
Hadrian no longer leaves stale .conf files in its package databases when rebuilding in the same build root with different settings (e.g. another flavour, or when hashes change with +hash-unit-ids) (#26661).

2.1.16. `ghc-pkg`¶

Removed traceId from the ghc-pkg executable.
Improve performance of ghc-pkg list command (#27275).

2.1.17. 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	10.0.0.20260917	The compiler itself
Cabal-syntax	3.18.1.0	Dependency of `ghc-pkg` utility
Cabal	3.18.1.0	Dependency of `ghc-pkg` utility
Win32	2.14.2.2	Dependency of `ghc` library
array	0.5.8.0	Dependency of `ghc` library
base	4.23.0.0	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	1.5.2.0	Dependency of `ghc` library
directory	1.3.11.0	Dependency of `ghc` library
exceptions	0.10.12	Dependency of `ghc` and `haskeline` library
file-io	0.2.0	Dependency of `directory` library
filepath	1.5.5.0	Dependency of `ghc` library
ghc-boot-th	10.0.0.20260917	Internal compiler library
ghc-boot	10.0.0.20260917	Internal compiler library
ghc-compact	0.1.0.0	Core library
ghc-heap	10.0.0.20260917	GHC heap-walking library
ghc-prim	0.14.0	Core library
ghci	10.0.0.20260917	The REPL interface
haskeline	0.8.5.0	Dependency of `ghci` executable
hpc	0.7.0.2	Dependency of `hpc` executable
integer-gmp	1.1	Core library
mtl	2.3.2	Dependency of `Cabal` library
os-string	2.0.11	Dependency of `filepath` library
parsec	3.1.18.0	Dependency of `Cabal` library
pretty	1.1.3.6	Dependency of `ghc` library
process	1.6.30.0	Dependency of `ghc` library
stm	2.5.3.1	Dependency of `haskeline` library
template-haskell	2.25.0.0	Core library
terminfo	0.4.1.7	Dependency of `haskeline` library
text	2.1.4	Dependency of `Cabal` library
time	1.16.0.1	Dependency of `ghc` library
transformers	0.6.3.0	Dependency of `ghc` library
unix	2.8.8.0	Dependency of `ghc` library
xhtml	3000.4.1.0	Dependency of `haddock` executable
haddock-api	2.34.0	Dependency of `haddock` executable
haddock-library	1.11.0	Dependency of `haddock` executable

2.1. Version 10.0.1¶

2.1.1. Language¶

2.1.2. Compiler¶

2.1.3. GHCi¶

2.1.4. JavaScript backend¶

2.1.5. WebAssembly backend¶

2.1.6. Runtime system¶

2.1.7. Cmm¶

2.1.8. base library¶

2.1.9. ghc-prim library¶

2.1.10. ghc library¶

2.1.11. ghc-heap library¶

2.1.12. ghc-internal library¶

2.1.13. ghc-experimental library¶

2.1.14. template-haskell library¶