GHC, using using GHC GHC's behaviour is controlled by options, which for historical reasons are also sometimes referred to as command-line flags or arguments. Options can be specified in three ways: structure, command-line command-linearguments argumentscommand-line An invocation of GHC takes the following form: ghc [argument...] Command-line arguments are either options or file names. Command-line options begin with -. They may not be grouped: -vO is different from -v -O. Options need not precede filenames: e.g., ghc *.o -o foo. All options are processed and then applied to all files; you cannot, for example, invoke ghc -c -O1 Foo.hs -O2 Bar.hs to apply different optimisation levels to the files Foo.hs and Bar.hs. source-file options Sometimes it is useful to make the connection between a source file and the command-line options it requires quite tight. For instance, if a Haskell source file uses GHC extensions, it will always need to be compiled with the -fglasgow-exts option. Rather than maintaining the list of per-file options in a Makefile, it is possible to do this directly in the source file using the OPTIONS_GHC pragma OPTIONS_GHC pragma: {-# OPTIONS_GHC -fglasgow-exts #-} module X where ... OPTIONS_GHC pragmas are only looked for at the top of your source files, upto the first (non-literate,non-empty) line not containing OPTIONS_GHC. Multiple OPTIONS_GHC pragmas are recognised. Do not put comments before, or on the same line as, the OPTIONS_GHC pragma. Note that your command shell does not get to the source file options, they are just included literally in the array of command-line arguments the compiler maintains internally, so you'll be desperately disappointed if you try to glob etc. inside OPTIONS_GHC. NOTE: the contents of OPTIONS_GHC are appended to the command-line options, so options given in the source file override those given on the command-line. It is not recommended to move all the contents of your Makefiles into your source files, but in some circumstances, the OPTIONS_GHC pragma is the Right Thing. (If you use -keep-hc-file and have OPTION flags in your module, the OPTIONS_GHC will get put into the generated .hc file). Options may also be modified from within GHCi, using the :set command. See for more details. staticoptions dynamicoptions modeoptions Each of GHC's command line options is classified as static, dynamic or mode: Mode flags For example, --make or -E. There may only be a single mode flag on the command line. The available modes are listed in . Dynamic Flags Most non-mode flags fall into this category. A dynamic flag may be used on the command line, in a GHC_OPTIONS pragma in a source file, or set using :set in GHCi. Static Flags A few flags are "static", which means they can only be used on the command-line, and remain in force over the entire GHC/GHCi run. The flag reference tables () lists the status of each flag. There are a few flags that are static except that they can also be used with GHCi's :set command; these are listed as “static/:set” in the table. suffixes, file file suffixes for GHC File names with “meaningful” suffixes (e.g., .lhs or .o) cause the “right thing” to happen to those files. .hs A Haskell module. .lhs lhs suffix A “literate Haskell” module. .hi A Haskell interface file, probably compiler-generated. .hc Intermediate C file produced by the Haskell compiler. .c A C file not produced by the Haskell compiler. .s An assembly-language source file, usually produced by the compiler. .o An object file, produced by an assembler. Files with other suffixes (or without suffixes) are passed straight to the linker. GHC's behaviour is firstly controlled by a mode flag. Only one of these flags may be given, but it does not necessarily need to be the first option on the command-line. The available modes are: ghc --interactive interactive mode ghci Interactive mode, which is also available as ghci. Interactive mode is described in more detail in . ghc --make make mode ––make In this mode, GHC will build a multi-module Haskell program automatically, figuring out dependencies for itself. If you have a straightforward Haskell program, this is likely to be much easier, and faster, than using make. Make mode is described in . ghc -e expr eval mode Expression-evaluation mode. This is very similar to interactive mode, except that there is a single expression to evaluate (expr) which is given on the command line. See for more details. ghc -E ghc -c ghc -S ghc -c -E -C -S -c This is the traditional batch-compiler mode, in which GHC can compile source files one at a time, or link objects together into an executable. This mode also applies if there is no other mode flag specified on the command line, in which case it means that the specified files should be compiled and then linked to form a program. See . ghc -M dependency-generation mode Dependency-generation mode. In this mode, GHC can be used to generate dependency information