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NAME

parse_args - Core-style argument parsing for scripts

SYNOPSIS

package require parse_args ?0.3.3?

parse_args::parse_args args argspec ?varname?

DESCRIPTION

The commands provided by the Tcl core follow a pattern of optional, named arguments (named with a leading “-” to indicate an option) in addition to positional arguments. They strongly prefer named arguments when the number of arguments and particularly optional arguments grows beyond about 3. Tcl scripts however have no access to standard argument parsing facilities to implement this pattern and have to build their named argument parsing machinery from scratch. This is slow to do in script, error prone, and creates noise in the code that distracts from the core task of the command implementation. This means that script-defined commands generally present half-baked argument handling APIs and tend to over-use positional arguments.

This package aims to address this weakness by providing a robust argument parsing mechanism to script code that can implement the full set of patterns established by the core Tcl commands, is terse and readable to specify the arguments, and is fast enough to use in any situation.

COMMANDS

  • parse_args::parse_args args argspec
    Parse the arguments supplied in args against the specification given in argspec, setting variables in the current scope as defined in the argspec. If the optional varname argument is supplied, it names a variable that will have a dictionary written to it containing the parsed arguments and no variables will be created for the arguments. See ARGUMENT SPECIFICATION below for the format of argspec.

ARGUMENT SPECIFICATION

The syntax of the argument specification (given in the argspec argument to the parse_args command) is a dictionary, with the keys corresponding to the names by which the arguments will be available to the caller, either as variables created by the parse_args command or keys in the dictionary written to varname if that was specified. If the first character of the key is a “-” character it defines a named option, otherwise a positional parameter. Like in proc argument definitions the name args is special, and will consume all remaining arguments.

The values of the dictionary contain the definition of the corresponding argument, such as -required, which marks that argument as being required (whether a named or positional argument). The options that are available to define the argument are:

  • -default val
    Provides a default value for the argument if it wasn’t supplied. Without this an argument that wasn’t supplied won’t have its corresponding variable (or dictionary key) set. val is permitted to be a value that is outside of the range of possible values that would be accepted from the caller, by design.
  • -required
    If this boolean option is given, the argument is flagged as required. Failing to supply the arg causes the parse_args to throw an exception similar to what would happen if a required positional argument to a proc was not provided.
  • -validate cmdprefix
    Append the value supplied for this argument to the cmdprefix and run it. The value is rejected if an exception is thrown or the value returned isn’t a Tcl boolean true value.
  • -name newname
    Change the name of the variable / dictionary key that will receive the value for this argument to newname. By default the name will be the argspec key (without the leading “-” for named arguments).
  • -boolean
    Flag this argument as a boolean, similar to -nocase to regexp. The variable will always be defined and be a boolean, true if the argument was supplied and false if it wasn’t.
  • -args count
    By default named arguments consume a single following argument as the value for that argument. This setting changes that to count, which will cause the following count arguments to be gathered into the variable holding this argument.
  • -enum possible_values
    Restrict the values that will be accepted for this argument to those in the possible_values list. Similar to using a validator that checks for inclusion in that list but with additional internal performance optimizations.
  • -# free_form_text
    Embed a comment for this argument. The supplied free_form_text is ignored by parse_args and serves only to document the argument to programmers reading the code. In some future version this text may be incorporated into a usage error message that is generated when argument parsing fails.
  • -multi name
    Group a set of defined arguments together, so they act like mutually exclusive flags. Used to implement patterns like the arguments -ascii, -dictionary, -integer and -real arguments to the core lsort command. name will be the name of the variable that holds the value of the flag that was supplied. A -default setting on any of the linked -multi arguments (those that share a name) will supply a default value in name if none were passed. A -required setting on any of the linked -multi arguments will require that the caller supply at least one of them. If multiple different flags were given, the one last in the argument list applies.
  • -alias
    Treat the value supplied for this argument as a variable name and bind the resulting variable for this argument to the variable of that name in the call frame above this one (upvar 1).

EXAMPLES

Mimic the argument handling of the core glob command:

proc glob args {
    parse_args::parse_args $args {
        -directory  {}
        -join       {-boolean}
        -nocomplain {-boolean}
        -path       {}
        -tails      {-boolean}
        -types      {-default {}}
        args        {-name patterns}
    }

    if {$join} {
        set patterns    [list [file join {*}$patterns]]
    }

    if {[llength $patterns] == 0 && $nocomplain} return

    foreach pattern $patterns {
        if {[info exists directory]} {
            ...
        }
    }
    ...
}

Mimic regex:

proc regexp args {
    parse_args::parse_args $args {
        -about      {-boolean}
        -expanded   {-boolean}
        -indices    {-boolean}
        -line       {-boolean}
        -linestop   {-boolean}
        -lineanchor {-boolean}
        -nocase     {-boolean}
        -all        {-boolean}
        -inline     {-boolean}
        -start      {-default 0}
        exp         {-required}
        string      {-required}
        matchvar    {}
        args        {-name submatchvars}
    }
    ...
}

lsort:

proc lsort args {
    parse_args::parse_args $args {
        -ascii      {-name compare_as -multi -default ascii}
        -dictionary {-name compare_as -multi}
        -integer    {-name compare_as -multi}
        -real       {-name compare_as -multi}

        -command    {}

        -increasing {-name order -multi -default increasing}
        -decreasing {-name order -multi}

        -indices    {-boolean}
        -index      {}
        -stride     {-default 1}
        -nocase     {-boolean}
        -unique     {-boolean}
        list        {-required}
    }

    if {![info exists command]} {
        switch -- $compare_as {
            ascii {
                set command {string compare}
                if {$nocase} {
                    lappend command -nocase
                }
            }
            dictionary {
                ...
            }
            integer - real {
                set command tcl::mathop::-
            }
        }
    }
}

lsearch:

proc lsearch args {
    parse_args::parse_args $args {
        -exact      {-name matchtype -multi}
        -glob       {-name matchtype -multi -default glob}
        -regexp     {-name matchtype -multi}

        -sorted     {-boolean}
        -all        {-boolean}
        -inline     {-boolean}
        -not        {-boolean}
        -start      {-default 0}

        -ascii      {-name compare_as -multi -default ascii}
        -dictionary {-name compare_as -multi}
        -integer    {-name compare_as -multi}
        -real       {-name compare_as -multi}

        -nocase     {-boolean}

        -decreasing {-name order -multi}
        -increasing {-name order -multi -default increasing}
        -bisect     {-boolean}

        -index      {}
        -subindices {-boolean}
    }

    if {$sorted && $matchtype in {glob regexp}} {
        error "-sorted is mutually exclusive with -glob and -regexp"
    }
}

A Tk widget - entry:

proc entry {widget args} {
    parse_args::parse_args $args {
        -disabledbackground {-default {}}
        -disabledforeground {-default {}}
        -invalidcommand     {-default {}}
        -readonlybackground {-default {}}
        -show               {}
        -state              {-default normal -enum {
            normal disabled readonly
        }}
        -validate           {-default none -enum {
            none focus focusin focusout key all
        }}
        -validatecommand    {-default {}}
        -width              {-default 0}
        -textvariable       {}
    }
}

SEE ALSO

The paper presented at the 2016 Tcl Conference which discusses the approach and design choices made in this package in much greater depth: https://www.tcl-lang.org/community/tcl2016/assets/talk33/parse_args-paper.pdf

BUGS

Please open an issue on the github tracker for the project if you encounter any problems: https://github.com/RubyLane/parse_args/issues

LICENSE

This package is Copyright 2021 Cyan Ogilvie, and is made available under the same license terms as the Tcl Core