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2. Introduction to GHC

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3. Release notes for version 8.0.1

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 7.10 branch.


Only Cabal versions 1.24 and newer will function properly with this release. (see Trac #11558). Consequently it will likely be necessary to recompile cabal-install before installing new packages.

The reason for this is a change in how packages are identified in GHC 8.0. While previous versions of Cabal identified packages to GHC with a package key (with GHC’s -this-package-key argument), GHC 8.0 and later uses installed package IDs in place of package keys.


Users compiling GHC on Mac OS X with XCode 7.3 will need to tell the build system to use the nm-classic command instead of Apple’s new nm implementation as the latter breaks POSIX compliance (see Trac #11744). This can be done by passing something like --with-nm=$(xcrun --find nm-classic) to configure.

3.1. Highlights

The highlights, since the 7.10 series, are:

3.2. Full details

3.2.1. Language

  • -XTypeInType supports universal type promotion and merges the type and kind language. This allows, for example, higher-rank kinds, along with kind families and type-level GADTs. Support is still experimental, and it is expected to improve over the next several releases. See Kind polymorphism and Type-in-Type for the details.

  • The parser now supports Haddock comments on GADT data constructors. For example

    data Expr a where
        -- | Just a normal sum
        Sum :: Int -> Int -> Expr Int
  • The new base constraint GHC.Stack.HasCallStack can be used by functions to request a partial call-stack. For example

    errorWithCallStack :: HasCallStack => String -> a
    errorWithCallStack msg = error (msg ++ "\n" ++ prettyCallStack callStack)
    ghci> errorWithCallStack "die"
    *** Exception: die
    CallStack (from HasCallStack):
      errorWithCallStack, called at <interactive>:2:1 in interactive:Ghci1

    prints the call-site of errorWithCallStack.

    See HasCallStack for a description of HasCallStack.

  • GHC now supports visible type application, allowing programmers to easily specify how type parameters should be instantiated when calling a function. See Visible type application for the details.

  • To conform to the common case, the default role assigned to parameters of datatypes declared in hs-boot files is representational. However, if the constructor(s) for the datatype are given, it makes sense to do normal role inference. This is now implemented, effectively making the default role for non-abstract datatypes in hs-boot files to be phantom, like it is in regular Haskell code.

  • Wildcards can be used in the type arguments of type/data family instance declarations to indicate that the name of a type variable doesn’t matter. They will be replaced with new unique type variables. See Data instance declarations for more details.

  • GHC now allows to declare type families as injective. Injectivity information can then be used by the typechecker. See Injective type families for details.

  • Due to a security issue, Safe Haskell now forbids annotations in programs marked as -XSafe.

  • Generic instances can be derived for data types whose constructors have arguments with certain unlifted types. See Generic programming for more details.

  • GHC generics can now provide strictness information for fields in a data constructor via the Selector type class.

  • The -XDeriveAnyClass extension now fills in associated type family default instances when deriving a class that contains them.

  • Users can now define record pattern synonyms. This allows pattern synonyms to behave more like normal data constructors. For example,

    pattern P :: a -> b -> (a, b)
    pattern P{x,y} = (x,y)

    will allow P to be used like a record data constructor and also defines selector functions x :: (a, b) -> a and y :: (a, b) -> b.

  • Pattern synonyms can now be bundled with type constructors. For a pattern synonym P and a type constructor T, P can be bundled with T so that when T is imported P is also imported. With this change a library author can provide either real data constructors or pattern synonyms in an opaque manner. See Import and export of pattern synonyms for details.

    -- Foo.hs
    module Foo ( T(P) ) where
    data T = T
    pattern P = T
    -- Baz.hs
    module Baz where
    -- P is imported
    import Foo (T(..))
  • Whenever a data instance is exported, the corresponding data family is exported, too. This allows one to write

    -- Foo.hs
    module Foo where
    data family T a
    -- Bar.hs
    module Bar where
    import Foo
    data instance T Int = MkT
    -- Baz.hs
    module Baz where
    import Bar (T(MkT))

    In previous versions of GHC, this required a workaround via an explicit export list in Bar.

  • GHC has grown a -XUndecidableSuperClasses language extension, which relaxes GHC’s recursive superclass check (see Trac #11318). This allows class definitions which have mutually recursive superclass constraints at the expense of potential non-termination in the solver.