suitable for use in a Makefile. See . ghc --mk-dll DLL-creation mode DLL-creation mode (Windows only). See . ghc --help ghc -? ––help Cause GHC to spew a long usage message to standard output and then exit. ghc --show-iface file ––--show-iface Read the interface in file and dump it as text to stdout. For example ghc --show-iface M.hi. ghc --supported-languages ––supported-languages Print the supported language extensions. ghc --info ––info Print information about the compiler. ghc --version ghc -V -V ––version Print a one-line string including GHC's version number. ghc --numeric-version ––numeric-version Print GHC's numeric version number only. ghc --print-libdir ––print-libdir Print the path to GHC's library directory. This is the top of the directory tree containing GHC's libraries, interfaces, and include files (usually something like /usr/local/lib/ghc-5.04 on Unix). This is the value of $libdirlibdir in the package configuration file (see ). ––make separate compilation When given the ––make option, GHC will build a multi-module Haskell program by following dependencies from one or more root modules (usually just Main). For example, if your Main module is in a file called Main.hs, you could compile and link the program like this: ghc ––make Main.hs The command line may contain any number of source file names or module names; GHC will figure out all the modules in the program by following the imports from these initial modules. It will then attempt to compile each module which is out of date, and finally, if there is a Main module, the program will also be linked into an executable. The main advantages to using ghc ––make over traditional Makefiles are: GHC doesn't have to be restarted for each compilation, which means it can cache information between compilations. Compiling a multi-module program with ghc ––make can be up to twice as fast as running ghc individually on each source file. You don't have to write a Makefile. Makefilesavoiding GHC re-calculates the dependencies each time it is invoked, so the dependencies never get out of sync with the source. Any of the command-line options described in the rest of this chapter can be used with ––make, but note that any options you give on the command line will apply to all the source files compiled, so if you want any options to apply to a single source file only, you'll need to use an OPTIONS_GHC pragma (see ). If the program needs to be linked with additional objects (say, some auxiliary C code), then the object files can be given on the command line and GHC will include them when linking the executable. Note that GHC can only follow dependencies if it has the source file available, so if your program includes a module for which there is no source file, even if you have an object and an interface file for the module, then GHC will complain. The exception to this rule is for package modules, which may or may not have source files. The source files for the program don't all need to be in the same directory; the -i option can be used to add directories to the search path (see ). This mode is very similar to interactive mode, except that there is a single expression to evaluate which is specified on the command line as an argument to the -e option: ghc -e expr Haskell source files may be named on the command line, and they will be loaded exactly as in interactive mode. The expression is evaluated in the context of the loaded modules. For example, to load and run a Haskell program containing a module Main, we might say ghc -e Main.main Main.hs or we can just use this mode to evaluate expressions in the context of the Prelude: $ ghc -e "interact (unlines.map reverse.lines)" hello olleh In batch mode, GHC will compile one or more source files given on the command line. The first phase to run is determined by each input-file suffix, and the last phase is determined by a flag. If no relevant flag is present, then go all the way through to linking. This table summarises: Phase of the compilation system Suffix saying “start here” Flag saying “stop after” (suffix of) output file literate pre-processor .lhs - .hs C pre-processor (opt.) .hs (with -cpp) -E .hspp Haskell compiler .hs -C, -S .hc, .s C compiler (opt.) .hc or .c -S .s assembler .s -c .o linker other - a.out -C -E -S -c Thus, a common invocation would be: ghc -c Foo.hs to compile the Haskell source file Foo.hs to an object file Foo.o. Note: What the Haskell compiler proper produces depends on whether a native-code generatornative-code generator is used (producing assembly language) or not (producing C). See for more details. Note: C pre-processing is optional, the -cpp-cpp flag turns it on. See for more details. Note: The option -E-E option runs just the pre-processing passes of the compiler, dumping the result in a file. As described above, the way in which a file is processed by GHC depends on its suffix. This behaviour can be overridden using the -x option: -x