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2. Introduction to GHC

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3. Release notes for version 8.4.1

The significant changes to the various parts of the compiler are listed in the following sections. There have also been numerous bug fixes and performance improvements over the 8.2.1 release.


This compiling this release requires GCC 4.7 or newer due to Trac #14244.

3.1. Highlights

The highlights, since the 8.2.1 release, are:

  • GHC is now capable of deriving more instances
  • More refinement of the -XTypeInType story and improvements in type error messages.
  • Further improvements in code generation
  • Incoporation of library changes including the Phase 2 of the Semigroup-Monoid Proposal
  • A variety of Windows compatibility improvements
  • Many, many bug fixes.

3.2. Full details

3.2.1. Language

  • Data families have been generalised a bit: a data family declaration can now end with a kind variable k instead of Type. Additionally, data/newtype instance no longer need to list all the patterns of the family if they don’t wish to; this is quite like how regular datatypes with a kind signature can omit some type variables.

  • There are now fewer restrictions regarding whether kind variables can appear on the right-hand sides of type and data family instances. Before, there was a strict requirements that all kind variables on the RHS had to be explicitly bound by type patterns on the LHS. Now, kind variables can be implicitly bound, which allows constructions like these:

    data family Nat :: k -> k -> *
    -- k is implicitly bound by an invisible kind pattern
    newtype instance Nat :: (k -> *) -> (k -> *) -> * where
      Nat :: (forall xx. f xx -> g xx) -> Nat f g
    class Funct f where
      type Codomain f :: *
    instance Funct ('KProxy :: KProxy o) where
      -- o is implicitly bound by the kind signature
      -- of the LHS type pattern ('KProxy)
      type Codomain 'KProxy = NatTr (Proxy :: o -> *)
  • Implicitly bidirectional pattern synonyms no longer allow bang patterns (!) or irrefutable patterns (~) on the right-hand side. Previously, this was allowed, although the bang patterns and irrefutable patterns would be silently ignored when used in an expression context. This is now a proper error, and explicitly bidirectional pattern synonyms should be used in their stead. That is, instead of using this (which is an error):

    data StrictJust a = Just !a

    Use this:

    data StrictJust a <- Just !a where
      StrictJust !a = Just a
  • GADTs with kind-polymorphic type arguments now require -XTypeInType. For instance, consider the following,

    data G :: k -> * where
      GInt   :: G Int
      GMaybe :: G Maybe

    In previous releases this would compile with -XPolyKinds alone due to bug Trac #13391. As of GHC 8.4, however, this requires -XTypeInType. Note that since GADT kind signatures aren’t generalized, this will also require that you provide a CUSK by explicitly quantifying over the kind argument, k,

    data G :: forall k. k -> * where
      GInt   :: G Int
      GMaybe :: G Maybe
  • The order in which type variables are quantified in GADT constructor type signatures has changed. Before, if you had MkT as below:

    data T a where
      MkT :: forall b a. b -> T a

    Then the type of MkT would (counterintuitively) be forall a b. b -> T a! Now, GHC quantifies the type variables in the order that the users writes them, so the type of MkT is now forall b a. b -> T a (this matters for -XTypeApplications).

  • The new -XEmptyDataDeriving extension allows deriving Eq, Ord, Read, and Show instances directly for empty data types, as in data Empty deriving Eq. (Previously, this would require the use of -XStandaloneDeriving to accomplish.)

    One can also now derive Data instances directly for empty data types (as in data Empty deriving Data) without needing to use -XStandaloneDeriving. However, since already requires a GHC extension (-XDeriveDataTypeable), one does not need to enable -XEmptyDataDeriving to do so. This also goes for other classes which require extensions to derive, such as -XDeriveFunctor.

  • Hexadecimal floating point literals (e.g. 0x0.1p4), enabled with -XHexFloatLiterals. See Hexadecimal floating point literals for the full details.

3.2.2. Compiler

  • LLVM code generator (e.g. -fllvm) compatible with LLVM releases in the 5.0 series.

  • Add warning flag -Wmissing-export-lists which causes the type checker to warn when a module does not include an explicit export list.

