2.1. Version 9.0.1¶

The significant changes to the various parts of the compiler are listed in the following sections.

2.1.1. Highlights¶

The LinearTypes extension enables linear function syntax a #-> b, as described in the Linear Types GHC proposal.

The GADT syntax can be used to define data types with linear and nonlinear fields.

This extension is considered experimental: it doesn’t implement the full proposal yet and the details are subject to change.
NCG
- The linear register allocator saw improvements reducing the number of redundant move instructions. Rare edge cases can see double digit improvements in runtime for inner loops.
  
  In the mean this improved runtime by about 0.8%. For details see ticket #17823.
Pattern-Match Coverage Checking
- The revamp of the pattern-match coverage checker that started in 8.10 concludes with this release and implements the novel *Lower Your Guards* algorithm.
- Compared to 8.10, end users might notice improvements to “long-distance information”:
```
f True = 1
f x    = ... case x of { False -> 2; True -> 3 } ...
```
  GHC is now able to detect the case alternative returning 3 as redundant.
- Some more performance improvements in edge cases.
Windows: Use the large address-space allocator.

This improves runtime but causes increased memory usage on Windows versions older than Win 8.1/Server 2012.
Big-number support
- GHC now relies on a new ghc-bignum package to provide Integer/Natural implementations. This package supports the following backends:
  - gmp: adapted from integer-gmp package that was used before
  - native: new Haskell implementation, faster than integer-simple which is not used anymore
- All backends now use the same representation for big numbers (the one that was previously used only by integer-gmp). It led to several compiler simplifications, performance improvements and bug fixes (e.g. #15262, #15286).
- All backends must provide exactly the same set of functions with deterministic results so that they can be tested one against the other (they can only differ in performance). As a consequence, some functions that were only provided by integer-gmp (prime test, secure powmod, etc.) are no longer provided by ghc-bignum. Note that other packages (e.g. hgmp) provide these functions.
- For now GHC still doesn’t allow dynamic selection of the ghc-bignum backend to use.
Breaking change: Template Haskell splices now act as separation points between constraint solving passes. It is no longer possible to use an instance of a class before a splice and define that instance after a splice. For example, this code now reports a missing instance for C Bool:
```
class C a where foo :: a
bar :: Bool
bar = foo
$(return [])
instance C Bool where foo = True
```

Support for 32-bit Windows has officially been dropped as Microsoft has formally discontinued new 32-bit Windows 10 releases in 2020. See #18487 for details.

2.1.2. Full details¶

2.1.2.1. Language¶

Record field selectors are now given type signatures that preserve the user-written order of quantified type variables. Moreover, field selector type signatures no longer make inferred type variables available for explicit type application. See Field selectors and TypeApplications for more details.

In certain situations, this will constitute a breaking change as this can affect TypeApplications. For instance, given the following definitions:
```
{-# LANGUAGE PolyKinds #-}

newtype P a = MkP { unP :: Proxy a }

newtype N :: Type -> Type -> Type where
  MkN :: forall b a. { unN :: Either a b } -> N a b
```
Previous versions of GHC would give the following types to unP and unN:
```
unP :: forall k (a :: k). P a -> Proxy a
unN :: forall a b. N a b -> Either a b
```
GHC will now give them the following types instead:
```
unP :: forall {k} (a :: k). P a -> Proxy a
unN :: forall b a. N a b -> Either a b
```
In obscure scenarios, GHC now rejects programs it previously accepted, but with unhelpful types. For example, if (with -XPartialTypeSignatures) you were to write x :: forall (f :: forall a (b :: a -> Type). b _). f _, GHC previously would have accepted x, but its type would have involved the mysterious Any internal type family. Now, GHC rejects, explaining the situation.
GHC now more faithfully implements the instance-lookup scheme described with QuantifiedConstraints. Previous bugs meant that programs like this were accepted:
```
data T (c :: Type -> Constraint)
instance (forall h. c h => Functor h) => Functor (T c)
instance (forall h. c h => Applicative h) => Applicative (T c)
```
Note that in the instance declaration for Applicative (T c), we cannot prove Functor (T c), because the quantified constraint shadows the global instance. There is an easy workaround, though: just include Functor (T c) as an assumption.
```
instance (forall h. c h => Applicative h, Functor (T c)) => Applicative (T c)
```
There is a chance we will tweak the lookup scheme in the future, to make this workaround unnecessary.
GHC now consistently does eager instantiation during type inference. As a consequence, visible type application (VTA) now only works when the head of the application is:
- A variable
- An expression with a type signature
For example (let x = blah in id) @Bool True no longer typechecks. You should write let x = blah in id @Bool True instead.

This change prepares the way for Quick Look impredicativity.
GHC now implements simplified subsumption, as described in GHC Proposal #287. This change simplifies the type system, and prevents the possibility of GHC silently changing the semantics of user programs, but it does mean that some libraries may need eta-expansion to typecheck. More info here: Subsumption.