  • The compiler is now a bit more conservative in solving constraints previously provided by superclasses (see Trac #11762). For instance, consider this program,:

    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE UndecidableInstances #-}
    class Super a
    class (Super a) => Left a
    class (Super a) => Right a
    instance (Left a) => Right a    -- this is now an error

    GHC now rejects this instance, claiming it cannot deduce the Super a superclass constraint of the Right typeclass. This stands in contrast to previous releases, which would accept this declaration, using the Super a constraint implied by the Left a constraint. To fix this simply add the needed superclass constraint explicitly,

    instance (Left a, Super a) => Right a
  • -XDeriveFoldable and -XDeriveTraversable now generate code without superfluous mempty or pure expressions. As a result, -XDeriveTraversable now works on datatypes that contain arguments which have unlifted types.

  • Note that the -XImpredicativeTypes extension, which has been known to be broken for many years, is even more broken than usual in this release (see Trac #11319, Trac #11675, and others). During pre-release testing we encountered a number of projects that broke with confusing type errors due to (often unnecessary) use of -XImpredicativeTypes. Users of -XImpredicativeTypes do so at their own risk!

3.2.2. Compiler

  • The LLVM code generator now supports only LLVM 3.7. This is in contrast to our previous policy where GHC would try to support a range of LLVM versions concurrently. We hope that by supporting a narrower range of versions we can provide more reliable support for each.

  • Warnings can now be controlled with -W(no-)... flags in addition to the old -f(no-)warn... ones. This was done as the first part of a rewrite of the warning system to provide better control over warnings, better warning messages, and more common syntax compared to other compilers. The old -f-based warning flags will remain functional for the forseeable future.

  • Added the option -dth-dec-file. This dumps out a .th.hs file of all Template Haskell declarations in a corresponding .hs file. The idea is that application developers can check this into their repository so that they can grep for identifiers used elsewhere that were defined in Template Haskell. This is similar to using -ddump-to-file with -ddump-splices but it always generates a file instead of being coupled to -ddump-to-file and only outputs code that does not exist in the .hs file and a comment for the splice location in the original file.

  • Added the option -fprint-expanded-types. When enabled, GHC also prints type-synonym-expanded types in type errors.

  • Added the option -fcpr-anal. When enabled, the demand analyser performs CPR analysis. It is implied by -O. Consequently, -fcpr-off is now removed, run with -fno-cpr-anal to get the old -fcpr-off behaviour.

  • Added the option -fworker-wrapper. When enabled, the worker-wrapper transformation is performed after a strictness analysis pass. It is implied by -O and by -fstrictness. It is disabled by -fno-strictness. Enabling -fworker-wrapper while strictness analysis is disabled (by -fno-strictness) has no effect.

  • -ddump-strsigs has been renamed to -ddump-str-signatures.

  • -XDeriveGeneric is now less picky about instantiating type arguments when deriving (Trac #11732). As a consequence, the following code is now legal (whereas before it would have been rejected).

    data T a b = T a b
    deriving instance Generic (T Int b)
  • Added the -fmax-pmcheck-iterations to control how many times the pattern match checker iterates. Since coverage checking is exponential in the general case, setting a default number of iterations prevents memory and performance blowups. By default, the number of iterations is set to 2000000 but it can be set with: -fmax-pmcheck-iterations=<n>. If the set number of iterations is exceeded, an informative warning is issued.

  • -this-package-key has been renamed again (hopefully for the last time!) to -this-unit-id. The renaming was motivated by the fact that the identifier you pass to GHC here doesn’t have much to do with packages: you may provide different unit IDs for libraries which are in the same package. -this-package-key is deprecated; you should use -this-unit-id or, if you need compatibility over multiple versions of GHC, -package-name.

  • When -fdefer-type-errors is enabled and an expression fails to typecheck, Control.Exception.TypeError will now be thrown instead of Control.Exception.ErrorCall. Warnings

