cmdargs-0.4: Command line argument processing




This module provides simple command line argument processing. The main function of interest is cmdArgs. A simple example 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 sample

Attributes are used to control a number of behaviours:

Supported Types: Each field in the record must be one of the supported atomic types (String, Int, Integer, Float, Double, Bool, an enumeration, a tuple of atomic types) or a list ([]) or Maybe wrapping at atomic type.

Missing Fields: If a field is shared by multiple modes, it may be omitted in subsequent modes, and will default to the previous value.

Purity: Values created with annotations are not pure - the first time they are computed they will include the annotations, but subsequently they will not. If you wish to run the above example in a more robust way:

sample = cmdArgsMode $ Sample{hello = ... -- as before
main = print =<< cmdArgsRun sample

Even using this scheme, sometimes GHC's optimisations may share values who have the same annotation. To disable sharing you may need to specify {-# OPTIONS_GHC -fno-cse #-} in the module you define the flags.


Running command lines

cmdArgs :: Data a => a -> IO aSource

Take impurely annotated records and run the corresponding command line. Shortcut for cmdArgsRun . cmdArgsMode.

cmdArgsMode :: Data a => a -> Mode (CmdArgs a)Source

Take impurely annotated records and turn them in to a Mode value, that can make use of the System.Console.CmdArgs.Explicit functions (i.e. process).

Annotated records are impure, and will only contain annotations on their first use. The result of this function is pure, and can be reused.

cmdArgsRun :: Mode (CmdArgs a) -> IO aSource

Run a Mode structure. This function reads the command line arguments and then performs as follows:

  • If invalid arguments are given, it will display the error message and exit.
  • If --help is given, it will display the help message and exit.
  • If --version is given, it will display the version and exit.
  • In all other circumstances the program will return a value.
  • Additionally, if either --quiet or --verbose is given (see verbosity) it will set the verbosity (see setVerbosity).

cmdArgs_ :: Data a => Annotate Ann -> IO aSource

Take purely annotated records and run the corresponding command line. Shortcut for cmdArgsRun . cmdArgsMode_.

cmdArgsMode_ :: Data a => Annotate Ann -> Mode (CmdArgs a)Source

Take purely annotated records and turn them in to a Mode value, that can make use of the System.Console.CmdArgs.Explicit functions (i.e. process).

cmdArgsApply :: CmdArgs a -> IO aSource

Perform the necessary actions dictated by a CmdArgs structure.

data CmdArgs a Source

A structure to store the additional data relating to --help, --version, --quiet and --verbose.




cmdArgsValue :: a

The underlying value being wrapped.

cmdArgsHelp :: Maybe String

Just if --help is given, then gives the help message for display.

cmdArgsVersion :: Maybe String

Just if --verion is given, then gives the version message for display.

cmdArgsVerbosity :: Maybe Verbosity

Just if --quiet or --verbose is given, then gives the verbosity to use.

cmdArgsPrivate :: CmdArgsPrivate

Private: Only exported due to Haddock limitations.


Constructing command lines

Attributes can work on a flag (inside a field), on a mode (outside the record), or on all modes (outside the modes call).

opt :: (Show a, Typeable a) => a -> AnnSource

Flag: "I want users to be able to omit the value for this flag."

Make the value of a flag optional. If --flag is given, it will be treated as --flag=this_argument.

 {hello = def &= opt "foo"}
   -h --hello[=VALUE]    (default=foo)

typ :: String -> AnnSource

Flag: "For this flag, users need to give something of type ..."

The the type of a flag's value, usually upper case. Only used for the help message. Commonly the type will be FILE (typFile) or DIR (typDir).

 {hello = def &= typ "MESSAGE"}
   -h --hello=MESSAGE

typFile :: AnnSource

Flag: "Users must give a file for this flag's value."

Alias for typ FILE.

typDir :: AnnSource

Flag: "Users must give a directory for this flag's value."

