GHC, using using GHC In this chapter you'll find a complete reference to the GHC command-line syntax, including all 400+ flags. It's a large and complex system, and there are lots of details, so it can be quite hard to figure out how to get started. With that in mind, this introductory section provides a quick introduction to the basic usage of GHC for compiling a Haskell program, before the following sections dive into the full syntax. Let's create a Hello World program, and compile and run it. First, create a file hello.hs containing the Haskell code: main = putStrLn "Hello, World!" To compile the program, use GHC like this: $ ghc hello.hs (where $ represents the prompt: don't type it). GHC will compile the source file hello.hs, producing an object file hello.o and an interface file hello.hi, and then it will link the object file to the libraries that come with GHC to produce an executable called hello on Unix/Linux/Mac, or hello.exe on Windows. By default GHC will be very quiet about what it is doing, only printing error messages. If you want to see in more detail what's going on behind the scenes, add -v to the command line. Then we can run the program like this: $ ./hello Hello World! If your program contains multiple modules, then you only need to tell GHC the name of the source file containing the Main module, and GHC will examine the import declarations to find the other modules that make up the program and find their source files. This means that, with the exception of the Main module, every source file should be named after the module name that it contains (with dots replaced by directory separators). For example, the module Data.Person would be in the file Data/Person.hs on Unix/Linux/Mac, or Data\Person.hs on Windows. 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 deliberately uses name shadowing, it should be compiled with the -fno-warn-name-shadowing 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 -fno-warn-name-shadowing #-} module X where ... OPTIONS_GHC is a file-header pragma (see ). Only dynamic flags can be used in an OPTIONS_GHC pragma (see ). 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 OPTIONS_GHC 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. .ll An llvm-intermediate-language source file, usually produced by the compiler. .bc An llvm-intermediate-language bitcode file, usually produced by the 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. If no mode flag is present, then GHC will enter make mode () if there are any Haskell source files given on the command line, or else it will link the objects named on the command line to produce an executable. The available mode flags 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 . This mode is the default if there are any Haskell source files mentioned on the command line, and in this case the --make option can be omitted. 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-extensions ghc --supported-languages --supported-extensions--supported-languages Print the supported language extensions. ghc --show-options --show-options Print the supported command line options. This flag can be used for autocompletion in a shell. 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 In this mode, 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 In fact, GHC enters make mode automatically if there are any Haskell source files on the command line and no other mode is specified, so in this case we could just type ghc Main.hs Any number of source file names or module names may be specified; 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. Using the -j flag, you can compile modules in parallel. Specify -jN to compile N jobs in parallel. 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 what backend code generator is used. 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 . -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-warnings-deprecations, -fwarn-amp, -fwarn-deprecated-flags, -fwarn-unrecognised-pragmas, -fwarn-pointless-pragmas, -fwarn-duplicate-constraints, -fwarn-duplicate-exports, -fwarn-overflowed-literals, -fwarn-empty-enumerations, -fwarn-missing-fields, -fwarn-missing-methods, -fwarn-lazy-unlifted-bindings, -fwarn-wrong-do-bind, -fwarn-unsupported-calling-conventions, -fwarn-dodgy-foreign-imports, -fwarn-inline-rule-shadowing, and -fwarn-unsupported-llvm-version. 