suffix -x Causes all files following this option on the command line to be processed as if they had the suffix suffix. For example, to compile a Haskell module in the file M.my-hs, use ghc -c -x hs M.my-hs. help options verbosity options See also the --help, --version, --numeric-version, and --print-libdir modes in . -n -n Does a dry-run, i.e. GHC goes through all the motions of compiling as normal, but does not actually run any external commands. -v -v The -v option makes GHC verbose: it reports its version number and shows (on stderr) exactly how it invokes each phase of the compilation system. Moreover, it passes the -v flag to most phases; each reports its version number (and possibly some other information). Please, oh please, use the -v option when reporting bugs! Knowing that you ran the right bits in the right order is always the first thing we want to verify. -vn -v To provide more control over the compiler's verbosity, the -v flag takes an optional numeric argument. Specifying -v on its own is equivalent to -v3, and the other levels have the following meanings: -v0 Disable all non-essential messages (this is the default). -v1 Minimal verbosity: print one line per compilation (this is the default when ––make or ––interactive is on). -v2 Print the name of each compilation phase as it is executed. (equivalent to -dshow-passes). -v3 The same as -v2, except that in addition the full command line (if appropriate) for each compilation phase is also printed. -v4 The same as -v3 except that the intermediate program representation after each compilation phase is also printed (excluding preprocessed and C/assembly files). -ferror-spans -ferror-spans Causes GHC to emit the full source span of the syntactic entity relating to an error message. Normally, GHC emits the source location of the start of the syntactic entity only. For example: test.hs:3:6: parse error on input `where' becomes: test296.hs:3:6-10: parse error on input `where' And multi-line spans are possible too: test.hs:(5,4)-(6,7): Conflicting definitions for `a' Bound at: test.hs:5:4 test.hs:6:7 In the binding group for: a, b, a Note that line numbers start counting at one, but column numbers start at zero. This choice was made to follow existing convention (i.e. this is how Emacs does it). -Hsize -H Set the minimum size of the heap to size. This option is equivalent to +RTS -Hsize, see . -Rghc-timing -Rghc-timing Prints a one-line summary of timing statistics for the GHC run. This option is equivalent to +RTS -tstderr, see . &separate; sanity-checking options warnings GHC has a number of options that select which types of non-fatal error messages, otherwise known as warnings, can be generated during compilation. By default, you get a standard set of warnings which are generally likely to indicate bugs in your program. These are: -fwarn-overlapping-patterns, -fwarn-deprecations, -fwarn-duplicate-exports, -fwarn-missing-fields, and -fwarn-missing-methods. The following flags are simple ways to select standard “packages” of warnings: -W: -W option Provides the standard warnings plus -fwarn-incomplete-patterns, -fwarn-dodgy-imports, -fwarn-unused-matches, -fwarn-unused-imports, and -fwarn-unused-binds. -Wall: -Wall Turns on all warning options that indicate potentially suspicious code. The warnings that are not enabled by -Wall are -fwarn-simple-patterns, -fwarn-tabs, -fwarn-incomplete-record-updates, -fwarn-monomorphism-restriction, and -fwarn-implicit-prelude. -w: -w Turns off all warnings, including the standard ones and those that -Wall doesn't enable. -Werror: -Werror Makes any warning into a fatal error. Useful so that you don't miss warnings when doing batch compilation. -Wwarn: -Wwarn Warnings are treated only as warnings, not as errors. This is the default, but can be useful to negate a -Werror flag. The full set of warning options is described below. To turn off any warning, simply give the corresponding -fno-warn-... option on the command line. -fwarn-deprecations: -fwarn-deprecations deprecations Causes a warning to be emitted when a deprecated function or type is used. Entities can be marked as deprecated using a pragma, see . This option is on by default. -fwarn-dodgy-imports: -fwarn-dodgy-imports Causes a warning to be emitted when a a datatype T is imported with all constructors, i.e. T(..), but has been exported abstractly, i.e. T. -fwarn-duplicate-exports: -fwarn-duplicate-exports duplicate exports, warning export lists, duplicates Have the compiler warn about duplicate entries in export lists. This is useful information if you maintain large export lists, and want to avoid the continued export of a definition after you've deleted (one) mention of it in the export list. This option is on by default. -fwarn-hi-shadowing: -fwarn-hi-shadowing shadowing interface files Causes the compiler to emit a warning when a module or interface file in the current directory is shadowing one with the same module