Command line interface

Creating your own CLI in Perl 6

Command line interface - an overview

The default command line interface of Perl 6 scripts consists of three parts:

This looks at the values in @*ARGS, interprets these according to some policy, and creates a Capture object out of that. An alternative way of parsing may be provided by the developer or installed using a module.

Standard multi dispatch is used to call the MAIN subroutine with the generated Capture object. This means that your MAIN subroutine may be a multi sub, each candidate of which is responsible for some part of processing the given command line arguments.

If multi dispatch failed, then the user of the script should be informed as well as possible as to why it failed. By default, this is done by inspecting the signature of each MAIN candidate sub, and any associated pod information. The result is then shown to the user on STDERR (or on STDOUT if --help was specified). An alternative way of generating the usage information may be provided by the developer or installed using a module.

sub MAIN

The sub with the special name MAIN will be executed after all relevant entry phasers (BEGIN, CHECK, INIT, PRE, ENTER) have been run and the mainline of the script has been executed. No error will occur if there is no MAIN sub: your script will then just have to do the work, such as argument parsing, in the mainline of the script.

Any normal exit from the MAIN sub will result in an exit code of 0, indicating success. Any return value of the MAIN sub will be ignored. If an exception is thrown that is not handled inside the MAIN sub, then the exit code will be 1. If the dispatch to MAIN failed, a usage message will be displayed on STDERR and the exit code will be 2.

The command line parameters are present in the @*ARGS dynamic variable and may be altered in the mainline of the script before the MAIN unit is called.

The signature of (the candidates of the multi) sub MAIN determines which candidate will actually be called using the standard multi dispatch semantics.

A simple example:

# inside file 'hello.p6' 
sub MAIN($name{
    say "Hello $name, how are you?"
}

If you call that script without any parameters:

$ perl6 hello.p6
Usage:
  hello.p6 <name>

However, if you give a default value for the parameter, running the script either with or without specifying a name will always work:

# inside file 'hello.p6' 
sub MAIN($name = 'bashful'{
    say "Hello $name, how are you?"
}
$ perl6 hello.p6
Hello bashfulhow are you?
$ perl6 hello.p6 Liz
Hello Lizhow are you?

Another way to do this is to make sub MAIN a multi sub:

# inside file 'hello.p6' 
multi sub MAIN()      { say "Hello bashful, how are you?" }
multi sub MAIN($name{ say "Hello $name, how are you?"   }

Which would give the same output as the examples above. Whether you should use either method to achieve the desired goal is entirely up to you.

A more complicated example using a single positional and multiple named parameters:

    # inside "frobnicate.p6" 
    sub MAIN(
      Str   $file where *.IO.f = 'file.dat',
      Int  :$length = 24,
      Bool :$verbose
    ) {
        say $length if $length.defined;
        say $file   if $file.defined;
        say 'Verbosity ', ($verbose ?? 'on' !! 'off');
    }

With file.dat present, this will work this way:

$ perl6 frobnicate.p6
24
file.dat
Verbosity off

Or this way with --verbose:

$ perl6 frobnicate.p6 -v
24
file.dat
Verbosity on

If the file file.dat is not present, or you've specified another filename that doesn't exist, you would get the standard usage message created from introspection of the MAIN sub:

$ perl6 frobnicate.p6 doesntexist.dat
Usage:
  frobnicate.p6 [--length=<Int>] [--verbose] [<file>]

Although you don't have to do anything in your code to do this, it may still be regarded as a bit terse. But there's an easy way to make that usage message better by providing hints using pod features:

    # inside "frobnicate.p6" 
    sub MAIN(
      Str   $file where *.IO.f = 'file.dat',  #= an existing file to frobnicate 
      Int  :$length = 24,                     #= length needed for frobnication 
      Bool :$verbose,                         #= required verbosity 
    ) {
        say $length if $length.defined;
        say $file   if $file.defined;
        say 'Verbosity ', ($verbose ?? 'on' !! 'off');
    }

Which would improve the usage message like this:

$ perl6 frobnicate.p6 doesntexist.dat
Usage:
  frobnicate.p6 [--length=<Int>] [--verbose] [<file>]
 
    [<file>]          an existing file to frobnicate
    --length=<Int>    length needed for frobnication
    --verbose         required verbosity

%*SUB-MAIN-OPTS

It's possible to alter how arguments are processed before they're passed to sub MAIN {} by setting options in the %*SUB-MAIN-OPTS hash. Due to the nature of dynamic variables, it is required to set up the %*SUB-MAIN-OPTS hash and fill it with the appropriate settings. For instance:

my %*SUB-MAIN-OPTS =
  :named-anywhere,    # allow named variables at any location 
  # other possible future options / custom options 
;
sub MAIN ($a$b:$c:$d{
    say "Accepted!"
}

Available options are:

named-anywhere

By default, named arguments passed to the program (i.e., MAIN) cannot appear after any positional argument. However, if %*SUB-MAIN-OPTS<named-anywhere> is set to a true value, named arguments can be specified anywhere, even after positional parameter. For example, the above program can be called with:

$ perl6 example.p6 1 --c=2 3 --d=4

is hidden-from-USAGE

Sometimes you want to exclude a MAIN candidate from being shown in any automatically generated usage message. This can be achieved by adding a hidden-from-USAGE trait to the specfication of the MAIN candidate you do not want to show. Expanding on an earlier example:

# inside file 'hello.p6' 
multi sub MAIN() is hidden-from-USAGE {
    say "Hello bashful, how are you?"
}
multi sub MAIN($name{  #= the name by which you would like to be called 
    say "Hello $name, how are you?"
}

So, if you would call this script with just a named variable, you would get the following usage:

$ perl6 hello.p6 --verbose
Usage:
  hello.p6 <name> -- the name by which you would like to be called

Without the hidden-from-USAGE trait on the first candidate, it would have looked like this:

$ perl6 hello.p6 --verbose
Usage:
  hello.p6
  hello.p6 <name> -- the name by which you would like to be called

Which, although technically correct, doesn't read as well.