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-exports, -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-tabs, -fwarn-incomplete-uni-patterns, -fwarn-incomplete-record-updates, -fwarn-monomorphism-restriction, -fwarn-auto-orphans, -fwarn-implicit-prelude, -fwarn-missing-local-sigs, -fwarn-missing-import-lists. -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. -fdefer-type-errors: -fdefer-type-errors warnings Defer as many type errors as possible until runtime. At compile time you get a warning (instead of an error). At runtime, if you use a value that depends on a type error, you get a runtime error; but you can run any type-correct parts of your code just fine. See -fhelpful-errors: -fhelpful-errors warnings When a name or package is not found in scope, make suggestions for the name or package you might have meant instead. This option is on by default. -fwarn-unrecognised-pragmas: -fwarn-unrecognised-pragmas warnings pragmas Causes a warning to be emitted when a pragma that GHC doesn't recognise is used. As well as pragmas that GHC itself uses, GHC also recognises pragmas known to be used by other tools, e.g. OPTIONS_HUGS and DERIVE. This option is on by default. -fwarn-pointless-pragmas: -fwarn-pointless-pragmas warnings pragmas Causes a warning to be emitted when GHC detects that a module contains a pragma that has no effect. This option is on by default. -fwarn-warnings-deprecations: -fwarn-warnings-deprecations warnings deprecations Causes a warning to be emitted when a module, function or type with a WARNING or DEPRECATED pragma is used. See for more details on the pragmas. This option is on by default. -fwarn-amp: -fwarn-amp amp applicative-monad proposal Causes a warning to be emitted when a definition is in conflict with the AMP (Applicative-Monad proosal), namely: 1. Instance of Monad without Applicative; 2. Instance of MonadPlus without Alternative; 3. Custom definitions of join/pure/<*> This option is on by default. -fwarn-deprecated-flags: -fwarn-deprecated-flags deprecated-flags Causes a warning to be emitted when a deprecated commandline flag is used. This option is on by default. -fwarn-unsupported-calling-conventions: -fwarn-unsupported-calling-conventions Causes a warning to be emitted for foreign declarations that use unsupported calling conventions. In particular, if the stdcall calling convention is used on an architecture other than i386 then it will be treated as ccall. -fwarn-dodgy-foreign-imports: -fwarn-dodgy-foreign-imports Causes a warning to be emitted for foreign imports of the following form: foreign import "f" f :: FunPtr t on the grounds that it probably should be foreign import "&f" f :: FunPtr t The first form declares that `f` is a (pure) C function that takes no arguments and returns a pointer to a C function with type `t`, whereas the second form declares that `f` itself is a C function with type `t`. The first declaration is usually a mistake, and one that is hard to debug because it results in a crash, hence this warning. -fwarn-dodgy-exports: -fwarn-dodgy-exports Causes a warning to be emitted when a datatype T is exported with all constructors, i.e. T(..), but is it just a type synonym. Also causes a warning to be emitted when a module is re-exported, but that module exports nothing. -fwarn-dodgy-imports: -fwarn-dodgy-imports Causes a warning to be emitted in the following cases: When a datatype T is imported with all constructors, i.e. T(..), but has been exported abstractly, i.e. T. When an import statement hides an entity that is not exported. -fwarn-overflowed-literals: -fwarn-overflowed-literals Causes a warning to be emitted if a literal will overflow, e.g. 300 :: Word8. -fwarn-empty-enumerations: -fwarn-empty-enumerations Causes a warning to be emitted if an enumeration is empty, e.g. [5 .. 3]. -fwarn-lazy-unlifted-bindings: -fwarn-lazy-unlifted-bindings Causes a warning to be emitted when an unlifted type is bound in a way that looks lazy, e.g. where (I# x) = .... Use where !(I# x) = ... instead. This will be an error, rather than a warning, in GHC 7.2. -fwarn-duplicate-constraints: -fwarn-duplicate-constraints duplicate constraints, warning Have the compiler warn about duplicate constraints in a type signature. For example f :: (Eq a, Show a, Eq a) => a -> a The warning will indicate the duplicated Eq a constraint. This option is on by default. -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-identities: -fwarn-identities Causes the compiler to emit a warning when a Prelude numeric conversion converts a type T to the same type T; such calls are probably no-ops and can be omitted. The functions checked for are: toInteger, toRational, fromIntegral, and realToFrac. -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 -XNoImplicitPrelude or a LANGUAGE NoImplicitPrelude pragma). Note that no warning is given for syntax that implicitly refers to the Prelude, even if -XNoImplicitPrelude 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-uni-patterns: -fwarn-incomplete-patterns -fwarn-incomplete-uni-patterns incomplete patterns, warning patterns, incomplete The option -fwarn-incomplete-patterns warns about places where a pattern-match might fail at runtime. 