X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fusing.xml;h=af7950cd80b2d1c23203cc81efb8e273f74f993e;hb=eca1e7e7becd3d444be45850031e3fa35bccd55d;hp=e26cbddc1e08c52eb247571af72734a5d948245e;hpb=2f979bf83bad9b012efa225c38cfcd7ea91b2b29;p=ghc-hetmet.git diff --git a/docs/users_guide/using.xml b/docs/users_guide/using.xml index e26cbdd..af7950c 100644 --- a/docs/users_guide/using.xml +++ b/docs/users_guide/using.xml @@ -46,26 +46,25 @@ 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 @@ -121,7 +120,7 @@ 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. @@ -845,7 +844,9 @@ ghc -c Foo.hs , , , - , and + , + , + , and . The following flags are simple ways to select standard “packages” of warnings: @@ -859,6 +860,7 @@ ghc -c Foo.hs -W option Provides the standard warnings plus , + , , , , and @@ -877,7 +879,8 @@ ghc -c Foo.hs , , , - , and + , + , and . @@ -919,6 +922,23 @@ 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 +992,25 @@ 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 +1018,20 @@ 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 6.14. + + + + + : @@ -1159,6 +1207,11 @@ 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 +1382,56 @@ 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 @@ -1707,7 +1810,9 @@ f "2" = 2 &phases; - + + &shared_libs; + Using Concurrent Haskell Concurrent Haskellusing @@ -1765,17 +1870,60 @@ 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 @@ -1787,9 +1935,19 @@ 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. @@ -1844,19 +2002,20 @@ 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. @@ -1906,7 +2065,7 @@ 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