  • When printing an out-of-scope error message, GHC will give helpful advice if the error might be caused by too restrictive imports.
  • Warning messages now mention the name of the warning flag which the message is controlled by (Trac #10752) If the flag was implied via a warning group then the name of the group will also be shown if -fshow-warning-groups is used.
  • Added the -Weverything warning group, along with its opposite -Wno-everything. This group includes all warnings supported by GHC. This is in contrast to -Wall which excludes some stylistic or otherwise controversial warnings.
  • Added the -Wdefault warning group, along with its opposite -Wno-default. This group is defined to be the set of warnings which ghc enables by default (e.g. when no additional -W flags are used).
  • Added the -Wcompat warning group (Trac #11000), along with its opposite -Wno-compat. Turns on warnings that will be enabled by default in the future, but remain off in normal compilations for the time being. This allows library authors eager to make their code future compatible to adapt to new features before they even generate warnings.
  • Added the -Wmissing-monadfail-instances flag. When enabled, this will issue a warning if a failable pattern is used in a context that does not have a MonadFail constraint. This flag represents phase 1 of the MonadFail Proposal (MFP).
  • Added the -Wsemigroup flag. When enabled, this will issue a warning if a type is an instance of Monoid but not Semigroup, and when a custom definition (<>) is made. Fixing these warnings makes sure the definition of Semigroup as a superclass of Monoid does not break any code.
  • After a long hiatus (see Trac #10935) the -Wmonomorphism-restriction (formerly -fwarn-monomorphism-restriction) flag has returned. The functionality of this flag was inadvertently removed by a commit in 2010; this has been fixed and the flag should now issue warnings as it previously did.
  • Added the options -Wmissed-specialisations and -Wall-missed-specialisations. When enabled, the simplifier will produce a warning when a overloaded imported function cannot be specialised (typically due to a missing INLINEABLE pragma). This is intended to alert users to cases where they apply INLINEABLE but may not get the speed-up they expect.
  • Added the option -Wnoncanonical-monad-instances and -Wnoncanonical-monadfail-instances which help detect noncanonical Applicative/Monad/MonadFail instance definitions. See flag description in Warnings and sanity-checking for more details.
  • Added the option -Wnoncanonical-semigroup-instances which warns of Monoid instances where mappend is not defined in term of Semigroup‘s (<>) operation.
  • Added the -Wmissing-pattern-synonym-signatures flag. When enabled, this will issue a warning when a pattern synonym definition doesn’t have a type signature. It is turned off by default but enabled by -Wall.
  • Added the -Wunused-type-patterns flag to report unused type variables in data and type family instances. This flag is not implied by -Wall, since -Wunused-type-patterns will warn about unused type variables even if the types themselves are intended to be used as documentation. If -Wunused-type-patterns is enabled, one can prefix or replace unused type variables with underscores to avoid warnings.
  • Split off the new flag -Wunused-foralls from the previously existing flag -Wunused-matches. -Wunused-foralls emits a warning in the specific case that a user writes explicit forall syntax with unused type variables, while -Wunused-matches only warns in the case of unused term-level patterns. Both flags are implied by -W.
  • -Wmissing-local-sigs is now deprecated in favor of -Wmissing-local-signatures.
  • -Wmissing-exported-sigs is now deprecated in favor of -Wmissing-exported-signatures.

3.2.3. GHCi

  • Main with an explicit module header but without main is now an error (Trac #7765).
  • The :back and :forward commands now take an optional count allowing the user to move forward or backward in history several steps at a time.
  • Added commands :load! and :reload!, effectively setting -fdefer-type-errors before loading a module and unsetting it after loading if it has not been set before (Trac #8353).
  • ghci -e now behaves like ghc -e (Trac #9360).
  • Added support for top-level function declarations (Trac #7253).
  • The new commands :all-types, :loc-at, :type-at, and :uses designed for editor-integration (such as Emacs’ haskell-mode) originally premiered by ghci-ng have been integrated into GHCi (Trac #10874).

3.2.4. Template Haskell

  • The new -XTemplateHaskellQuotes flag allows to use the quotes (not quasi-quotes) subset of TemplateHaskell. This is particularly useful for use with a stage 1 compiler (i.e. GHC without interpreter support). Also, -XTemplateHaskellQuotes is considered safe under Safe Haskell.
  • The __GLASGOW_HASKELL_TH__ CPP constant denoting support for -XTemplateHaskell introduced in GHC 7.10.1 has been changed to use the values 1/0 instead of the previous YES/NO values.
  • Partial type signatures can now be used in splices, see Where can they occur?.
  • Template Haskell now fully supports typed holes and quoting unbound variables. This means it is now possible to use pattern splices nested inside quotation brackets.
  • Template Haskell now supports the use of UInfixT in types to resolve infix operator fixities, in the same vein as UInfixP and UInfixE in patterns and expressions. ParensT and InfixT have also been introduced, serving the same functions as their pattern and expression counterparts.
  • Template Haskell has now explicit support for representing GADTs. Until now GADTs were encoded using NormalC, RecC (record syntax) and ForallC constructors. Two new constructors - GadtC and RecGadtC - are now supported during quoting, splicing and reification.
  • Primitive chars (e.g., [| 'a'# |]) and primitive strings (e.g., [| "abc"# |]) can now be quoted with Template Haskell. The Lit data type also has a new constructor, CharPrimL, for primitive char literals.
  • addTopDecls now accepts annotation pragmas.
  • Internally, the implementation of quasi-quotes has been unified with that of normal Template Haskell splices. Under the previous implementation, top-level declaration quasi-quotes did not cause a break in the declaration groups, unlike splices of the form $(...). This behavior has been preserved under the new implementation, and is now recognized and documented in Syntax.
  • The Lift class is now derivable via the -XDeriveLift extension. See Deriving Lift instances for more information.
  • The FamilyD data constructor and FamFlavour data type have been removed. Data families are now represented by DataFamilyD and open type families are now represented by OpenTypeFamilyD instead of FamilyD. Common elements of OpenTypeFamilyD and ClosedTypeFamilyD have been moved to TypeFamilyHead.
  • The representation of data, newtype, data instance, and newtype instance declarations has been changed to allow for multi-parameter type classes in the deriving clause. In particular, dataD and newtypeD now take a CxtQ instead of a [Name] for the list of derived classes.
  • isExtEnabled can now be used to determine whether a language extension is enabled in the Q monad. Similarly, extsEnabled can be used to list all enabled language extensions.
  • One can now reify the strictness information of a constructors’ fields using Template Haskell’s reifyConStrictness function, which takes into account whether flags such as -XStrictData or -funbox-strict-fields are enabled.
  • Previously, quoting a type signature like a -> a would produce the abstract syntax for forall a. a -> a. This behavior remains, but it is extended to kinds, too, meaning that Proxy a -> Proxy a becomes forall k (a :: k). Proxy a -> Proxy a. This change is not intentional, but is forced by the fact that GHC has a hard time telling kinds apart from types. The effect of this change is that round-tripping kind- polymorphic types will now require -XTypeInType.

