cmdargs: Command line argument processing

[ bsd3, console, library ] [ Propose Tags ]

This library provides an easy way to define command line parsers. Most users will want to use the System.Console.CmdArgs.Implicit module, whose documentation contains an example.

For a general reference on what command line flags are commonly used, see

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Dependencies base (>=4.4 && <4.10), filepath, process (>=1.0), template-haskell, transformers (>=0.2) [details]
License BSD-3-Clause
Copyright Neil Mitchell 2009-2015
Author Neil Mitchell <>
Maintainer Neil Mitchell <>
Revised Revision 3 made by NeilMitchell at 2018-03-24T15:18:34Z
Category Console
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Uploaded by NeilMitchell at 2015-05-22T15:18:18Z
Distributions Arch:0.10.20, Debian:0.10.20, Fedora:0.10.20, FreeBSD:0.10.13, LTSHaskell:0.10.20, NixOS:0.10.20, Stackage:0.10.20, openSUSE:0.10.20
Executables cmdargs
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CmdArgs: Easy Command Line Processing Hackage version Build Status

<p> CmdArgs is a Haskell library for defining command line parsers. The two features that make it a better choice than the standard <a href="">getopt library</a> are: </p> <ol> <li>It's very concise to use. The HLint command line handling is three times shorter with CmdArgs.</li> <li>It supports programs with multiple modes, such as <a href="">darcs</a> or <a href="">Cabal</a>.</li> </ol> <p> A very simple example of a command line processor is: </p> <pre> data Sample = Sample {hello :: String} deriving (Show, Data, Typeable)

sample = Sample{hello = def &= help "World argument" &= opt "world"} &= summary "Sample v1"

main = print =<< cmdArgs sample </pre> <p> Despite being very concise, this processor is already fairly well featured: </p> <pre> $ runhaskell Sample.hs --hello=world Sample {hello = "world"}

$ runhaskell Sample.hs --help Sample v1, (C) Neil Mitchell 2009

sample [FLAG]

-? --help[=FORMAT] Show usage information (optional format) -V --version Show version information -v --verbose Higher verbosity -q --quiet Lower verbosity -h --hello=VALUE World argument (default=world) </pre>

<h2>User Manual</h2> <p> The rest of this document explains how to write the "hello world" of command line processors, then how to extend it with features into a complex command line processor. Finally this document gives three samples, which the <tt>cmdargs</tt> program can run. The three samples are: </p> <ol> <li><tt>hlint</tt> - the <a href="">HLint</a> program.</li> <li><tt>diffy</tt> - a program to compare the differences between directories.</li> <li><tt>maker</tt> - a make style program.</li> </ol> <p> For each example you are encouraged to look at it's source (see the <a href="">darcs repo</a>, or the bottom of this document) and run it (try <tt>cmdargs hlint --help</tt>). The HLint program is fairly standard in terms of it's argument processing, and previously used the <a href="">System.Console.GetOpt</a> library. Using GetOpt required 90 lines and a reasonable amount of duplication. Using CmdArgs the code requires 30 lines, and the logic is much simpler. </p> <h3>Acknowledgements</h3> <p> Thanks to Kevin Quick for substantial patches, and additional code contributions from Sebastian Fischer and Daniel Schoepe. </p> <h2>Hello World Example</h2> <p> The following code defines a complete command line argument processor: </p>
{-# LANGUAGE DeriveDataTypeable #-}
module Sample where
import System.Console.CmdArgs

data Sample = Sample {hello :: String}
              deriving (Show, Data, Typeable)

sample = Sample{hello = def}

main = print =<< cmdArgs sample
<p> To use the CmdArgs library there are three steps: </p> <ol> <li>Define a record data type (<tt>Sample</tt>) that contains a field for each argument. This type needs to have instances for <tt>Show</tt>, <tt>Data</tt> and <tt>Typeable</tt>.</li> <li>Give a value of that type (<tt>sample</tt>) with default values (<tt>def</tt> is a default value of any type, but I could also have written <tt>""</tt>). This value is turned into a command line by calling the <tt>cmdArgs</tt> function.</li> </ol> <p> Now we have a reasonably functional command line argument processor. Some sample interactions are: </p> <pre> $ runhaskell Sample.hs --hello=world Sample {hello = "world"}

$ runhaskell Sample.hs --version The sample program

$ runhaskell Sample.hs --help The sample program

sample [OPTIONS]

-? --help Display help message -V --version Print version information -h --hello=ITEM </pre> <p> CmdArgs uses defaults to automatically infer a command line parser for a value, and provides annotations to override any of the the defaults. CmdArgs automatically supports <tt>--help</tt> and <tt>--version</tt> flags, and optionally supports verbosity flags. </p>