Unit-scoped definition of MAIN

If the entire program body resides within MAIN, you can use the unit declarator as follows (adapting an earlier example):

unit sub MAIN(
  Str   $file where *.IO.f = 'file.dat',
  Int  :$length = 24,
  Bool :$verbose,
);  # <- note semicolon here 
 
say $length if $length.defined;
say $file   if $file.defined;
say 'Verbosity ', ($verbose ?? 'on' !! 'off');
# rest of script is part of MAIN 

Note that this is only appropriate if you can get by with just a single (only) sub MAIN.

sub USAGE

If no multi candidate of MAIN is found for the given command line parameters, the sub USAGE is called. If no such method is found, the compiler will output a default generated usage message.

#|(is it the answer) 
multi MAIN(Int $i{ say $i == 42 ?? 'answer' !! 'dunno' }
#|(divide two numbers) 
multi MAIN($a$b){ say $a/$b }
 
sub USAGE() {
    print Q:c:to/EOH/; 
    Usage: {$*PROGRAM-NAME} [number]
 
    Prints the answer or 'dunno'.
EOH
}

The default usage message is available inside sub USAGE via the read-only $*USAGE variable. It will be generated based on available sub MAIN candidates and their parameters. As shown before, you can specify an additional extended description for each candidate using a #|(...) Pod block to set WHY.

Intercepting CLI argument parsing (2018.10, v6.d and later)

You can replace or augment the default way of argument parsing by supplying a ARGS-TO-CAPTURE subroutine yourself, or by importing one from any of the Getopt modules available in the ecosystem.

sub ARGS-TO-CAPTURE

The ARGS-TO-CAPTURE subroutine should accept two parameters: a Callable representing the MAIN unit to be executed (so it can be introspected if necessary) and an array with the arguments from the command line. It should return a Capture object that will be used to dispatch the MAIN unit. A very contrived example that will create a Capture depending on some keyword that was entered (which can be handy during testing of a command line interface of a script):

sub ARGS-TO-CAPTURE(&main@args --> Capture{
    # if we only specified "frobnicate" as an argument 
    @args == 1 && @args[0eq 'frobnicate'
      # then dispatch as MAIN("foo","bar",verbose => 2) 
      ?? Capture.newlist => <foo bar>hash => { verbose => 2 } )
      # otherwise, use default processing of args 
      !! &*ARGS-TO-CAPTURE(&main@args)
}

Note that the dynamic variable &*ARGS-TO-CAPTURE is available to perform the default command line arguments to Capture processing so you don't have to reinvent the whole wheel if you don't want to.

&*ARGS-TO-CAPTURE

A dynamic variable available inside any custom ARGS-TO-CAPTURE subroutine that can be used to perform the default argument parsing. Takes the same parameters as are expected of the custom ARGS-TO-CAPTURE subroutine.

Intercepting usage message generation (2018.10, v6.d and later)

You can replace or augment the default way of usage message generation (after a failed dispatch to MAIN) by supplying a GENERATE-USAGE subroutine yourself, or by importing one from any of the Getopt modules available in the ecosystem.

sub GENERATE-USAGE

The GENERATE-USAGE subroutine should accept a Callable representing the MAIN subroutine that didn't get executed because the dispatch failed. This can be used for introspection. All the other parameters are the parameters that were set up to be sent to MAIN. It should return the string of the usage information you want to be shown to the user. An example that will just recreate the Capture that was created from processing the arguments:

sub GENERATE-USAGE(&main|capture{
    capture<foo>:exists
      ?? "You're not allowed to specify a --foo"
      !! &*GENERATE-USAGE(&main|capture)
}

You can also use multi subroutines to create the same effect:

multi sub GENERATE-USAGE(&main:$foo!{
    "You're not allowed to specify a --foo"
}
multi sub GENERATE-USAGE(&main|capture{
    &*GENERATE-USAGE(&main|capture)
}

Note that the dynamic variable &*GENERATE-USAGE is available to perform the default usage message generation so you don't have to reinvent the whole wheel if you don't want to.

&*GENERATE-USAGE

A dynamic variable available inside any custom GENERATE-USAGE subroutine that can be used to perform the default usage message creation. Takes the same parameters as are expected of the custom GENERATE-USAGE subroutine.

Intercepting MAIN calling (before 2018.10, v6.e)

An older interface enabled one to intercept the calling to MAIN completely. This depended on the existence of a MAIN_HELPER subroutine that would be called if a MAIN subroutine was found in the mainline of a program.

This interface was never documented. However, any programs using this undocumented interface will continue to function until v6.e. From v6.d onward, the use of the undocumented API will cause a DEPRECATED message.

Ecosystem modules can provide both the new and the old interface for compatibility with older versions of Perl 6: if a newer Perl 6 recognizes the new (documented) interface, it will use that. If there is no new interface subroutine available, but the old MAIN_HELPER interface is, then it will use the old interface.

If a module developer decides to only offer a module for v6.d or higher, then the support for the old interface can be removed from the module.