1.5. Release notes for version 7.10.1

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 7.8 branch.

1.5.1. Highlights

The highlights, since the 7.8 branch, are:


1.5.2. Full details Language

  • There is a new extension, StaticPointers, which allows you to create pointers to expressions which remain valid across processes. This is useful for referencing higher-order values in distributed systems. The pointers are created with a new keyword static as in x = static ("abc" ++ "123") :: StaticPtr String. All processes which dereference x get the same result, that is, the body of the static form.

  • Added support for binary integer literals

  • Simplified rules for implicit quantification. In previous versions of GHC, it was possible to use the => arrow to quantify over type variables in data and type declarations without a forall quantifier. For example, data Fun = Fun (Ord a => a -> b) was identical to data Fun = Fun (forall a b. Ord a => a -> b), while data Fun = Fun (a -> b) caused a not-in-scope error. This implicit quantification is now deprecated, and variables in higher-rank constructors should be quantified with forall regardless of whether a class context is present or not. GHC 7.10 raises a warning (controlled by -fwarn-context-quantification, enabled by default) and GHC 7.12 will raise an error. See examples in GHC documentation.

    The change also applies to Template Haskell splices such as [t|Ord a => a|], which should be written as [t|forall a. Ord a => a|].

  • Instance contexts inferred while processing deriving directives attached to data and newtype declarations now forbid equality constraints. This is a regression in obscure cases, but it will yield better error messages in more common cases. Users caught by the regression can simply use standalone-deriving, where you specify the context yourself. Compiler

  • GHC now checks that all the language extensions required for the inferred type signatures are explicitly enabled. This means that if any of the type signatures inferred in your program requires some language extension you will need to enable it. The motivation is that adding a missing type signature inferred by GHC should yield a program that typechecks. Previously this was not the case.

    This is a breaking change. Code that used to compile in the past might fail with an error message requiring some particular language extension (most likely -XTypeFamilies, -XGADTs or -XFlexibleContexts).

  • -fwarn-tabs warning flag is turned on by default with this release of GHC. It can be suppressed either by using GHC_OPTIONS pragma or by specifying -fno-warn-tabs flag.

  • A new warning flag, -fwarn-trustworthy-safe has been added and is turned on with -Wall. It warns when a module that is compiled with -XTrustworthy is actually infered as an -XSafe module. This lets the module author know that they can tighten their Safe Haskell bounds if desired.

  • The -fwarn-safe and -fwarn-unsafe that warn if a module was infered as Safe or Unsafe have been improved to work with all Safe Haskell module types. Previously, they only worked for unmarked modules where the compiler was infering the modules Safe Haskell type. They now work even for modules marked as -XTrustworthy or -XUnsafe. This is useful either to have GHC check your assumptions, or to generate a list of reasons easily why a module is regarded as Unsafe.

    For many use cases, the new -fwarn-trustworthy-safe flag is better suited than either of these two.

  • -ddump-simpl-phases and -ddump-core-pipeline flags have been removed.

  • Many more options have learned to respect the -ddump-to-file. For example you can use -ddump-to-file with -ddump-splices to produce a .dump-splices file for each file that uses Template Haskell. This should be much easier to understand on a larger project than having everything being dumped to stdout.

  • Compiler plugins (with the -fplugin flag) may now modify the behaviour of the constraint solver, to add new functionality to GHC's typechecker. See Section 9.3.4, “Typechecker plugins” for more details.

  • A new warning flag, -fwarn-missing-exported-sigs has been added. The behavior is similar to -fwarn-missing-signatures but GHC will only flag exported values. This flag takes precedence over -fwarn-missing-signatures so it can be used in conjunction with -Wall. GHCi

  • TODO FIXME Template Haskell

  • Added support for generating LINE pragma declarations (Section 7.22.7, “LINE pragma”).

  • The type Pred (which stores a type constraint) is now a synonym for Type, in order to work with the ConstraintKinds extension. This is a breaking change and may require some rewriting of Template Haskell code.

  • Pattern splices now work.