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, and it is switched on by -W. The flag -fwarn-incomplete-uni-patterns is similar, except that it applies only to lambda-expressions and pattern bindings, constructs that only allow a single pattern: h = \[] -> 2 Just k = f y -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-import-lists: -fwarn-import-lists missing import lists, warning import lists, missing This flag warns if you use an unqualified import declaration that does not explicitly list the entities brought into scope. For example module M where import X( f ) import Y import qualified Z p x = f x x The -fwarn-import-lists flag will warn about the import of Y but not X If module Y is later changed to export (say) f, then the reference to f in M will become ambiguous. No warning is produced for the import of Z because extending Z's exports would be unlikely to produce ambiguity in M. -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. The MINIMAL pragma can be used to change which combination of methods will be required for instances of a particular class. See . -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-missing-local-sigs: -fwarn-missing-local-sigs type signatures, missing If you use the -fwarn-missing-local-sigs flag GHC will warn you about any polymorphic local bindings. 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 .... The warning is suppressed for names beginning with an underscore. For example f x = do { _ignore <- this; _ignore <- that; return (the other) } -fwarn-orphans, -fwarn-auto-orphans: -fwarn-orphans -fwarn-auto-orphans orphan instances, warning orphan rules, warning These flags cause 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. See for details. The flag -fwarn-orphans warns about user-written orphan rules or instances. The flag -fwarn-auto-orphans warns about automatically-generated orphan rules, notably as a result of specialising functions, for type classes (Specialise) or argument values (SpecConstr). -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-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 and later 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 -fwarn-unused-do-bind: -fwarn-unused-do-bind unused do binding, warning do binding, unused Report expressions occurring in do and mdo blocks that appear to silently throw information away. For instance do { mapM popInt xs ; return 10 } would report the first statement in the do block as suspicious, as it has the type StackM [Int] and not StackM (), but that [Int] value is not bound to anything. The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away: do { _ <- mapM popInt xs ; return 10 } Of course, in this particular situation you can do even better: do { mapM_ popInt xs ; return 10 } -fwarn-wrong-do-bind: -fwarn-wrong-do-bind apparently erroneous do binding, warning do binding, apparently erroneous Report expressions occurring in do and mdo blocks that appear to lack a binding. For instance do { return (popInt 10) ; return 10 } would report the first statement in the do block as suspicious, as it has the type StackM (StackM Int) (which consists of two nested applications of the same monad constructor), but which is not then "unpacked" by binding the result. The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away: do { _ <- return (popInt 10) ; return 10 } For almost all sensible programs this will indicate a bug, and you probably intended to write: do { popInt 10 ; return 10 } 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. 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 (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). A flag -fwombat can be negated by saying -fno-wombat. The flags below are off by default, except where noted below. See for a compact list. -favoid-vect Part of Data Parallel Haskell (DPH). Off by default. Enable the vectorisation avoidance optimisation. This optimisation only works when used in combination with the -fvectorise transformation. While vectorisation of code using DPH is often a big win, it can also produce worse results for some kinds of code. This optimisation modifies the vectorisation transformation to try to determine if a function would be better of unvectorised and if so, do just that. -fcase-merge On by default. Merge immediately-nested case expressions that scrutinse the same variable. Example case x of Red -> e1 _ -> case x of Blue -> e2 Green -> e3 ==> case x of Red -> e1 Blue -> e2 Green -> e2 -fcse -fcse On by default.. Enables the common-sub-expression elimination optimisation. Switching this off can be useful if you have some unsafePerformIO expressions that you don't want commoned-up. -fdicts-cheap A very experimental flag that makes dictionary-valued expressions seem cheap to the optimiser. -fdo-lambda-eta-expansion On by default. Eta-expand let-bindings to increase their arity. -fdo-eta-reduction On by default. Eta-reduce lambda expressions, if doing so gets rid of a whole group of lambdas. -feager-blackholing Usually GHC black-holes a thunk only when it switches threads. This flag makes it do so as soon as the thunk is entered. See Haskell on a shared-memory multiprocessor. -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. Note that the 32-bit x86 native code generator only supports excess-precision mode, so neither -fexcess-precision nor -fno-excess-precision has any