name in a library or other directory. -fwarn-implicit-prelude: -fwarn-implicit-prelude implicit prelude, warning Have the compiler warn if the Prelude is implicitly imported. This happens unless either the Prelude module is explicitly imported with an import ... Prelude ... line, or this implicit import is disabled (either by -fno-implicit-prelude or a LANGUAGE NoImplicitPrelude pragma). Note that no warning is given for syntax that implicitly refers to the Prelude, even if -fno-implicit-prelude would change whether it refers to the Prelude. For example, no warning is given when 368 means Prelude.fromInteger (368::Prelude.Integer) (where Prelude refers to the actual Prelude module, regardless of the imports of the module being compiled). This warning is off by default. -fwarn-incomplete-patterns: -fwarn-incomplete-patterns incomplete patterns, warning patterns, incomplete Similarly for incomplete patterns, the function g below will fail when applied to non-empty lists, so the compiler will emit a warning about this when -fwarn-incomplete-patterns is enabled. g [] = 2 This option isn't enabled by default because it can be a bit noisy, and it doesn't always indicate a bug in the program. However, it's generally considered good practice to cover all the cases in your functions. -fwarn-incomplete-record-updates: -fwarn-incomplete-record-updates incomplete record updates, warning record updates, incomplete The function f below will fail when applied to Bar, so the compiler will emit a warning about this when -fwarn-incomplete-record-updates is enabled. data Foo = Foo { x :: Int } | Bar f :: Foo -> Foo f foo = foo { x = 6 } This option isn't enabled by default because it can be very noisy, and it often doesn't indicate a bug in the program. -fwarn-missing-fields: -fwarn-missing-fields missing fields, warning fields, missing This option is on by default, and warns you whenever the construction of a labelled field constructor isn't complete, missing initializers for one or more fields. While not an error (the missing fields are initialised with bottoms), it is often an indication of a programmer error. -fwarn-missing-methods: -fwarn-missing-methods missing methods, warning methods, missing This option is on by default, and warns you whenever an instance declaration is missing one or more methods, and the corresponding class declaration has no default declaration for them. The warning is suppressed if the method name begins with an underscore. Here's an example where this is useful: class C a where _simpleFn :: a -> String complexFn :: a -> a -> String complexFn x y = ... _simpleFn ... The idea is that: (a) users of the class will only call complexFn; never _simpleFn; and (b) instance declarations can define either complexFn or _simpleFn. -fwarn-missing-signatures: -fwarn-missing-signatures type signatures, missing If you would like GHC to check that every top-level function/value has a type signature, use the -fwarn-missing-signatures option. As part of the warning GHC also reports the inferred type. The option is off by default. -fwarn-name-shadowing: -fwarn-name-shadowing shadowing, warning This option causes a warning to be emitted whenever an inner-scope value has the same name as an outer-scope value, i.e. the inner value shadows the outer one. This can catch typographical errors that turn into hard-to-find bugs, e.g., in the inadvertent capture of what would be a recursive call in f = ... let f = id in ... f .... -fwarn-orphans: -fwarn-orphans orphan instances, warning orphan rules, warning This option causes a warning to be emitted whenever the module contains an "orphan" instance declaration or rewrite rule. An instance declaration is an orphan if it appears in a module in which neither the class nor the type being instanced are declared in the same module. A rule is an orphan if it is a rule for a function declared in another module. A module containing any orphans is called an orphan module. The trouble with orphans is that GHC must pro-actively read the interface files for all orphan modules, just in case their instances or rules play a role, whether or not the module's interface would otherwise be of any use. Other things being equal, avoid orphan modules. -fwarn-overlapping-patterns: -fwarn-overlapping-patterns overlapping patterns, warning patterns, overlapping By default, the compiler will warn you if a set of patterns are overlapping, e.g., f :: String -> Int f [] = 0 f (_:xs) = 1 f "2" = 2 where the last pattern match in f won't ever be reached, as the second pattern overlaps it. More often than not, redundant patterns is a programmer mistake/error, so this option is enabled by default. -fwarn-simple-patterns: -fwarn-simple-patterns Causes the compiler to warn about lambda-bound patterns that can fail, eg. \(x:xs)->.... Normally, these aren't treated as incomplete patterns by -fwarn-incomplete-patterns. “Lambda-bound patterns” includes all places where