2.1. Version 9.6.1¶
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.
The LLVM backend
of this release is to be used with LLVM
10, 11, 12, 13, or 14.
2.1.1. Language¶
Record updates for GADTs and other existential datatypes are now fully supported.
For example:
data D b where MkD :: { fld1 :: a -> a, fld2 :: a -> (), fld3 :: b } -> D b foo :: D b -> D b foo d = d { fld1 = id, fld2 = const () }
In this example, we have an existential variable
a
, and we update all fields whose type involvesa
at once, so the update is valid.A side-effect of this change is that GHC now rejects some record updates involving fields whose types contain type families (these record updates were previously erroneously accepted).
Example:
type family F a where F Int = Char F Float = Char data T b = MkT { x :: [Int], y :: [F b] } emptyT :: forall b. T b emptyT = MkT [] [] bar :: T Int bar = emptyT { x = [3] }
In this example, we can’t infer the type of
emptyT
inbar
: it could beT Int
, but it could also beT Float
because the type familyF
is not injective andT Float ~ T Int
. Indeed, the following typechecksbaz :: T Int baz = case ( emptyT :: T Float ) of { MkT _ y -> MkT [3] y }
This means that the type of
emptyT
is ambiguous in the definition ofbar
above, and thus GHC rejects the record update:Couldn't match type `F b0' with `Char' Expected: [F Int] Actual: [F b0] NB: ‘F’ is a non-injective type family The type variable ‘b0’ is ambiguous
To fix these issues, add a type signature to the expression that the record update is applied to (
emptyT
in the example above), or add an injectivity annotation to the type family in the case that the type family is in fact injective.Error messages are now assigned unique error codes, of the form
[GHC-12345]
.GHC Proposal #106 has been implemented, introducing a new language extension
TypeData
. This extension permitstype data
declarations as a more fine-grained alternative toDataKinds
.GHC now does a better job of solving constraints in the presence of multiple matching quantified constraints. For example, if we want to solve
C a b Int
and we have matching quantified constraints:forall x y z. (Ord x, Enum y, Num z) => C x y z forall u v. (Enum v, Eq u) => C u v Int
Then GHC will use the second quantified constraint to solve
C a b Int
, as it has a strictly weaker precondition.GHC proposal #170 Unrestricted OverloadedLabels has been implemented. This extends the variety syntax for constructing labels under
OverloadedLabels
. Examples of newly allowed syntax: - Leading capital letters: #Foo equivalant to getLabel @”Foo” - Numeric characters: #1728 equivalent to getLabel @”1728” - Arbitrary strings: #”Hello, World!” equivalent to getLabel @”Hello, World!”
2.1.2. Compiler¶
- The WebAssembly backend
has been merged. This allows GHC to be built as a cross-compiler
that targets
wasm32-wasi
and compiles Haskell code to self-contained WebAssembly modules that can be executed on a variety of different runtimes. There are a few caveats to be aware of:- To use the WebAssembly backend, one would need to follow the
instructions on ghc-wasm-meta. The WebAssembly
backend is not included in the GHC release bindists for the time
being, nor is it supported by
ghcup
orstack
yet. - The WebAssembly backend is still under active development. It’s presented in this GHC version as a technology preview, bugs and missing features are expected.
- To use the WebAssembly backend, one would need to follow the
instructions on ghc-wasm-meta. The WebAssembly
backend is not included in the GHC release bindists for the time
being, nor is it supported by
- The
TypeInType
is now marked as deprecated. Its meaning has been included inPolyKinds
andDataKinds
. - The
-Woperator-whitespace
warning no longer ignores constructor symbols (operators starting with:
). - The
-Wstar-is-type
warning is now enabled by default, continuing the implementation of GHC proposal #143.
2.1.3. GHCi¶
GHCi will now accept any file-header pragmas it finds, such as
{-# OPTIONS_GHC ... #-}
and{-# LANGUAGE ... #-}
(see Pragmas). For example, instead of using:set
to enable-Wmissing-signatures
, you could instead write:ghci> {-# OPTIONS_GHC -Wmissing-signatures #-}
This can be convenient when pasting large multi-line blocks of code into GHCi.
