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 6.12 branch. Due to changes in the runtime system, version 2.0 or later (preferably version 2.2 or later) of the parallel library should be used, or parallelism will be lost. The full Haskell import syntax can now been used to bring modules into scope in GHCi, e.g. Prelude> import Data.List as L Prelude Data.List> L.length "foo" 3 GHC now understands the Haskell98 and Haskell2010 languages. These get processed before the language extension pragmas, and define the default sets of extensions that are enabled. If neither is specified, then the default is Haskell2010 plus the MonoPatBinds extension. GHC now supports the DoAndIfThenElse extension, which is part of the Haskell 2010 standard. Datatype contexts, such as the Eq a in data Eq a => Set a = NilSet | ConsSet a (Set a) are now treated as an extension DatatypeContexts (on by default) by GHC. GHC's support for unicode source has been improved, including removing support for U+22EF for the .. symbol. See for more details. Pragmas are now reread after preprocessing. In particular, this means that if a pragma is used to turn CPP on, then other pragmas can be put in CPP conditionals. The TypeOperators extension now allows instance heads to use infix syntax. The PackageImports extension now understands this to mean the current package. The INLINE and NOINLINE pragmas can now take a CONLIKE modifier, which indicates that the right hand side is cheap to compute, and can thus be duplicated more freely. See for more details. A ForceSpecConstr annotation on a type, e.g. import SpecConstr {-# ANN type SPEC ForceSpecConstr #-} can be used to force GHC to fully specialise argument of that type. A NoSpecConstr annotation on a type, e.g. import SpecConstr {-# ANN type T NoSpecConstr #-} can be used to prevent SpecConstr from specialising on arguments of that type. There is are two experimental new extensions AlternativeLayoutRule and AlternativeLayoutRuleTransitional, which are for exploring alternative layout rules in Haskell'. The details are subject to change, so we advise against using them in real code for now. The NewQualifiedOperators extension has been deprecated, as it was rejected by the Haskell' committee. There is now a warning for missing import lists, controlled by the new -fwarn-missing-import-lists flag. GHC will now warn about SPECIALISE and UNPACK pragmas that have no effect. Shared libraries are once again supported on Windows. Shared libraries are now supported on OS X, both on x86 and on PowerPC. The new -dylib-install-name GHC flag is used to set the location of the dynamic library. See for more details. For security reasons, by default, the only RTS flag that programs accept is +RTS --info. If you want the full range of RTS flags then you need to link with the new -rtsopts flag. See for more details. On POSIX platforms, the RTS now has a new I/O manager based on epoll/kqueue/poll, which allows it to scale to a much larger number (100k+) of open file descriptors. The RTS now exports a function setKeepCAFs which is important when loading Haskell DLLs dynamically, as a DLL may refer to CAFs that hae already been GCed. The garbage collector no longer allows you to specify a number of steps; there are now always 2. The -T RTS flag has thus been removed. A new RTS flag -H causes the RTS to use a larger nursery, but without exceeding the amount of memory that the application is already using. It makes some programs go slower, but others go faster. GHC now returns memory to the OS, if memory usage peaks and then drops again. This is mainly useful for long running processes which normally use very little memory, but occasionally need a lot of memory for a short period of time. On OS X, eventLog events are now available as DTrace probes. The PAPI support has been improved. The new RTS flag -a#0x40000000 can be used to tell the RTS to collect the native PAPI event 0x40000000. GHC now defaults to --make mode, i.e. GHC will chase dependencies for you automatically by default. GHC now includes an LLVM code generator. This includes a number of new flags: a flag to tell GHC to use LLVM, -fllvm; a flag to dump the LLVM input ,-ddump-llvm; flags to keep the LLVM intermediate files, -keep-llvm-file and -keep-llvm-files; flags to set the location and options for the LLVM assembler, optimiser and compiler, -pgmla, -pgmlo, -pgmlc, -optla, -optlo and -optlc. It is now possible to use -fno-code with --make. The new flag -dsuppress-coercions controls whether GHC prints coercions in core dumps. The inliner has been overhauled. The most significant user-visible change is that only saturated functions are inlined, e.g. (.) f g x = f (g x) would only be inlined if (.) is applied to 3 arguments, while (.) f g = \x -> f (g x) will be inlined if only applied to 2 arguments. The -finline-if-enough-args flag is no longer supported. Column numbers in warnings and error messages now start at 1, as is more standard, rather than 0. GHCi now understands most linker scripts. In particular, this means that GHCi is able to load the C pthread library. The ghc --info output has been updated: It now includes the location of the global package database, in the Global Package DB field. It now includes the build, host and target platforms, in the Build platform, Host platform and Target platform fields. It now includes a Have llvm code generator field. The Win32 DLLs field has been removed. The registerised via-C backend, and the -fvia-C, have been deprecated. The poor floating-point performance in the x86 native code generator has now been fixed, so we don't believe there is still any reason to use the via-C backend. There is now a new flag --supported-extensions, which currently behaves the same as --supported-languages. GHC progress output such as [ 1 of 5] Compiling Foo ( Foo.hs, Foo.o ) is now sent to stdout rather than stderr. The new flag -fexpose-all-unfoldings makes GHC put unfoldings for everything in the interface file. There are two new flags, -fno-specialise and -fno-float-in, for disabling the specialise and float-in passes. The new flag -fstrictness-before=n tells GHC to run an additional strictness analysis pass before simplifier phase n. There is a new flag -funfolding-dict-discount for tweaking the optimiser's behaviour. The -fspec-inline-join-points flag has been removed. GHCi now understands the layout of multiline-commands, so this now works: Prelude> :{ Prelude| let x = 1 Prelude| y = 2 in x + y Prelude| :} 3 It is now possible to quasi-quote patterns with [p| ... |]. It is no longer to use a $ before the name of a quasi-quoter, e.g. one can now say [expr| ... |] rather than [$expr| ... |]. It is now possible to use a quasi-quoter for types, e.g. f :: [$qq| ... |] It is now possible to quasi-quote existentials and GADTs. There are now Data and Typeable instances for the HsSyn typed. As language extensions are not applied until after the base language (Haskell98, Haskell2010 or the default) has been selected, it is now necessary to tell the GHC API the point at which the extension flags should be processed. Normally this is done by calling DynFlags.flattenExtensionFlags once all the flags and pragmas have been read.