System.Console.CmdArgs.Implicit

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

To use cmdArgs with custom command line arguments see withArgs.

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.

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).

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

To use cmdArgs_ with custom command line arguments see withArgs.

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).

Perform the necessary actions dictated by a CmdArgs structure.

• If cmdArgsHelp is Just, it will display the help message and exit.

• If cmdArgsVersion is Just, it will display the version and exit.

• In all other circumstances it will return a value.

• Additionally, if cmdArgsVerbosity is Just (see verbosity) it will set the verbosity (see setVerbosity).

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

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}

Flag: "Put non-flag arguments here."

All argument flags not captured by argPos are returned by args.

{hello = def &= args}

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{..}]

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

Suffix to be added to the help message.

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

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"}
  --foo=VALUE

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"}
Welcomes
  -h --hello=VALUE

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

Descriptive text used in the help output.

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

Modes: "Customise the help argument."

Add extra options to a help argument, such as help, name, ignore or explicit.

Sample{..} &= helpArg [explicit, name "h"]

Flag/Mode: "Ignore this field, don't let the user set it."

A mode or field is not dealt with by CmdArgs.

{hello = def, extra = def &= ignore}
  --hello=VALUE

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

Add flags which trigger this option.

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

Program: "Turn off @ expansion."

Usually arguments starting with @ are treated as a file containing a set of arguments. This annotation turns off that behaviour.

Sample{..} &= noAtExpand

Flag: "I want users to be able to omit the value associated with 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)

Note that all flags in CmdArgs are optional, and if omitted will use their default value. Those annotated with opt also allow the flag to be present without an associated value. As an example:

{hello = "DEFAULT" &= opt "OPTIONAL"}

$ main
{hello = "DEFAULT"}
$ main --hello
{hello = "OPTIONAL"}
$ main --hello=VALUE
{hello = "VALUE"}

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"

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. If the string contains a version number component will also provide --numeric-version.

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

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

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

Alias for typ DIR.

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

Alias for typ FILE.

Modes: "My program needs verbosity flags."

Add --verbose and --quiet flags.

Modes: "Customise the verbosity arguments."

Add extra options to a verbosity arguments (--verbose and --quiet), such as help, name, ignore or explicit. The verbose options come first, followed by the quiet options.

Sample{..} &= verbosityArgs [ignore] [name "silent", explicit]

Modes: "Customise the version argument."

Add extra options to a version argument, such as help, name, ignore, summary or explicit.

Sample{..} &= versionArg [ignore]

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

A mode. Do not use the Mode constructor directly, instead use mode to construct the Mode and then record updates. Each mode has three main features:

• A list of submodes (modeGroupModes)

• A list of flags (modeGroupFlags)

• Optionally an unnamed argument (modeArgs)

To produce the help information for a mode, either use helpText or show.

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 within 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.

The class Typeable allows a concrete representation of a type to be calculated.