X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fusing.xml;h=cb1fb650b0c344203cfa65d3ed32a43792330b2e;hb=05e0eac7ced255a7bcc2904ca8499b26275be8d0;hp=e26cbddc1e08c52eb247571af72734a5d948245e;hpb=2f979bf83bad9b012efa225c38cfcd7ea91b2b29;p=ghc-hetmet.git diff --git a/docs/users_guide/using.xml b/docs/users_guide/using.xml index e26cbdd..cb1fb65 100644 --- a/docs/users_guide/using.xml +++ b/docs/users_guide/using.xml @@ -6,6 +6,77 @@ using GHC + Getting started: compiling programs + + + 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 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. + + + + Options overview GHC's behaviour is controlled by @@ -46,26 +117,25 @@ ghc [argument...] 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 - option. Rather than maintaining + tight. For instance, if a Haskell source file deliberately + uses name shadowing, it should be compiled with the + 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 #-} +{-# OPTIONS_GHC -fno-warn-name-shadowing #-} 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. + 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 @@ -111,7 +181,7 @@ module X where Mode flags - For example, or . + For example, or . There may only be a single mode flag on the command line. The available modes are listed in . @@ -121,7 +191,7 @@ module X where 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 + OPTIONS_GHC pragma in a source file, or set using :set in GHCi. @@ -221,10 +291,20 @@ module X where Modes of operation - 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'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: @@ -243,7 +323,7 @@ module X where - ghc --make + ghc ––make make mode @@ -255,6 +335,12 @@ module X where 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 option can be omitted. + @@ -356,9 +442,10 @@ module X where + ghc --supported-extensions ghc --supported-languages - + Print the supported language extensions. @@ -429,8 +516,7 @@ module X where separate compilation - When given the option, - GHC will build a multi-module Haskell program by following + 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 @@ -441,12 +527,22 @@ module X where 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. + + 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 @@ -845,7 +941,9 @@ ghc -c Foo.hs , , , - , and + , + , + , and . The following flags are simple ways to select standard “packages” of warnings: @@ -859,6 +957,7 @@ ghc -c Foo.hs -W option Provides the standard warnings plus , + , , , , and @@ -877,7 +976,8 @@ ghc -c Foo.hs , , , - , and + , + , and . @@ -919,6 +1019,23 @@ ghc -c Foo.hs + : + + + + 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. + + + + : @@ -972,11 +1089,25 @@ foreign import "&f" f :: FunPtr t + : + + + + 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. + + + + : - Causes a warning to be emitted when a a datatype + Causes a warning to be emitted when a datatype T is imported with all constructors, i.e. T(..), but has been exported abstractly, i.e. T. @@ -984,6 +1115,20 @@ foreign import "&f" f :: FunPtr t + : + + + + 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 7.0. + + + + + : @@ -1159,6 +1304,11 @@ f foo = foo { x = 6 } 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) } + + @@ -1329,6 +1479,56 @@ f "2" = 2 + + : + + + unused do binding, warning + do binding, unused + + Report expressions occuring 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 } + + + + + + + : + + + apparently erroneous do binding, warning + do binding, apparently erroneous + + Report expressions occuring 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 @@ -1564,6 +1764,26 @@ f "2" = 2 + + + + + Turns off the float-in transformation. + + + + + + + + + + Turns off the automatic specialisation of overloaded functions. + + + + + @@ -1707,7 +1927,9 @@ f "2" = 2 &phases; - + + &shared_libs; + Using Concurrent Haskell Concurrent Haskellusing @@ -1716,7 +1938,7 @@ f "2" = 2 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. + url="&libraryBaseLocation;/Control-Concurrent.html">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 @@ -1765,17 +1987,60 @@ f "2" = 2 linkend="lang-parallel" /> we describe the language features that affect parallelism. - - Options for SMP parallelism + + Compile-time options for SMP parallelism In order to make use of multiple CPUs, your program must be linked with the option (see ). Then, to run a program on multiple - CPUs, use the RTS option: + linkend="options-linker" />). Additionally, the following + compiler options affect parallelism: + + + + + + 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 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 + flag. + + + + + + + + RTS options for SMP parallelism + + To run a program on multiple CPUs, use the + RTS option: + + - + RTS option Use x simultaneous threads when @@ -1787,12 +2052,26 @@ f "2" = 2 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 also has the effect of - setting (the number of OS threads to - use for garbage collection) to the same value. + enabling the parallel garbage collector (see + ). There is no means (currently) by which this value may vary after the program has started. + + The current value of the option + is available to the Haskell program + via GHC.Conc.numCapabilities. @@ -1802,6 +2081,17 @@ f "2" = 2 + + RTS + 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! + + + RTS option @@ -1809,9 +2099,16 @@ f "2" = 2 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. It is probably - only of use if you are explicitly scheduling threads onto - CPUs with GHC.Conc.forkOnIO. + 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 GHC.Conc.forkOnIO. + @@ -1844,19 +2141,20 @@ f "2" = 2 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. + 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. @@ -1873,9 +2171,27 @@ f "2" = 2 + : + + + (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). + + + SSE2 is unconditionally used on x86-64 platforms. + + + + + : - (iX86 machines)-monly-N-regs + (x86 only)-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 @@ -1906,7 +2222,7 @@ statements or clauses. 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 + .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 @@ -1944,7 +2260,6 @@ statements or clauses.