effect. This is a known bug, see . -fexpose-all-unfoldings An experimental flag to expose all unfoldings, even for very large or recursive functions. This allows for all functions to be inlined while usually GHC would avoid inlining larger functions. -ffloat-in On by default. Float let-bindings inwards, nearer their binding site. See Let-floating: moving bindings to give faster programs (ICFP'96). This optimisation moves let bindings closer to their use site. The benefit here is that this may avoid unnecessary allocation if the branch the let is now on is never executed. It also enables other optimisation passes to work more effectively as they have more information locally. This optimisation isn't always beneficial though (so GHC applies some heuristics to decide when to apply it). The details get complicated but a simple example is that it is often beneficial to move let bindings outwards so that multiple let bindings can be grouped into a larger single let binding, effectively batching their allocation and helping the garbage collector and allocator. -ffull-laziness -ffull-laziness On by default. Run the full laziness optimisation (also known as let-floating), which floats let-bindings outside enclosing lambdas, in the hope they will be thereby be computed less often. See Let-floating: moving bindings to give faster programs (ICFP'96). 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. -ffun-to-thunk -fignore-asserts Worker-wrapper removes unused arguments, but usually we do not remove them all, lest it turn a function closure into a thunk, thereby perhaps creating a space leak and/or disrupting inlining. This flag allows worker/wrapper to remove all value lambdas. Off by default. -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. -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. -flate-dmd-anal -flate-dmd-anal Off by default.Run demand analysis again, at the end of the simplification pipeline. We found some opportunities for discovering strictness that were not visible earlier; and optimisations like SpecConstr can create functions with unused arguments which are eliminated by late demand analysis. Improvements are modest, but so is the cost. See notes on the Trac wiki page. -fliberate-case -fliberate-case Off by default, but enabled by -O2. Turn on the liberate-case transformation. This unrolls recursive function once in its own RHS, to avoid repeated case analysis of free variables. It's a bit like the call-pattern specialiser (-fspec-constr) but for free variables rather than arguments. -fliberate-case-threshold=N -fliberate-case-threshold Set the size threshold for the liberate-case transformation. -fmax-relevant-bindings=N -fmax-relevant-bindings The type checker sometimes displays a fragment of the type environment in error messages, but only up to some maximum number, set by this flag. The default is 6. Turning it off with -fno-max-relevant-bindings gives an unlimited number. Syntactically top-level bindings are also usually excluded (since they may be numerous), but -fno-max-relevant-bindings includes them too. -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). -fomit-yields -fomit-yields On by default. Tells GHC to omit heap checks when no allocation is being performed. While this improves binary sizes by about 5%, it also means that threads run in tight non-allocating loops will not get preempted in a timely fashion. If it is important to always be able to interrupt such threads, you should turn this optimization off. Consider also recompiling all libraries with this optimization turned off, if you need to guarantee interruptibility. -fpedantic-bottoms -fpedantic-bottoms Make GHC be more precise about its treatment of bottom (but see also -fno-state-hack). In particular, stop GHC eta-expanding through a case expression, which is good for performance, but bad if you are using seq on partial applications. -fregs-graph Off by default, but enabled by -O2. Only applies in combination with the native code generator. Use the graph colouring register allocator for register allocation in the native code generator. By default, GHC uses a simpler, faster linear register allocator. The downside being that the linear register allocator usually generates worse code. -fregs-iterative Off by default, only applies in combination with the native code generator. Use the iterative coalescing graph colouring register allocator for register allocation in the native code generator. This is the same register allocator as the -freg-graph one but also enables iterative coalescing during register allocation. -fsimpl-tick-factor=n -fsimpl-tick-factor GHC's optimiser can diverge if you write rewrite rules ( ) that don't terminate, or (less satisfactorily) if you code up recursion through data types (). To avoid making the compiler fall into an infinite loop, the optimiser carries a "tick count" and stops inlining and applying rewrite rules when this count is exceeded. The limit is set as a multiple