Alias for typ DIR.

help :: String -> AnnSource

Flag/Mode: "The help message is ..."

Descriptive text used in the help output.

 {hello = def &= help "Help message"}
   -h --hello=VALUE      Help message

name :: String -> AnnSource

Flag: "Use this flag name for this field."

Add flags which trigger this option.

 {hello = def &= name "foo"}
   -h --hello --foo=VALUE

args :: AnnSource

Flag: "Put non-flag arguments here."

 {hello = def &= args}

argPos :: Int -> AnnSource

Flag: "Put the nth non-flag argument here."

This field should be used to store a particular argument position (0-based).

 {hello = def &= argPos 0}

groupname :: String -> AnnSource

Flag/Mode: "Give these flags/modes a group name in the help output."

This mode will be used for all following modes/flags, until the next groupname.

 {hello = def &= groupname "Welcomes"}
   -h --hello=VALUE

details :: [String] -> AnnSource

Mode: "A longer description of this mode is ..."

Suffix to be added to the help message.

 Sample{..} &= details ["More details on the website"]

summary :: String -> AnnSource

Modes: "My program name/version/copyright is ..."

One line summary of the entire program, the first line of --help and the only line of --version.

 Sample{..} &= summary "CmdArgs v0.0, (C) Neil Mitchell 1981"

auto :: AnnSource

Mode: "If the user doesn't give a mode, use this one."

This mode is the default. If no mode is specified and a mode has this attribute then that mode is selected, otherwise an error is raised.

 modes [Mode1{..}, Mode2{..} &= auto, Mode3{..}]

program :: String -> AnnSource

Modes: "My program executable is named ..."

This is the name of the program executable. Only used in the help message. Defaults to the type of the mode.

 Sample{..} &= program "sample"

explicit :: AnnSource

Flag: "Don't guess any names for this field."

A field should not have any flag names guessed for it. All flag names must be specified by flag.

 {hello = def &= explicit &= name "foo"}

verbosity :: AnnSource

Modes: "My program needs verbosity flags."

Add --verbose and --quiet flags.


(&=) :: Data val => val -> Ann -> valSource

Add an annotation to a value. Note that if the value is evaluated more than once the annotation will only be available the first time.

modes :: Data val => [val] -> valSource

Modes: "I want a program with multiple modes, like darcs or cabal."

Takes a list of modes, and creates a mode which includes them all. If you want one of the modes to be chosen by default, see auto.

 data Modes = Mode1 | Mode2 | Mode3 deriving Data
 cmdArgs $ modes [Mode1,Mode2,Mode3]

enum :: Data val => [val] -> valSource

Flag: "I want several different flags to set this one field to different values."

This annotation takes a type which is an enumeration, and provides multiple separate flags to set the field to each value.

 data State = On | Off deriving Data
 data Mode = Mode {state :: State}
 cmdArgs $ Mode {state = enum [On &= help "Turn on",Off &= help "Turn off"]}
   --on   Turn on
   --off  Turn off


(+=) :: Annotate ann -> ann -> Annotate annSource

Add an annotation to a value.

record :: Data a => a -> [Annotate b] -> Annotate bSource

Create a constructor/record. The first argument should be the type of field, the second should be a list of fields constructed originally defined by := or :=+.

This operation is not type safe, and may raise an exception at runtime if any field has the wrong type or label.

atom :: Data val => val -> Annotate annSource

Lift a pure value to an annotation.

data Annotate ann Source

This type represents an annotated value. The type of the underlying value is not specified.


forall c f . (Data c, Data f) => (c -> f) := f

Construct a field, fieldname := value.

enum_ :: (Data c, Data f) => (c -> f) -> [Annotate Ann] -> Annotate AnnSource

Re-exported for convenience

Provides a few opaque types (for writing type signatures), verbosity control, default values with def and the Data/Typeable type classes.

data Ann Source

The general type of annotations that can be associated with a value.


data Mode a Source

A mode. Each mode has three main features:


class Typeable a => Data a

The Data class comprehends a fundamental primitive gfoldl for folding over constructor applications, say terms. This primitive can be instantiated in several ways to map over the immediate subterms of a term; see the gmap combinators later in this class. Indeed, a generic programmer does not necessarily need to use the ingenious gfoldl primitive but rather the intuitive gmap combinators. The gfoldl primitive is completed by means to query top-level constructors, to turn constructor representations into proper terms, and to list all possible datatype constructors. This completion allows us to serve generic programming scenarios like read, show, equality, term generation.