there is a single pattern, including list comprehensions and do-notation. In these cases, a pattern-match failure is quite legitimate, and triggers filtering (list comprehensions) or the monad fail operation (monads). For example: f :: [Maybe a] -> [a] f xs = [y | Just y <- xs] Switching on -fwarn-simple-patterns will elicit warnings about these probably-innocent cases, which is why the flag is off by default. -fwarn-tabs: -fwarn-tabs tabs, warning Have the compiler warn if there are tabs in your source file. This warning is off by default. -fwarn-type-defaults: -fwarn-type-defaults defaulting mechanism, warning Have the compiler warn/inform you where in your source the Haskell defaulting mechanism for numeric types kicks in. This is useful information when converting code from a context that assumed one default into one with another, e.g., the ‘default default’ for Haskell 1.4 caused the otherwise unconstrained value 1 to be given the type Int, whereas Haskell 98 defaults it to Integer. This may lead to differences in performance and behaviour, hence the usefulness of being non-silent about this. This warning is off by default. -fwarn-monomorphism-restriction: -fwarn-monomorphism-restriction monomorphism restriction, warning Have the compiler warn/inform you where in your source the Haskell Monomorphism Restriction is applied. If applied silently the MR can give rise to unexpected behaviour, so it can be helpful to have an explicit warning that it is being applied. This warning is off by default. -fwarn-unused-binds: -fwarn-unused-binds unused binds, warning binds, unused Report any function definitions (and local bindings) which are unused. For top-level functions, the warning is only given if the binding is not exported. A definition is regarded as "used" if (a) it is exported, or (b) it is mentioned in the right hand side of another definition that is used, or (c) the function it defines begins with an underscore. The last case provides a way to suppress unused-binding warnings selectively. Notice that a variable is reported as unused even if it appears in the right-hand side of another unused binding. -fwarn-unused-imports: -fwarn-unused-imports unused imports, warning imports, unused Report any modules that are explicitly imported but never used. However, the form import M() is never reported as an unused import, because it is a useful idiom for importing instance declarations, which are anonymous in Haskell. -fwarn-unused-matches: -fwarn-unused-matches unused matches, warning matches, unused Report all unused variables which arise from pattern matches, including patterns consisting of a single variable. For instance f x y = [] would report x and y as unused. The warning is suppressed if the variable name begins with an underscore, thus: f _x = True If you're feeling really paranoid, the -dcore-lint option-dcore-lint is a good choice. It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's sanity, not yours.) &packages; optimisation improvement, code The -O* options specify convenient “packages” of optimisation flags; the -f* options described later on specify individual optimisations to be turned on/off; the -m* options specify machine-specific optimisations to be turned on/off. There are many options that affect the quality of code produced by GHC. Most people only have a general goal, something like “Compile quickly” or “Make my program run like greased lightning.” The following “packages” of optimisations (or lack thereof) should suffice. Note that higher optimisation levels cause more cross-module optimisation to be performed, which can have an impact on how much of your program needs to be recompiled when you change something. This is one reason to stick to no-optimisation when developing code. No -O*-type option specified: -O* not specified This is taken to mean: “Please compile quickly; I'm not over-bothered about compiled-code quality.” So, for example: ghc -c Foo.hs -O0: -O0 Means “turn off all optimisation”, reverting to the same settings as if no -O options had been specified. Saying -O0 can be useful if eg. make has inserted a -O on the command line already. -O or -O1: -O option -O1 option optimisenormally Means: “Generate good-quality code without taking too long about it.” Thus, for example: ghc -c -O Main.lhs -O2: -O2 option optimiseaggressively Means: “Apply every non-dangerous optimisation, even if it means significantly longer compile times.” The avoided “dangerous” optimisations are those that can make runtime or space worse if you're unlucky. They are normally turned on or off individually. At the moment, -O2 is unlikely to produce better code than -O. -Ofile <file>: -Ofile <file> option optimising, customised (NOTE: not supported since GHC 4.x. Please ask if you're interested in this.) For those who need absolute control over exactly what options are used (e.g., compiler writers, sometimes :-), a list of options can be put in a file and