This change also prepares the way for Quick Look impredicativity.
GHC now allows users to manually define the specificity of type variable binders. By marking a variable with braces {tyvar} or {tyvar :: kind}, it becomes inferred despite appearing in a type signature. This feature effectively allows users to choose which variables can or can’t be instantiated through visible type application. More information can be found here: Manually defining inferred variables.
GADT constructor types now properly adhere to The forall-or-nothing rule. As a result, GHC will now reject some GADT constructors that previous versions of GHC would accept, such as the following:
```
data T where
  MkT1 :: (forall a. a -> b -> T)
  MkT2 :: (forall a. a -> T)
```
MkT1 and MkT2 are rejected because the lack of an outermost forall triggers implicit quantification, making the explicit foralls nested. Furthermore, GADT constructors do not permit the use of nested foralls, as explained in Formal syntax for GADTs.

In addition to rejecting nested foralls, GHC is now more stringent about rejecting uses of nested contexts in GADT constructors. For example, the following example, which previous versions of GHC would accept, is now rejected:
```
data U a where
  MkU :: (Show a => U a)
```
GHC more strictly enforces the rule that the type in the top of an instance declaration is not permitted to contain nested foralls or contexts, as documented in Formal syntax for instance declaration types. For example, the following examples, which previous versions of GHC would accept, are now rejected:

instance (forall a. C a) where … instance (Show a => C a) where …

In addition, GHC now enforces the rule that the types in deriving clauses and via types (for instances derived with DerivingVia) cannot contain nested foralls or contexts. For example, the following examples, which previous versions of GHC would accept, are now rejected:
```
data T = MkT deriving (C1, (forall x. C2 x))
deriving via (forall x. V x) instance C (S x)
```

A new language extension QualifiedDo is implemented, allowing to qualify a do block to control which operations to use for desugaring do syntax.

{-# LANGUAGE QualifiedDo #-}
import qualified SomeModule as M

f x = M.do           -- desugars to:
  y <- M.return x    -- M.return x M.>>= \y ->
  M.return y         -- M.return y M.>>
  M.return y         -- M.return y

See Qualified do-notation for more details.

LexicalNegation is a new extension that detects whether the minus sign stands for negation during lexical analysis by checking for the surrounding whitespace:
```
a = x - y  -- subtraction
b = f -x   -- negation

f = (- x)  -- operator section
c = (-x)   -- negation
```
The behavior of NegativeLiterals changed, and now we require that a negative literal must not be preceded by a closing token (see GHC Proposal #229 for the definition of a closing token). In other words, we parse f -123 as f (-123), but x-123 as (-) x 123. Before this amendment, NegativeLiterals caused x-123 to be parsed as x(-123).

2.1.2.2. Compiler¶

A new flag -flink-rts to enable linking the RTS when linking shared libraries.

2.1.2.3. GHCi¶

The :script command now allows for file names that contain spaces to passed as arguments: either by enclosing the file names in double quotes or by escaping spaces in file names with a backslash. (#18027)
The GHCi debugger syntax :break <qualified.name> now allows to set breakpoints on all functions. The restrictions top-Level and exported have been removed. Hence it’s now possible to use this syntax to set breakpoints on functions defined in nested where or let clauses.

2.1.2.4. Runtime system¶

-N without a count now tries to respect the number of processors in the process’s affinity mask, making GHC’s behavior more predictable in containerized settings (#14781).
Support for Windows Vista has been dropped. GHC-compiled programs now require Windows 7 or later.
Windows now uses the large address space allocator by default. In extreme cases we saw improvements by up to 3% decreased runtime.

The downside is that haskell apps run on older (Pre Win-8.1/Server 2012) systems will have higher memory footprints.

2.1.2.5. Template Haskell¶

Implement the Overloaded Quotations proposal (#246). The type of all quotation forms have now been generalised in terms of a minimal interface necessary (the Quote type class) for the implementation rather than the overapproximation of the Q monad.
Template Haskell quotes now handle fixity declarations in let and where bindings properly. Previously, such fixity declarations would be dropped when quoted due to a Template Haskell bug.
The -XTemplateHaskellQuotes extension now allows nested splices as nested splices do not lead directly to compile-time evaluation. (Merge request !2288)

2.1.2.6. Arrow notation¶

When combined with Arrows, the LambdaCase extension now additionally allows \case syntax to be used as a command in proc notation.
When combined with Arrows, the effects of the BlockArguments extension now also apply to applications of arrow control operators in (| banana brackets |):
```
(| untilA (increment -< x + y) do
     within 0.5 -< x
     ... |)
```

2.1.2.7. Haddock¶

Parsing is now more robust to insufficiently indented Haddock comments:

class C a where
  f :: a -> a
-- ^ This comment used to trigger a parse error
  g :: a -> a