of the program size, so bigger programs get more ticks. The -fsimpl-tick-factor flag lets you change the multiplier. The default is 100; numbers larger than 100 give more ticks, and numbers smaller than 100 give fewer. If the tick-count expires, GHC summarises what simplifier steps it has done; you can use -fddump-simpl-stats to generate a much more detailed list. Usually that identifies the loop quite accurately, because some numbers are very large. -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 (aka inlining) that unfolding 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 -funfolding-use-threshold 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 (aka inlining): 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). The difference between this and -funfolding-creation-threshold is that this one determines if a function definition will be inlined at a call site. The other option determines if a function definition will be kept around at all for potential inlining. -fvectorise Part of Data Parallel Haskell (DPH). Off by default. Enable the vectorisation optimisation transformation. This optimisation transforms the nested data parallelism code of programs using DPH into flat data parallelism. Flat data parallel programs should have better load balancing, enable SIMD parallelism and friendlier cache behaviour. -fspec-constr -fspec-constr Off by default, but enabled by -O2. Turn on call-pattern specialisation; see Call-pattern specialisation for Haskell programs. This optimisation specializes recursive functions according to their argument "shapes". This is best explained by example so consider: last :: [a] -> a last [] = error "last" last (x : []) = x last (x : xs) = last xs In this code, once we pass the initial check for an empty list we know that in the recursive case this pattern match is redundant. As such -fspec-constr will transform the above code to: last :: [a] -> a last [] = error "last" last (x : xs) = last' x xs where last' x [] = x last' x (y : ys) = last' y ys As well avoid unnecessary pattern matching it also helps avoid unnecessary allocation. This applies when a argument is strict in the recursive call to itself but not on the initial entry. As strict recursive branch of the function is created similar to the above example. -fspecialise -fspecialise On by default. Specialise each type-class-overloaded function defined in this module for the types at which it is called in this module. Also specialise imported functions that have an INLINABLE pragma () for the types at which they are called in this module. -fstatic-argument-transformation -fstatic-argument-transformation Turn on the static argument transformation, which turns a recursive function into a non-recursive one with a local recursive loop. See Chapter 7 of Andre Santos's PhD thesis -fstrictness On by default.. Switch on the strictness analyser. There is a very old paper about GHC's strictness analyser, Measuring the effectiveness of a simple strictness analyser, but the current one is quite a bit different. The strictness analyser figures out when arguments and variables in a function can be treated 'strictly' (that is they are always evaluated in the function at some point). This allow GHC to apply certain optimisations such as unboxing that otherwise don't apply as they change the semantics of the program when applied to lazy arguments. -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 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. An alternative is to use -funbox-strict-fields to turn on unboxing by default but disable it for certain constructor fields using the NOUNPACK pragma (see ). -funbox-small-strict-fields: -funbox-small-strict-fields strict constructor fields constructor fields, strict On by default.. This option causes all constructor fields which are marked strict (i.e. “!”) and which representation is smaller or equal to the size of a pointer to be unpacked, if possible. It is equivalent to adding an UNPACK pragma (see ) to every strict constructor field that fulfils the size restriction. For example, the constructor fields in the following data types data A = A !Int data B = B !A newtype C = C B data D = D !C would all be represented by a single Int# (see ) value with -funbox-small-strict-fields enabled. This option is less of a sledgehammer than -funbox-strict-fields: it should rarely make things worse. If you use -funbox-small-strict-fields to turn on unboxing by default you can disable it for certain constructor fields using the NOUNPACK pragma (see ). Note that for consistency Double, Word64, and Int64 constructor fields are unpacked on 32-bit platforms, even though they are technically larger than a pointer on those platforms. &code-gens; &phases; &shared_libs; 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. Optionally, the program may be linked with the -threaded option (see . This provides two benefits: It enables the -N-NxRTS option RTS option to be used, which allows threads to run in parallelparallelism on a multiprocessormultiprocessorSMP or multicoremulticore machine. See . If a thread makes a foreign call (and the call is not marked unsafe), then other Haskell threads in the program will continue to run while the foreign call is in progress. Additionally, foreign exported Haskell functions may be called from multiple OS threads simultaneously. See . 