<h2>Specifying Attributes</h2> <p> In order to control the behaviour we can add attributes. For example to add an attribute specifying the help text for the <tt>--hello</tt> argument we can write: </p> <pre> sample = Sample{hello = def &= help "Who to say hello to"} </pre> <p> We can add additional attributes, for example to specify the type of the value expected by hello: </p> <pre> sample = Sample {hello = def &= help "Who to say hello to" &= typ "WORLD"} </pre> <p> Now when running <tt>--help</tt> the final line is: </p> <pre> -h --hello=WORLD Who to say hello to </pre> <p> There are many more attributes, detailed in the <a href="">Haddock documentation</a>. </p> <h2>Multiple Modes</h2> <p> To specify a program with multiple modes, similar to <a href="">darcs</a>, we can supply a data type with multiple constructors, for example: </p>
data Sample = Hello {whom :: String}
            | Goodbye
              deriving (Show, Data, Typeable)

hello = Hello{whom = def}
goodbye = Goodbye

main = print =<< cmdArgs (modes [hello,goodbye])
<p> Compared to the first example, we now have multiple constructors, and a sample value for each constructor is passed to <tt>cmdArgs</tt>. Some sample interactions with this command line are: </p> <pre> $ runhaskell Sample.hs hello --whom=world Hello {whom = "world"}

$ runhaskell Sample.hs goodbye Goodbye

$ runhaskell Sample.hs --help The sample program

sample [OPTIONS]

Common flags -? --help Display help message -V --version Print version information

sample hello [OPTIONS]

-w --whom=ITEM

sample goodbye [OPTIONS] </pre> <p> As before, the behaviour can be customised using attributes. </p>

<h2>Larger Examples</h2> <p> For each of the following examples we first explain the purpose of the program, then give the source code, and finally the output of <tt>--help=HTML</tt>. The programs are intended to show sample uses of CmdArgs, and are available to experiment with through <tt>cmdargs <i>progname</i></tt>. </p> <h3>HLint</h3> <p> The <a href="">HLint</a> program analyses a list of files, using various options to control the analysis. The command line processing is simple, but a few interesting points are: </p> <ul> <li>The <tt>--report</tt> flag can be used to output a report in a standard location, but giving the flag a value changes where the file is output.</li> <li>The <tt>color</tt> field is assigned two flag aliases, <tt>--colour</tt> and <tt>-c</tt>. Assigning the <tt>-c</tt> short flag explicitly stops either of the CPP fields using it.</li> <li>The <tt>show_</tt> field would clash with <tt>show</tt> if given the expected name, but CmdArgs automatically strips the trailing underscore.</li> <li>The <tt>cpp_define</tt> field has an underscore in it's name, which is transformed into a hyphen for the flag name.</li> </ul> <!-- BEGIN code hlint --> <pre> {-# LANGUAGE DeriveDataTypeable #-} module HLint where import System.Console.CmdArgs

data HLint = HLint {report :: [FilePath] ,hint :: [FilePath] ,color :: Bool ,ignore_ :: [String] ,show_ :: Bool ,extension :: [String] ,language :: [String] ,utf8 :: Bool ,encoding :: String ,find :: [FilePath] ,test_ :: Bool ,datadir :: [FilePath] ,cpp_define :: [String] ,cpp_include :: [FilePath] ,files :: [FilePath] } deriving (Data,Typeable,Show,Eq)

hlint = HLint {report = def &= opt "report.html" &= typFile &= help "Generate a report in HTML" ,hint = def &= typFile &= help "Hint/ignore file to use" ,color = def &= name "c" &= name "colour" &= help "Color the output (requires ANSI terminal)" ,ignore_ = def &= typ "MESSAGE" &= help "Ignore a particular hint" ,show_ = def &= help "Show all ignored ideas" ,extension = def &= typ "EXT" &= help "File extensions to search (defaults to hs and lhs)" ,language = def &= name "X" &= typ "LANG" &= help "Language extension (Arrows, NoCPP)" ,utf8 = def &= help "Use UTF-8 text encoding" ,encoding = def &= typ "ENC" &= help "Choose the text encoding" ,find = def &= typFile &= help "Find hints in a Haskell file" ,test_ = def &= help "Run in test mode" ,datadir = def &= typDir &= help "Override the data directory" ,cpp_define = def &= typ "NAME[=VALUE]" &= help "CPP #define" ,cpp_include = def &= typDir &= help "CPP include path" ,files = def &= args &= typ "FILES/DIRS" } &= verbosity &= help "Suggest improvements to Haskell source code" &= summary "HLint v0.0.0, (C) Neil Mitchell" &= details ["Hlint gives hints on how to improve Haskell code","" ,"To check all Haskell files in 'src' and generate a report type:"," hlint src --report"]