  • The configure script now no longer accepts --with-TOOL flags (e.g. --with-nm, --with-ld, etc.). Instead, these are taken from environment variables, as is typical in autoconf scripts. For instance, ./configure --with-nm=/usr/local/bin/nm turns into ./configure NM=/usr/local/bin/nm.

  • Derived Functor, Foldable, and Traversable instances are now optimized when their last type parameters have phantom roles. Specifically,

    fmap _ = coerce
    traverse _ x = pure (coerce x)
    foldMap _ _ = mempty

    These definitions of foldMap and traverse are lazier than the ones we would otherwise derive, as they may produce results without inspecting their arguments at all.

    See also Deriving Functor instances, Deriving Foldable instances, and Deriving Traversable instances.

  • Derived instances for empty data types are now substantially different than before. Here is an overview of what has changed. These examples will use a running example of data Empty a to describe what happens when an instance is derived for Empty:

    • Derived Eq and Ord instances would previously emit code that used error:

      instance Eq (Empty a) where
        (==) = error "Void =="
      instance Ord (Empty a) where
        compare = error "Void compare"

      Now, they emit code that uses maximally defined, lazier semantics:

      instance Eq (Empty a) where
        _ == _ = True
      instance Ord (Empty a) where
        compare _ _ = EQ
    • Derived Read instances would previous emit code that used parens:

      instance Read (Empty a) where
        readPrec = parens pfail

      But parens forces parts of the parsed string that it doesn’t need to. Now, the derived instance will not use parens (that it, parsing Empty will always fail, without reading any input):

      instance Read (Empty a) where
        readPrec = pfail
    • Derived Show instances would previously emit code that used error:

      instance Show (Empty a) where
        showsPrec = error "Void showsPrec"

      Now, they emit code that inspects the argument. That is, if the argument diverges, then showing it will also diverge:

      instance Show (Empty a) where
        showsPrec _ x = case x of {}
    • Derived Functor, Foldable, Traversable, Generic, Generic1, Lift, and Data instances previously emitted code that used error:

      instance Functor Empty where
        fmap = error "Void fmap"
      instance Foldable Empty where
        foldMap = error "Void foldMap"
      instance Traversable Empty where
        traverse = error "Void traverse"
      instance Generic (Empty a) where
        from = M1 (error "No generic representation for empty datatype Empty")
        to (M1 _) = error "No values for empty datatype Empty"
      -- Similarly for Generic1
      instance Lift (Empty a) where
        lift _ = error "Can't lift value of empty datatype Empty"
      instance Data a => Data (Empty a) where
        gfoldl _ _ _ = error "Void gfoldl"
        toConstr _ = error "Void toConstr"

      Now, derived Functor, Traversable, ``Generic, Generic1, Lift, and Data instances emit code which inspects their arguments:

      instance Functor Empty where
        fmap _ x = case x of {}
      instance Traversable Empty where
        traverse _ x = pure (case x of {})
      instance Generic (Empty a) where
        from x = M1 (case x of {})
        to (M1 x) = case x of {}
      -- Similarly for Generic1
      instance Lift (Empty a) where
        lift x = pure (case x of {})
      instance Data a => Data (Empty a) where
        gfoldl _ x = case x of {}
        toConstr x = case x of {}

      Derived Foldable instances now are maximally lazy:

      instance Foldable Empty where
        foldMap _ _ = mempty
  • Derived Foldable instances now derive custom definitions for null instead of using the default one. This leads to asymptotically better performance for recursive types not shaped like cons-lists, and allows null to terminate for more (but not all) infinitely large structures.

  • Configure on Windows now supports the --enable-distro-toolchain configure flag, which can be used to build a GHC using compilers on your PATH instead of using the bundled bindist. See Trac #13792

  • GHC now enables -fllvm-pass-vectors-in-regs by default. This means that GHC will now use native vector registers to pass vector arguments across function calls.

  • The optional instance keyword is now usable in type family instance declarations. See Trac #13747

  • Lots of other bugs. See Trac for a complete list.