  • reifyInstances now treats unbound type variables as univerally quantified, allowing lookup of, say, the instance for Eq [a].

  • More kind annotations appear in reified types, in order to disambiguate types that would otherwise be ambiguous in the presence of PolyKinds. In particular, all reified TyVarBndrs are now KindedTVs. (This does not affect Template Haskell quotations, just calls to reify.)

  • Various features unsupported in quotations were previously silently ignored. These now cause errors.

  • Lift instances were added for many more types: all of the IntXX and WordXX types, Ratio a, (), Float, and Double.

  • All Template Haskell datatypes now have Generic and Ord instances.

  • Ppr instances were added for Lit and Loc.

  • Two new declaration forms are now supported: standalone-deriving declarations and generic method signatures (written using default in a class). This means an expansion to the Dec type.

  • Template Haskell is now more pedantic about splicing in bogus variable names, like those containing whitespace. If you use bogus names in your Template Haskell code, this may break your program. Runtime system

  • The linker API is now thread-safe. The main user-facing impact of this change is that you must now call initLinker before calling loadObj or any of the other linker APIs. Build system

  • ghc-pkg now respects --user and --global when modifying packages (e.g. changing exposed/trust flag or unregistering). Previously, ghc-pkg would ignore these flags and modify whichever package it found first on the database stack. To recover the old behavior, simply omit these flags.

  • ghc-pkg accepts a --user-package-db flag which allows a user to override the location of the user package database. Unlike databases specified using --package-db, a user package database configured this way respects the --user flag. Package system

  • TODO: cover module renaming, thinning, re-export etc

  • ghc-pkg (and ghc) have dropped support for single-file style package databases. Since version 6.12, ghc-pkg has defaulted to a new database format (using a directory of files, one per package plus a binary cache).

    This change will not affect programs and scripts that use ghc-pkg init to create package databases.

    This will affect scripts that create package databases using tricks like

    echo "[]" > package.conf

    Such scripts will need to be modified to use ghc-pkg init, and to delete databases by directory removal, rather than simple file delete.

1.5.3. Libraries array

  • Version number XXXXX (was base

  • Version number XXXXX (was

  • GHC has had its internal Unicode database for parsing updated to the Unicode 7.0 standard.

  • Attempting to access a portion of the result of System.IO.hGetContents that was not yet read when the handle was closed now throws an exception. Previously, a lazy read from a closed handle would simply end the result string, leading to silent or delayed failures. bin-package-db

  • This is an internal package, and should not be used. binary

  • Version number XXXXX (was bytestring

  • Version number XXXXX (was Cabal

  • Version number XXXXX (was containers

  • Version number XXXXX (was deepseq

  • Version number XXXXX (was directory

  • Version number XXXXX (was filepath

  • Version number XXXXX (was ghc

  • Many internal functions in GHC related to package IDs have been renamed to refer to package keys, e.g. PackageId is now PackageKey, the wired-in names such as primPackageId are now primPackageKey, etc. This reflects a distinction that we are now making: a package ID is, as before, the user-visible ID from Cabal foo-1.0; a package key is now a compiler-internal entity used for generating linking symbols, and may not correspond at all to the package ID. In particular, there may be multiple package keys per package ID.

  • The ghc library no longer depends on the Cabal library. This means that users of the ghc library are no longer forced to use the same version of Cabal as ghc did. It also means that Cabal is freed up to be able to depend on packages that ghc does not want to depend on (which for example may enable improvements to Cabal's parsing infrastructure). ghc-prim

  • Version number XXXXX (was haskell98

  • Version number XXXXX (was haskell2010

  • Version number XXXXX (was hoopl

  • Version number XXXXX (was hpc

  • Version number XXXXX (was integer-gmp

  • Version number XXXXX (was old-locale

  • Version number XXXXX (was old-time

  • Version number XXXXX (was process

  • Version number XXXXX (was template-haskell

  • Version number XXXXX (was time

  • Version number XXXXX (was 1.4.1) unix

  • Version number XXXXX (was Win32

  • Version number XXXXX (was

1.5.4. Known bugs