3.2.5. Runtime system

  • We have a shiny new two-step memory allocator for 64-bit platforms (see Trac #9706). In addition to simplifying the runtime system’s implementation this may significantly improve garbage collector performance. Note, however, that Haskell processes will have an apparent virtual memory footprint of a terabyte or so. Don’t worry though, most of this amount is merely mapped but uncommitted address space which is not backed by physical memory.
  • Support for performance monitoring with PAPI has been dropped.
  • -maxN⟨x⟩ flag added to complement -N. It will choose to use at most ⟨x⟩ capabilities, limited by the number of processors as -N is.
  • The runtime linker is no longer greedy and will load only the needed objects from archives. This means particularly on Windows packages requiring e.g. C99 support will now function properly. As part of this the RTS on Windows no longer re-exports deprecated POSIX functions under the undeprecated names (see Trac #11223).
  • A number of subtle but serious issues in the runtime linker were resolved on ARM (summarized in Trac #11206).

3.2.6. Build system

  • Nothing changed here.

3.2.7. Package system

  • Various internal changes but nothing user-facing.

3.2.8. hsc2hs

  • hsc2hs now supports the #alignment macro, which can be used to calculate the alignment of a struct in bytes. Previously, #alignment had to be implemented manually via a #let directive, e.g.,

    #let alignment t = "%lu", (unsigned long)offsetof(struct {char x__; t (y__); }, y__)

    As a result, if you have the above directive in your code, it will now emit a warning when compiled with GHC 8.0.

    Module.hsc:24:0: warning: "hsc_alignment" redefined [enabled by default]
    In file included from dist/build/Module_hsc_make.c:1:0:
    /path/to/ghc/lib/template-hsc.h:88:0: note: this is the location of the previous definition
     #define hsc_alignment(t...) \

    To make your code free of warnings on GHC 8.0 and still support earlier versions, surround the directive with a pragma checking for the right GHC version.

    #if __GLASGOW_HASKELL__ < 800
    #let alignment t = "%lu", (unsigned long)offsetof(struct {char x__; t (y__); }, y__)

3.3. Libraries

3.3.1. array

  • Version number (was

3.3.2. base

See in the base package for full release notes.

  • Version number (was

  • GHC.Stack exports two new types SrcLoc and CallStack. A SrcLoc contains package, module, and file names, as well as start and end positions. A CallStack is essentially a [(String, SrcLoc)], sorted by most-recent call.

  • error and undefined will now report a partial stack-trace using the new CallStack feature (and the -prof stack if available).

  • A new function, interruptible, was added to GHC.IO allowing an IO action to be run such that it can be interrupted by an asynchronous exception, even if exceptions are masked (except if masked with interruptibleMask).

    This was introduced to fix the behavior of allowInterrupt, which would previously incorrectly allow exceptions in uninterruptible regions (see Trac #9516).

  • Per-thread allocation counters (setAllocationCounter and getAllocationCounter) and limits (enableAllocationLimit, disableAllocationLimit are now available from System.Mem. Previously this functionality was only available from GHC.Conc.

  • forever, filterM, mapAndUnzipM, zipWithM, zipWithM_, replicateM, and replicateM were generalized from Monad to Applicative. If this causes performance regressions, try to make the implementation of (*>) match that of (>>) (see Trac #10168).

