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:

  • GHC has implemented "The Applicative Monad Proposal", meaning the Applicative typeclass is now a superclass of Monad. This is a breaking change and your programs will need to be updated. Please see the GHC 7.10 Migration Guide on the GHC wiki.

  • GHC has implemented the "Burning Bridges Proposal", meaning that many of the combinators in Prelude are now re-exported from more generic modules (such as Data.Traversable), rather than exporting custom, less-generic versions. This is a change that may require updates to your program. Please see the GHC 7.10 Migration Guide on the GHC wiki.

  • GHC now has support for 'partial type signatures', which give you the ability to add 'holes' to a type signature that the compiler can later infer. For more, see Section 7.15, “Partial Type Signatures”.

  • The integer-gmp package has been completely rewritten from the ground up. The primary change in this rewrite is that GHC-compiled programs that link to GMP no longer 'hook' GMP allocation routines, to create an Integer on the raw Haskell heap. Instead, integer-gmp now allocates all memory in Haskell code, and talks to GMP via normal FFI imports like other C code.

    The practical side effect of this is that C libraries which bind to GMP (such as MPFR or FLINT) no longer need careful (or impossible) hacks to be used inside a GHC-compiled program via the FFI; GMP is treated just like any other C library, with no special accomodations.

  • GHC now has support for plugins which modify the type checker. This allows external users to interface with GHC and write type-checking plugins to solve constraints and equalities generated by the typechecker.

    This feature is experimental and will likely change in the future.

  • GHC now has support for a new extension, -XStaticPointers, that allows you to (de)reference and serialize pointers to known, closed expressions. This is primarily aimed at making distributed programming (via interfaces like Cloud Haskell) easier. For more, see Section 7.21, “Static pointers ”.

    This feature is experimental and will likely change in the future.

  • GHC now has preliminary support for DWARF-based debugging, when compiling programs with the new -g option. This will embed DWARF information into the module object files, which can then be read by tools like GDB for backtraces or single-stepping.

    This feature is highly experimental and will likely change in the future, but should still be useful today.

1.5.2. Full details Language

  • Implicit parameters of the new base type GHC.Stack.CallStack are treated specially, and automatically solved for the current source location. For example

                          f = print (?stk :: CallStack)

    will print the singleton stack containing the occurrence of ?stk. If there is another CallStack implicit in-scope, the new location will be appended to the existing stack, e.g.

                          f :: (?stk :: CallStack) => IO ()
                          f = print (?stk :: CallStack)

    will print the occurrence of ?stk and the call-site of f. The name of the implicit parameter does not matter.

    See the release notes for base for a description of the CallStack type.

  • 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).

  • The solvers for both type family reductions and Coercible instances have been improved. This should lead to faster compilation of type-family-heavy code and more Coercible instances to be found. However, some bugs remain: see 'Known Bugs' below.

  • -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.

  • The new -fdefer-typed-holes flag turns typed hole errors into typed hole warnings that produce runtime errors when evaluated. The -fno-warn-typed-holes flag was repurposed to silence the warnings produced when -fdefer-typed-holes is used. As a result, it is no longer possible to disable typed holes like it was in GHC 7.8. This only turned a self-explanatory error into a cryptic parse error and was thus not very useful. For more details, consult Section 7.14, “Typed Holes” and Section 7.16, “Deferring type errors to runtime”.

  • 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.

  • A new warning flag, -fwarn-unticked-promoted-constructors has been added. This flag causes GHC to warn when you use a promoted constructor without using a "tick" preceding its name.

    For example:

    data Nat = Succ Nat | Zero
    data Vec n s where
      Nil  :: Vec Zero a
      Cons :: a -> Vec n a -> Vec (Succ n) a

    Will raise two warnings because Zero and Succ are not written as 'Zero and 'Succ.

    This warning is enabled by default in -Wall mode.

  • 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 fi GHCi

  • It's now possible to use :set -l{foo} in GHCi to link against a foreign library after startup.

  • Pattern synonyms are now supported in GHCi. 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

  • 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 (was base

  • Version number (was

  • A new module GHC.SrcLoc was added, exporting a new type SrcLoc. A SrcLoc contains package, module, and file names, as well as start and end positions.

  • A new type CallStack was added for use with the new implicit callstack parameters. A CallStack is a [(String, SrcLoc)], sorted by most-recent call.

  • 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 (was bytestring

  • Version number (was Cabal

  • Version number (was containers

  • Version number (was deepseq

  • Version number (was directory

  • Version number (was filepath

  • Version number (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 (was

  • The low-level prefetch API exported by GHC.Prim (added in GHC 7.8) has been overhauled to use State# parameters to serialize and thread state around.

    This API is still considered experimental, and will be prone to change. haskeline

  • Version number (was hoopl

  • Version number (was hpc

  • Version number (was

  • The hpc command supports a new flag, --verbosity=n, which controls the verbosity level of subcommands. integer-gmp

  • Version number (was

  • The integer-gmp package has been completely rewritten to be more efficient and interoperate more sanely with the GMP library. Specifically, GHC no longer needs to 'hook' the GMP memory allocators to make allocations exist on the Haskell heap, a complication which makes GMP-dependent C libraries difficult. This means external libraries that use GMP (such as MPFR or FLINT) can now be trivially FFI'd to without any complication. pretty

  • Version number (was process

  • Version number (was template-haskell

  • Version number (was terminfo

  • Version number (was time

  • Version number (was 1.4.2) transformers

  • Version number (was unix

  • Version number (was Win32

  • Version number (was xhtml

  • Version number remained at 3000.2.1

1.5.4. Known bugs

  • For issues dealing with language changes, please see the GHC 7.10 Migration Guide on the GHC wiki.

  • GHC's LLVM backend does not support LLVM 3.4 (issue #9929)

  • On Mac OS X, the -threaded Garbage Collector currently suffers from a large performance penalty due to a lack of system-specific optimization (issue #7602).

  • GHC's LLVM backend is currently incompatible with LLVM 3.4 (issue #9929).

  • GHCi fails to appropriately load .dyn_o files (issue #8736).

  • Not all cases of non-terminating type-level computation (with both recursive type families and recursive newtypes) are caught. This means that GHC might hang, but it should do so only when the program is ill-typed (due to non-terminating type-level features). The bugs are reported as #7788 and #10139. There also remain certain obscure scenarios where the solver for Coercible instances is known to be still incomplete. See comments in #10079.