then slurped in with -Ofile. In that file, comments are of the #-to-end-of-line variety; blank lines and most whitespace is ignored. Please ask if you are baffled and would like an example of -Ofile! We don't use a -O* flag for day-to-day work. We use -O to get respectable speed; e.g., when we want to measure something. When we want to go for broke, we tend to use -O2 -fvia-C (and we go for lots of coffee breaks). The easiest way to see what -O (etc.) “really mean” is to run with -v, then stand back in amazement. -f* options (GHC) -fno-* options (GHC) These flags turn on and off individual optimisations. They are normally set via the -O options described above, and as such, you shouldn't need to set any of them explicitly (indeed, doing so could lead to unexpected results). However, there are one or two that may be of interest: -fexcess-precision: -fexcess-precision When this option is given, intermediate floating point values can have a greater precision/range than the final type. Generally this is a good thing, but some programs may rely on the exact precision/range of Float/Double values and should not use this option for their compilation. -fignore-asserts: -fignore-asserts Causes GHC to ignore uses of the function Exception.assert in source code (in other words, rewriting Exception.assert p e to e (see ). This flag is turned on by -O. -fno-cse -fno-cse Turns off the common-sub-expression elimination optimisation. Can be useful if you have some unsafePerformIO expressions that you don't want commoned-up. -fno-strictness -fno-strictness Turns off the strictness analyser; sometimes it eats too many cycles. -fno-full-laziness -fno-full-laziness Turns off the full laziness optimisation (also known as let-floating). Full laziness increases sharing, which can lead to increased memory residency. NOTE: GHC doesn't implement complete full-laziness. When optimisation in on, and -fno-full-laziness is not given, some transformations that increase sharing are performed, such as extracting repeated computations from a loop. These are the same transformations that a fully lazy implementation would do, the difference is that GHC doesn't consistently apply full-laziness, so don't rely on it. -fspec-constr -fspec-constr Turn on call-pattern specialisation. -fliberate-case -fliberate-case Turn on the liberate-case transformation. -fstatic-argument-transformation -fstatic-argument-transformation Turn on the static argument transformation. -fno-state-hack -fno-state-hack Turn off the "state hack" whereby any lambda with a State# token as argument is considered to be single-entry, hence it is considered OK to inline things inside it. This can improve performance of IO and ST monad code, but it runs the risk of reducing sharing. -fomit-interface-pragmas -fomit-interface-pragmas Tells GHC to omit all inessential information from the interface file generated for the module being compiled (say M). This means that a module importing M will see only the types of the functions that M exports, but not their unfoldings, strictness info, etc. Hence, for example, no function exported by M will be inlined into an importing module. The benefit is that modules that import M will need to be recompiled less often (only when M's exports change their type, not when they change their implementation). -fignore-interface-pragmas -fignore-interface-pragmas Tells GHC to ignore all inessential information when reading interface files. That is, even if M.hi contains unfolding or strictness information for a function, GHC will ignore that information. -funbox-strict-fields: -funbox-strict-fields strict constructor fields constructor fields, strict This option causes all constructor fields which are marked strict (i.e. “!”) to be unboxed or unpacked if possible. It is equivalent to adding an UNPACK pragma to every strict constructor field (see ). This option is a bit of a sledgehammer: it might sometimes make things worse. Selectively unboxing fields by using UNPACK pragmas might be better. -funfolding-creation-threshold=n: -funfolding-creation-threshold inlining, controlling unfolding, controlling (Default: 45) Governs the maximum size that GHC will allow a function unfolding to be. (An unfolding has a “size” that reflects the cost in terms of “code bloat” of expanding that unfolding at at a call site. A bigger function would be assigned a bigger cost.) Consequences: (a) nothing larger than this will be inlined (unless it has an INLINE pragma); (b) nothing larger than this will be spewed into an interface file. Increasing this figure is more likely to result in longer compile times than faster code. The next option is more useful: -funfolding-use-threshold=n -funfolding-use-threshold inlining, controlling unfolding, controlling (Default: 8) This is the magic cut-off figure for unfolding: below this size, a function definition will be unfolded at the call-site, any bigger and it won't. The size computed for