  • Add URec, UAddr, UChar, UDouble, UFloat, UInt, and UWord to GHC.Generics as part of making GHC generics capable of handling unlifted types (Trac #10868)

  • Expand Floating class to include operations that allow for better precision: log1p, expm1, log1pexp and log1mexp. These are not available from Prelude, but the full class is exported from Numeric.

  • Add Data.List.NonEmpty and Data.Semigroup (to become super-class of Monoid in the future). These modules were provided by the semigroups package previously. (Trac #10365)

  • Add GHC.TypeLits.TypeError and ErrorMessage to allow users to define custom compile-time error messages. (see Custom compile-time errors and the original proposal).

  • The datatypes in GHC.Generics now have Enum, Bounded, Ix, Functor, Applicative, Monad, MonadFix, MonadPlus, MonadZip, Foldable, Foldable, Traversable, Generic1, and Data instances as appropriate. (Trac #9043)

  • The Generic instance for Proxy is now poly-kinded (see Trac #10775)

  • The IsString instance for [Char] has been modified to eliminate ambiguity arising from overloaded strings and functions like (++).

  • Move Const from Control.Applicative to its own module in Data.Functor.Const. (see Trac #11135)

  • Enable PolyKinds in the Data.Functor.Const module to give Const the kind * -> k -> * (see Trac #10039).

  • Add the TypeError datatype to Control.Exception, which represents the error that is thrown when an expression fails to typecheck when run using -fdefer-type-errors. (see Trac #10284)

3.3.3. binary

  • Version number (was

3.3.4. bytestring

  • Version number (was

3.3.5. Cabal

  • Version number 1.24.0 (was

3.3.6. containers

  • Version number (was

3.3.7. deepseq

  • Version number (was

3.3.8. directory

  • Version number (was

3.3.9. filepath

  • Version number (was

3.3.10. ghc

  • The HsBang type has been removed in favour of HsSrcBang and HsImplBang. Data constructors now always carry around their strictness annotations as the user wrote them, whether from an imported module or not.
  • Moved startsVarSym, startsVarId, startsConSym, startsConId, startsVarSymASCII, and isVarSymChar from Lexeme to the GHC.Lemexe module of the ghc-boot library.
  • Add isImport, isDecl, and isStmt functions.
  • The mi_fix_fn field of ModIface had its type changed from OccName -> Fixity to OccName -> Maybe Fixity, where a returned value of Nothing indicates a cache miss. As a result, the types of mkIfaceFixCache and emptyIfaceFixCache were also changed to have a return type of Maybe Fixity, and a new mi_fix :: OccName -> Fixity function was introduced which invokes mi_fix_fn but returns defaultFixity upon a cache miss.

3.3.11. ghc-boot

  • This is an internal package. Use with caution.
  • Version number 8.0.1
  • This package was renamed from bin-package-db to reflect its new purpose of containing intra-GHC functionality that needs to be shared across multiple GHC boot libraries.

3.3.12. ghc-boot-th

  • This is an internal package. Use with caution.
  • Version number 8.0.1
  • This package was created to share types and utilities between the ghc and template-haskell packages
  • Added GHC.Lexeme, which contains functions for determining if a character can be the first letter of a variable or data constructor in Haskell, as defined by GHC. (These functions were moved from Lexeme in ghc.)
  • Added GHC.LanguageExtensions which contains a type listing all supported language extensions.

3.3.13. ghc-prim

  • Version number (was

3.3.14. haskell98

  • No longer shipped

3.3.15. haskell2010

  • No longer shipped. We hope that this package or something like it will be restored in a future GHC release.

3.3.16. hoopl

  • Version number (was

3.3.17. hpc

  • Version number (was

3.3.18. integer-gmp

  • Version number (was

3.4. old-locale

  • No longer shipped

3.4.1. old-time

  • No longer shipped

3.4.2. process

  • Version number (was

3.4.3. template-haskell

  • Version number (was
  • The Lift type class for lifting values into Template Haskell splices now has a default signature lift :: Data a => a -> Q Exp, which means that you do not have to provide an explicit implementation of lift for types which have a Data instance. To manually use this default implementation, you can use the liftData function which is now exported from Language.Haskell.TH.Syntax.
  • Info‘s constructors no longer have Fixity fields. A qReifyFixity function was added to the Quasi type class (as well as the reifyFixity function, specialized for Q) to allow lookup of fixity information for any given Name.

3.4.4. time

  • Version number (was

3.4.5. unix

  • Version number (was

3.4.6. Win32

  • Version number (was

3.5. Known bugs