mode = cmdArgsMode hlint </pre>

HLint v0.0.0, (C) Neil Mitchell

Suggest improvements to Haskell source code

Common flags:
  -r --report[=FILE]            Generate a report in HTML
  -h --hint=FILE                Hint/ignore file to use
  -c --colour --color           Color the output (requires ANSI terminal)
  -i --ignore=MESSAGE           Ignore a particular hint
  -s --show                     Show all ignored ideas
     --extension=EXT            File extensions to search (defaults to hs and lhs)
  -X --language=LANG            Language extension (Arrows, NoCPP)
  -u --utf8                     Use UTF-8 text encoding
     --encoding=ENC             Choose the text encoding
  -f --find=FILE                Find hints in a Haskell file
  -t --test                     Run in test mode
  -d --datadir=DIR              Override the data directory
     --cpp-define=NAME[=VALUE]  CPP #define
     --cpp-include=DIR          CPP include path
  -? --help                     Display help message
  -V --version                  Print version information
  -v --verbose                  Loud verbosity
  -q --quiet                    Quiet verbosity

Hlint gives hints on how to improve Haskell code

To check all Haskell files in 'src' and generate a report type:
  hlint src --report
<h3>Diffy</h3> <p> The Diffy sample is a based on the idea of creating directory listings and comparing them. The tool can operate in two separate modes, <tt>create</tt> or <tt>diff</tt>. This sample is fictional, but the ideas are drawn from a real program. A few notable features: </p> <ul> <li>There are multiple modes of execution, creating and diffing.</li> <li>The diff mode takes exactly two arguments, the old file and the new file.</li> <li>Default values are given for the <tt>out</tt> field, which are different in both modes.</li> </ul> <!-- BEGIN code diffy --> <pre> {-# LANGUAGE DeriveDataTypeable #-} module Diffy where import System.Console.CmdArgs

data Diffy = Create {src :: Maybe FilePath, out :: FilePath} | Diff {old :: FilePath, new :: FilePath, out :: FilePath} deriving (Data,Typeable,Show,Eq)

outFlags x = x &= help "Output file" &= typFile

create = Create {src = def &= help "Source directory" &= typDir ,out = outFlags "ls.txt" } &= help "Create a fingerprint"

diff = Diff {old = def &= typ "OLDFILE" &= argPos 0 ,new = def &= typ "NEWFILE" &= argPos 1 ,out = outFlags "diff.txt" } &= help "Perform a diff"

mode = cmdArgsMode $ modes [create,diff] &= help "Create and compare differences" &= program "diffy" &= summary "Diffy v1.0" </pre> <!-- END --> <!-- BEGIN help diffy --> Diffy v1.0

diffy [COMMAND] ... [OPTIONS]
  Create and compare differences
Common flags:
  -o --out=FILE  Output file
  -? --help      Display help message
  -V --version   Print version information
diffy create [OPTIONS]
  Create a fingerprint
  -s  --src=DIR  Source directory
  Perform a diff
<!-- END --> <h3>Maker</h3> <p> The Maker sample is based around a build system, where we can either build a project, clean the temporary files, or run a test. Some interesting features are: </p> <ul> <li>The build mode is the default, so <tt>maker</tt> on it's own will be interpretted as a build command.</li> <li>The build method is an enumeration.</li> <li>The <tt>threads</tt> field is in two of the constructors, but not all three. It is given the short flag <tt>-j</tt>, rather than the default <tt>-t</tt>.</li> </ul> <!-- BEGIN code maker --> <pre> {-# LANGUAGE DeriveDataTypeable #-} module Maker where import System.Console.CmdArgs

data Method = Debug | Release | Profile deriving (Data,Typeable,Show,Eq)

data Maker = Wipe | Test {threads :: Int, extra :: [String]} | Build {threads :: Int, method :: Method, files :: [FilePath]} deriving (Data,Typeable,Show,Eq)

threadsMsg x = x &= help "Number of threads to use" &= name "j" &= typ "NUM"

wipe = Wipe &= help "Clean all build objects"

test_ = Test {threads = threadsMsg def ,extra = def &= typ "ANY" &= args } &= help "Run the test suite"

build = Build {threads = threadsMsg def ,method = enum [Release &= help "Release build" ,Debug &= help "Debug build" ,Profile &= help "Profile build"] ,files = def &= args } &= help "Build the project" &= auto

mode = cmdArgsMode $ modes [build,wipe,test_] &= help "Build helper program" &= program "maker" &= summary "Maker v1.0\nMake it" </pre> <!-- END --> <!-- BEGIN help maker --> Maker v1.0 Make it

maker [COMMAND] ... [OPTIONS]
  Build helper program
Common flags:
  -? --help     Display help message
  -V --version  Print version information
maker [build] [OPTIONS] [ITEM]
  Build the project
  -j --threads=NUM  Number of threads to use
  -r --release      Release build
  -d --debug        Debug build
  -p --profile      Profile build
maker wipe [OPTIONS]
  Clean all build objects
maker test [OPTIONS] [ANY]
  Run the test suite
  -j --threads=NUM  Number of threads to use
<!-- END -->