-Winvalid-haddock is a new warning that reports discarded Haddock comments that cannot be associated with AST elements:
```
myValue =
  -- | Invalid (discarded) comment in an expression
  2 + 2
```
When faced with several comments for a data constructor or a data constructor field, Haddock now picks the first one instead of the last one. The extraneous comment is reported as invalid when -Winvalid-haddock is enabled:
```
data T
  -- | First comment
  = MkT
  -- ^ Second comment (rejected)
```
Haddock is now more relaxed about the placement of comments in types relative to the function arrow ->, allowing more formatting styles:
```
f :: Int ->   -- ^ comment on Int (no longer a parse error)
     Bool     -- ^ comment on Bool
```
Haddock can now parse the documentation comment for the first declaration in a module without a module header (#17561):
```
-- | This comment used to trigger a parse error
main = putStrLn "Hello"
```

2.1.2.8. `ghc-prim` library¶

Add a known-key cstringLength# to GHC.CString that is eligible for constant folding by a built-in rule.

2.1.2.9. `ghc` library¶

The type of the getAnnotations function has changed to better reflect the fact that it returns two different kinds of annotations, those on names and those on modules:

getAnnotations :: Typeable a
               => ([Word8] -> a) -> ModGuts
               -> CoreM (ModuleEnv [a], NameEnv [a])

The meaning of the hs_fixds field of HsGroup has changed slightly. It now only contains fixity signatures defined for top-level declarations and class methods defined outside of the class itself. Previously, hs_fixds would also contain fixity signatures for class methods defined inside the class, such as the fixity signature for m in the following example:
```
class C a where
  infixl 4 `m`
  m :: a -> a -> a
```
If you wish to attain the previous behavior of hs_fixds, use the new hsGroupTopLevelFixitySigs function, which collects all top-level fixity signatures, including those for class methods defined inside classes.
The Exception module was boiled down acknowledging the existence of the exceptions dependency. In particular, the ExceptionMonad class is not a proper class anymore, but a mere synonym for MonadThrow, MonadCatch, MonadMask (all from exceptions) and MonadIO. All of g*-functions from the module (gtry, gcatch, etc.) are erased, and their exceptions-alternatives are meant to be used in the GHC code instead.
parseModule is now the only parser entry point that deals with Haddock comments. The other entry points (parseDeclaration, parseExpression, etc) do not insert the Haddock comments into the AST.

2.1.2.10. `base` library¶

ForeignPtrContents has a new nullary data constructor FinalPtr. FinalPtr is intended for turning a primitive string literal into a ForeignPtr. Unlike PlainForeignPtr, FinalPtr does not have a finalizer. Replacing PlainForeignPtr that has NoFinalizers with FinalPtr reduces allocations, reduces the size of compiled binaries, and unlocks important Core-to-Core optimizations. FinalPtr will be used in an upcoming bytestring release to improve the performance of ByteString literals created with OverloadedStrings.

2.1.2.11. Build system¶

2.1.3. Bootstrapping requirements¶

GHC now requires a C compiler which supports __atomic_op_n builtins. This raises the requirement for GCC to 4.7.

2.1.4. 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	9.0.0.20200925	The compiler itself
Cabal	3.4.0.0	Dependency of `ghc-pkg` utility
Win32	2.10.0.0	Dependency of `ghc` library
array	0.5.4.0	Dependency of `ghc` library
base	4.15.0.0	Core library
binary	0.8.8.0	Dependency of `ghc` library
bytestring	0.10.12.0	Dependency of `ghc` library
containers	0.6.2.1	Dependency of `ghc` library
deepseq	1.4.4.0	Dependency of `ghc` library
directory	1.3.6.1	Dependency of `ghc` library
exceptions	0.10.4	Dependency of `ghc` and `haskeline` library
filepath	1.4.2.1	Dependency of `ghc` library
ghc-boot-th	9.0.0.20200925	Internal compiler library
ghc-boot	9.0.0.20200925	Internal compiler library
ghc-compact	0.1.0.0	Core library
ghc-heap	9.0.0.20200925	GHC heap-walking library
ghc-prim	0.7.0	Core library
ghci	9.0.0.20200925	The REPL interface
haskeline	0.8.1.0	Dependency of `ghci` executable
hpc	0.6.1.0	Dependency of `hpc` executable
integer-gmp	1.1	Core library
libiserv	9.0.0.20200925	Internal compiler library
mtl	2.2.2	Dependency of `Cabal` library
parsec	3.1.14.0	Dependency of `Cabal` library
pretty	1.1.3.6	Dependency of `ghc` library
process	1.6.10.0	Dependency of `ghc` library
stm	2.5.0.0	Dependency of `haskeline` library
template-haskell	2.17.0.0	Core library
terminfo	0.4.1.4	Dependency of `haskeline` library
text	1.2.4.0	Dependency of `Cabal` library
time	1.9.3	Dependency of `ghc` library
transformers	0.5.6.2	Dependency of `ghc` library
unix	2.7.2.2	Dependency of `ghc` library
xhtml	3000.2.2.1	Dependency of `haddock` executable