2.1.4. Runtime system¶
- The Delimited continuation primops
proposal has been implemented, adding native support for first-class,
delimited continuations to the RTS. For the reasons given in the proposal,
no safe API to access this functionality is provided anywhere in
base
. Instead, theprompt#
andcontrol0#
primops are intended to be consumed by library authors directly, who may wrap them a safe API that maintains the necessary invariants. See the documentation inGHC.Prim
for more details. - The behaviour of the
-M
flag has been made more strict. It will now trigger a heap overflow if the total amount of memory used by the Haskell heap exceeds the limit. Previously only live blocks were taken into account. This makes it more likely to trigger promptly when the heap is highly fragmented. - Fixed a bug that sometimes caused live sparks to be GC’ed too early either during minor GC or major GC with workstealing disabled. See #22528.
2.1.5. base
library¶
- Exceptions thrown by weak pointer finalizers are now caught and reported
via a global exception handler. By default this handler reports the error
to
stderr
although this can be changed usingGHC.Weak.Finalize.setFinalizerExceptionHandler
. - GHC now provides a set of operations for introspecting on the threads of a
program,
GHC.Conc.listThreads
, as well as operations for querying a thread’s label (GHC.Conc.threadLabel) and status (GHC.Conc.threadStatus). - Change default
Ord
implementation of(>=)
,(>)
, and(<)
to use(<=)
instead ofcompare
per CLC proposal: https://github.com/haskell/core-libraries-committee/issues/24 - Updated to Unicode 15.0.0.
- Add standard Unicode case predicates Data.Char.isUpperCase and Data.Char.isLowerCase. These predicates use the standard Unicode case properties and are more intuitive than Data.Char.isUpper and Data.Char.isLower.
2.1.6. ghc-prim
library¶
2.1.7. ghc
library¶
- Add Foreign.C.Types.ConstPtr was added to encode
const
-qualified pointer return types in foreign declarations when usingCApiFFI
extension.
2.1.8. ghc-heap
library¶
2.1.8.1. 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 | 9.6.0.20230210 | The compiler itself |
Cabal-syntax | 3.9.0.0 | Dependency of ghc-pkg utility |
Cabal | 3.9.0.0 | Dependency of ghc-pkg utility |
Win32 | 2.13.3.0 | Dependency of ghc library |
array | 0.5.4.0 | Dependency of ghc library |
base | 4.18.0.0 | Core library |
binary | 0.8.9.1 | Dependency of ghc library |
bytestring | 0.11.4.0 | Dependency of ghc library |
containers | 0.6.7 | Dependency of ghc library |
deepseq | 1.4.8.0 | Dependency of ghc library |
directory | 1.3.8.0 | Dependency of ghc library |
exceptions | 0.10.7 | Dependency of ghc and haskeline library |
filepath | 1.4.100.0 | Dependency of ghc library |
ghc-boot-th | 9.6.0.20230210 | Internal compiler library |
ghc-boot | 9.6.0.20230210 | Internal compiler library |
ghc-compact | 0.1.0.0 | Core library |
ghc-heap | 9.6.0.20230210 | GHC heap-walking library |
ghc-prim | 0.10.0 | Core library |
ghci | 9.6.0.20230210 | The REPL interface |
haskeline | 0.8.2 | Dependency of ghci executable |
hpc | 0.6.2.0 | Dependency of hpc executable |
integer-gmp | 1.1 | Core library |
libiserv | 9.6.0.20230210 | Internal compiler library |
mtl | 2.3.1 | Dependency of Cabal library |
parsec | 3.1.16.1 | Dependency of Cabal library |
pretty | 1.1.3.6 | Dependency of ghc library |
process | 1.6.16.0 | Dependency of ghc library |
stm | 2.5.1.0 | Dependency of haskeline library |
template-haskell | 2.20.0.0 | Core library |
terminfo | 0.4.1.5 | Dependency of haskeline library |
text | 2.0.1 | Dependency of Cabal library |
time | 1.12.2 | Dependency of ghc library |
transformers | 0.6.1.0 | Dependency of ghc library |
unix | 2.8.0.0 | Dependency of ghc library |
xhtml | 3000.2.2.1 | Dependency of haddock executable |