a function depends on two things: the actual size of the expression minus any discounts that apply (see -funfolding-con-discount). &phases; Concurrent Haskellusing GHC supports Concurrent Haskell by default, without requiring a special option or libraries compiled in a certain way. To get access to the support libraries for Concurrent Haskell, just import Control.Concurrent. More information on Concurrent Haskell is provided in the documentation for that module. The following RTS option(s) affect the behaviour of Concurrent Haskell programs:RTS options, concurrent -Cs -CsRTS option Sets the context switch interval to s seconds. A context switch will occur at the next heap block allocation after the timer expires (a heap block allocation occurs every 4k of allocation). With -C0 or -C, context switches will occur as often as possible (at every heap block allocation). By default, context switches occur every 20ms. parallelism SMP GHC supports running Haskell programs in parallel on an SMP (symmetric multiprocessor). There's a fine distinction between concurrency and parallelism: parallelism is all about making your program run faster by making use of multiple processors simultaneously. Concurrency, on the other hand, is a means of abstraction: it is a convenient way to structure a program that must respond to multiple asynchronous events. However, the two terms are certainly related. By making use of multiple CPUs it is possible to run concurrent threads in parallel, and this is exactly what GHC's SMP parallelism support does. But it is also possible to obtain performance improvements with parallelism on programs that do not use concurrency. This section describes how to use GHC to compile and run parallel programs, in we describe the language features that affect parallelism. In order to make use of multiple CPUs, your program must be linked with the -threaded option (see ). Then, to run a program on multiple CPUs, use the RTS -N option: -Nx -NxRTS option Use x simultaneous threads when running the program. Normally x should be chosen to match the number of CPU cores on the machine. There is no means (currently) by which this value may vary after the program has started. For example, on a dual-core machine we would probably use +RTS -N2 -RTS. Whether hyperthreading cores should be counted or not is an open question; please feel free to experiment and let us know what results you find. Add the -sstderr RTS option when running the program to see timing stats, which will help to tell you whether your program got faster by using more CPUs or not. If the user time is greater than the elapsed time, then the program used more than one CPU. You should also run the program without -N for comparison. GHC's parallelism support is new and experimental. It may make your program go faster, or it might slow it down - either way, we'd be interested to hear from you. One significant limitation with the current implementation is that the garbage collector is still single-threaded, and all execution must stop when GC takes place. This can be a significant bottleneck in a parallel program, especially if your program does a lot of GC. If this happens to you, then try reducing the cost of GC by tweaking the GC settings (): enlarging the heap or the allocation area size is a good start. -m* options platform-specific options machine-specific options Some flags only make sense for particular target platforms. -monly-[32]-regs: (iX86 machines)-monly-N-regs option (iX86 only) GHC tries to “steal” four registers from GCC, for performance reasons; it almost always works. However, when GCC is compiling some modules with four stolen registers, it will crash, probably saying: Foo.hc:533: fixed or forbidden register was spilled. This may be due to a compiler bug or to impossible asm statements or clauses. Just give some registers back with -monly-N-regs. Try `3' first, then `2'. If `2' doesn't work, please report the bug to us. &runtime; intermediate code generation GHC can dump its optimized intermediate code (said to be in “Core” format) to a file as a side-effect of compilation. Non-GHC back-end tools can read and process Core files; these files have the suffix .hcr. The Core format is described in An External Representation for the GHC Core Language, and sample tools for manipulating Core files (in Haskell) are in the GHC source distribution directory under utils/ext-core. Note that the format of .hcr files is different from the Core output format that GHC generates for debugging purposes (), though the two formats appear somewhat similar. The Core format natively supports notes which you can add to your source code using the CORE pragma (see ). -fext-core -fext-core Generate .hcr files. Currently (as of version 6.8.2), GHC does not have the ability to read in External Core files as source. If you would like GHC to have this ability, please make your wishes known to the GHC Team. &debug; &flags;