  • New flags -fignore-optim-changes and -fignore-hpc-changes allow GHC to reuse previously compiled modules even if they were compiled with different optimisation or HPC flags. These options are enabled by default by --interactive. See Trac #13604

3.2.3. Runtime system

  • Function hs_add_root() was removed. It was a no-op since GHC-7.2.1 where module initialisation stopped requiring a call to hs_add_root().
  • Proper import library support added to GHC which can handle all of the libraries produced by dlltool. The limitation of them needing to be named with the suffix .dll.a is also removed. See Trac #13606, Trac #12499, Trac #12498
  • The GHCi runtime linker on Windows now supports the big-obj file format.
  • The runtime system’s native stack backtrace support on POSIX platforms is now triggered by SIGQUIT instead of SIGUSR2 as it was in previous releases. This change is to bring GHC’s behavior into compliance with the model set by the most Java virtual machine implementations.
  • The GHC runtime on Windows now uses Continue handlers instead of Vectorized handlers to trap exceptions. This change gives other exception handlers a chance to handle the exception before the runtime does. Furthermore The RTS flag --install-seh-handlers= Can be used on Wndows to completely disable the runtime’s handling of exceptions. See Trac #13911, Trac #12110.
  • The GHC runtime on Windows can now generate crash dumps on unhandled exceptions using the RTS flag --generate-crash-dumps.
  • The GHCi runtime linker now avoid calling GCC to find libraries as much as possible by caching the list of search directories of GCC and querying the file system directly. This results in much better performance, especially on Windows.
  • The GHC runtime on Windows can now generate stack traces on unhandled exceptions. When running in GHCi more information is displayed about the symbols if available. This behavior can be controlled with the RTS flag –generate-stack-traces=<yes|no>.

3.2.4. Template Haskell

  • Template Haskell now reifies data types with GADT syntax accurately. Previously, TH used heuristics to determine whether a data type should be reified using GADT syntax, which could lead to incorrect results, such as data T1 a = (a ~ Int) => MkT1 being reified as a GADT and data T2 a where MkT2 :: Show a => T2 a not being reified as a GADT.

    In addition, reified GADT constructors now more accurately track the order in which users write type variables. Before, if you reified MkT as below:

    data T a where
      MkT :: forall b a. b -> T a

    Then the reified type signature of MkT would have been headed by ForallC [PlainTV a, PlainTV b]. Now, reifying MkT will give a type headed by ForallC [PlainTV b, PlainTV a], as one would expect.

  • Language.Haskell.TH.FamFlavour, which was deprecated in GHC 8.2, has been removed.

3.2.5. base library

  • Blank strings can now be used as values for environment variables using the System.Environment.Blank module. See Trac #12494
  • Data.Type.Equality.== is now a closed type family. It works for all kinds out of the box. Any modules that previously declared instances of this family will need to remove them. Whereas the previous definition was somewhat ad hoc, the behavior is now completely uniform. As a result, some applications that used to reduce no longer do, and conversely. Most notably, (==) no longer treats the *, j -> k, or () kinds specially; equality is tested structurally in all cases.

3.2.6. Build system

  • dll-split has been removed and replaced with an automatic partitioning utility gen-dll. This utility can transparently split and compile any DLLs that require this. Note that the rts and base can not be split at this point because of the mutual recursion between base and rts. There is currently no explicit dependency between the two in the build system and such there is no way to notify base that the rts has been split, or vice versa. (see Trac #5987).

3.3. 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 8.4.1 The compiler itself
Cabal Dependency of ghc-pkg utility
Win32 Dependency of ghc library
array Dependency of ghc library
base 2.1 Core library
binary Dependency of ghc library
bytestring Deppendency of ghc library
containers Dependency of ghc library
deepseq Dependency of ghc library
directory Dependency of ghc library
filepath 1.4.2 Dependency of ghc library
ghc-boot 8.4.1 Internal compiler library
ghc-compact Core library
ghc-prim 2.1 Core library
ghci 8.4.1 The REPL interface
haskeline Dependency of ghci executable
hpc Dependency of hpc executable
integer-gmp Core library
mtl 2.2.2 Dependency of Cabal library
parsec 1.12 Dependency of Cabal library
process Dependency of ghc library
template-haskell Core library
text Dependency of Cabal library
time Dependency of ghc library
transformers Dependency of ghc library
unix Dependency of ghc library
xhtml 3000.2.2 Dependency of haddock executable

3.3.1. Win32


This release is a backwards incompatible release which corrects the type of certain APIs. See issue #24.