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 ). Additionally, the following compiler options affect parallelism: -feager-blackholing -feager-blackholing Blackholing is the act of marking a thunk (lazy computuation) as being under evaluation. It is useful for three reasons: firstly it lets us detect certain kinds of infinite loop (the NonTermination exception), secondly it avoids certain kinds of space leak, and thirdly it avoids repeating a computation in a parallel program, because we can tell when a computation is already in progress. The option -feager-blackholing causes each thunk to be blackholed as soon as evaluation begins. The default is "lazy blackholing", whereby thunks are only marked as being under evaluation when a thread is paused for some reason. Lazy blackholing is typically more efficient (by 1-2% or so), because most thunks don't need to be blackholed. However, eager blackholing can avoid more repeated computation in a parallel program, and this often turns out to be important for parallelism. We recommend compiling any code that is intended to be run in parallel with the -feager-blackholing flag. There are two ways to run a program on multiple processors: call Control.Concurrent.setNumCapabilities from your program, or 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 machineWhether hyperthreading cores should be counted or not is an open question; please feel free to experiment and let us know what results you find.. For example, on a dual-core machine we would probably use +RTS -N2 -RTS. Omitting x, i.e. +RTS -N -RTS, lets the runtime choose the value of x itself based on how many processors are in your machine. Be careful when using all the processors in your machine: if some of your processors are in use by other programs, this can actually harm performance rather than improve it. Setting -N also has the effect of enabling the parallel garbage collector (see ). The current value of the -N option is available to the Haskell program via Control.Concurrent.getNumCapabilities, and it may be changed while the program is running by calling Control.Concurrent.setNumCapabilities. The following options affect the way the runtime schedules threads on CPUs: -qa -qaRTS option Use the OS's affinity facilities to try to pin OS threads to CPU cores. This is an experimental feature, and may or may not be useful. Please let us know whether it helps for you! -qm -qmRTS option Disable automatic migration for load balancing. Normally the runtime will automatically try to schedule threads across the available CPUs to make use of idle CPUs; this option disables that behaviour. Note that migration only applies to threads; sparks created by par are load-balanced separately by work-stealing. This option is probably only of use for concurrent programs that explicitly schedule threads onto CPUs with Control.Concurrent.forkOn. Add the -s 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. The output of +RTS -s tells you how many “sparks” were created and executed during the run of the program (see ), which will give you an idea how well your par annotations are working. GHC's parallelism support has improved in 6.12.1 as a result of much experimentation and tuning in the runtime system. We'd still be interested to hear how well it works for you, and we're also interested in collecting parallel programs to add to our benchmarking suite. -m* options platform-specific options machine-specific options Some flags only make sense for particular target platforms. -msse2: (x86 only, added in GHC 7.0.1) Use the SSE2 registers and instruction set to implement floating point operations when using the native code generator. This gives a substantial performance improvement for floating point, but the resulting compiled code will only run on processors that support SSE2 (Intel Pentium 4 and later, or AMD Athlon 64 and later). The LLVM backend will also use SSE2 if your processor supports it but detects this automatically so no flag is required. SSE2 is unconditionally used on x86-64 platforms. -msse4.2: (x86 only, added in GHC 7.4.1) Use the SSE4.2 instruction set to implement some floating point and bit operations when using the native code generator. The resulting compiled code will only run on processors that support SSE4.2 (Intel Core i7 and later). The LLVM backend will also use SSE4.2 if your processor supports it but detects this automatically so no flag is required. &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 available in the extcore package on Hackage. 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;