sub MAIN

Documentation for sub MAIN assembled from the following types:

language documentation Command line interface

From Command line interface

(Command line interface) 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.

language documentation Functions

From Functions

(Functions) sub MAIN

Declaring a sub MAIN is not compulsory in Perl 6 scripts, but you can provide one to create a command line interface for your script.