The combinators gmapT, gmapQ, gmapM, etc are all provided with default definitions in terms of gfoldl, leaving open the opportunity to provide datatype-specific definitions. (The inclusion of the gmap combinators as members of class Data allows the programmer or the compiler to derive specialised, and maybe more efficient code per datatype. Note: gfoldl is more higher-order than the gmap combinators. This is subject to ongoing benchmarking experiments. It might turn out that the gmap combinators will be moved out of the class Data.)

Conceptually, the definition of the gmap combinators in terms of the primitive gfoldl requires the identification of the gfoldl function arguments. Technically, we also need to identify the type constructor c for the construction of the result type from the folded term type.

In the definition of gmapQx combinators, we use phantom type constructors for the c in the type of gfoldl because the result type of a query does not involve the (polymorphic) type of the term argument. In the definition of gmapQl we simply use the plain constant type constructor because gfoldl is left-associative anyway and so it is readily suited to fold a left-associative binary operation over the immediate subterms. In the definition of gmapQr, extra effort is needed. We use a higher-order accumulation trick to mediate between left-associative constructor application vs. right-associative binary operation (e.g., (:)). When the query is meant to compute a value of type r, then the result type withing generic folding is r -> r. So the result of folding is a function to which we finally pass the right unit.

With the -XDeriveDataTypeable option, GHC can generate instances of the Data class automatically. For example, given the declaration

data T a b = C1 a b | C2 deriving (Typeable, Data)

GHC will generate an instance that is equivalent to

instance (Data a, Data b) => Data (T a b) where gfoldl k z (C1 a b) = z C1 `k` a `k` b gfoldl k z C2 = z C2 gunfold k z c = case constrIndex c of 1 -> k (k (z C1)) 2 -> z C2 toConstr (C1 _ _) = con_C1 toConstr C2 = con_C2 dataTypeOf _ = ty_T con_C1 = mkConstr ty_T "C1" [] Prefix con_C2 = mkConstr ty_T "C2" [] Prefix ty_T = mkDataType "Module.T" [con_C1, con_C2]

This is suitable for datatypes that are exported transparently.


Data Bool 
Data Char 
Data Double 
Data Float 
Data Int 
Data Int8 
Data Int16 
Data Int32 
Data Int64 
Data Integer 
Data Ordering 
Data Word 
Data Word8 
Data Word16 
Data Word32 
Data Word64 
Data () 
Data Ann 
Data Verbosity 
Data CmdArgsPrivate 
Data a => Data [a] 
(Data a, Integral a) => Data (Ratio a) 
Typeable a => Data (Ptr a) 
Typeable a => Data (ForeignPtr a) 
Data a => Data (Maybe a) 
Data a => Data (CmdArgs a) 
(Data a, Data b) => Data (Either a b) 
(Data a, Data b) => Data (a, b) 
(Typeable a, Data b, Ix a) => Data (Array a b) 
(Data a, Data b, Data c) => Data (a, b, c) 
(Data a, Data b, Data c, Data d) => Data (a, b, c, d) 
(Data a, Data b, Data c, Data d, Data e) => Data (a, b, c, d, e) 
(Data a, Data b, Data c, Data d, Data e, Data f) => Data (a, b, c, d, e, f) 
(Data a, Data b, Data c, Data d, Data e, Data f, Data g) => Data (a, b, c, d, e, f, g)