CmdArgs is a Haskell library for defining command line parsers. The two features that make it a better choice than the standard getopt library are:
- It's very concise to use. The HLint command line handling is three times shorter with CmdArgs.
- It supports programs with multiple modes, such as darcs or Cabal.
A very simple example of a command line processor is:
data Sample = Sample {hello :: String} deriving (Show, Data, Typeable)
sample = Sample{hello = def &= help "World argument" &= opt "world"}
&= summary "Sample v1"
main = print =<< cmdArgs sampleDespite being very concise, this processor is already fairly well featured:
$ runghc Sample.hs --hello=world
Sample {hello = "world"}
$ runghc 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)
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 cmdargs program can run. The three samples are:
hlint- the HLint program.diffy- a program to compare the differences between directories.maker- a make style program.
For each example you are encouraged to look at it's source (in the repo) and run it (try cmdargs hlint --help). The HLint program is fairly standard in terms of it's argument processing, and previously used the System.Console.GetOpt 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.
Acknowledgements Thanks to Kevin Quick for substantial patches, and additional code contributions from Sebastian Fischer and Daniel Schoepe.
The following code defines a complete command line argument processor:
{-# LANGUAGE DeriveDataTypeable #-}
{-# OPTIONS_GHC -fno-cse #-}
module Sample where
import System.Console.CmdArgs
data Sample = Sample {hello :: String}
deriving (Show, Data, Typeable)
sample = Sample{hello = def}
main = print =<< cmdArgs sampleTo use the CmdArgs library there are three steps:
- Define a record data type (
Sample) that contains a field for each argument. This type needs to have instances forShow,DataandTypeable. - Give a value of that type (
sample) with default values (defis a default value of any type, but I could also have written""). This value is turned into a command line by calling thecmdArgsfunction. - To ensure GHC evalutes attributes the right number of times we disable the CSE optimisation on this module.
Now we have a reasonably functional command line argument processor. Some sample interactions are:
$ runghc Sample.hs --hello=world
Sample {hello = "world"}
$ runghc Sample.hs --version
The sample program
$ runghc Sample.hs --help
The sample program
sample [OPTIONS]
-? --help Display help message
-V --version Print version information
-h --hello=ITEM
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 --help and --version flags, and optionally supports verbosity flags.
In order to control the behaviour we can add attributes. For example to add an attribute specifying the help text for the --hello argument we can write:
sample = Sample{hello = def &= help "Who to say hello to"}We can add additional attributes, for example to specify the type of the value expected by hello:
sample = Sample {hello = def &= help "Who to say hello to" &= typ "WORLD"}Now when running --help the final line is:
-h --hello=WORLD Who to say hello to
There are many more attributes, detailed in the Haddock documentation.
To specify a program with multiple modes, similar to darcs, we can supply a data type with multiple constructors, for example:
data Sample = Hello {whom :: String}
| Goodbye
deriving (Show, Data, Typeable)
hello = Hello{whom = def}
goodbye = Goodbye
main = print =<< cmdArgs (modes [hello,goodbye])Compared to the first example, we now have multiple constructors, and a sample value for each constructor is passed to cmdArgs. Some sample interactions with this command line are:
$ runghc Sample.hs hello --whom=world
Hello {whom = "world"}
$ runghc Sample.hs goodbye
Goodbye
$ runghc 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]
As before, the behaviour can be customised using attributes.
For each of the following examples we first explain the purpose of the program, then give the source code, and finally the output of --help. The programs are intended to show sample uses of CmdArgs, and are available to experiment with through cmdargs progname.
The HLint 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:
- The
--reportflag can be used to output a report in a standard location, but giving the flag a value changes where the file is output. - The
colorfield is assigned two flag aliases,--colourand-c. Assigning the-cshort flag explicitly stops either of the CPP fields using it. - The
show_field would clash withshowif given the expected name, but CmdArgs automatically strips the trailing underscore. - The
cpp_definefield has an underscore in it's name, which is transformed into a hyphen for the flag name.
The code is:
{-# 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 hlintProduces the --help output:
HLint v0.0.0, (C) Neil Mitchell
hlint [OPTIONS] [FILES/DIRS]
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
The Diffy sample is a based on the idea of creating directory listings and comparing them. The tool can operate in two separate modes, create or diff. This sample is fictional, but the ideas are drawn from a real program. A few notable features:
- There are multiple modes of execution, creating and diffing.
- The diff mode takes exactly two arguments, the old file and the new file.
- Default values are given for the
outfield, which are different in both modes.
The code is:
{-# 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"And --help produces:
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
diffy diff [OPTIONS] OLDFILE NEWFILE
Perform a diff
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:
- The build mode is the default, so
makeron it's own will be interpreted as a build command. - The build method is an enumeration.
- The
threadsfield is in two of the constructors, but not all three. It is given the short flag-j, rather than the default-t.
The code is:
{-# 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"And --help produces:
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
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