Devel::Declare - (DEPRECATED) Adding keywords to perl, in perl

      use Method::Signatures;
      # or ...
      use MooseX::Declare;
      # etc.

      # Use some new and exciting syntax like:
      method hello (Str :$who, Int :$age where { $_ > 0 }) {
        $self->say("Hello ${who}, I am ${age} years old!");

    Devel::Declare can install subroutines called declarators which locally
    take over Perl's parser, allowing the creation of new syntax.

    This document describes how to create a simple declarator.

    Warning: Devel::Declare is a giant bag of crack originally implemented
    by mst with the goal of upsetting the perl core developers so much by
    its very existence that they implemented proper keyword handling in the

    As of perl5 version 14, this goal has been achieved, and modules such as
    Devel::CallParser, Function::Parameters, and Keyword::Simple provide
    mechanisms to mangle perl syntax that don't require hallucinogenic drugs
    to interpret the error messages they produce.

    If you are using something that uses Devel::Declare, please for the love
    of kittens use something else:

    *   Instead of TryCatch, use Try::Tiny

    *   Instead of Method::Signatures, use real subroutine signatures
        (requires perl 5.22) or Moops

    We'll demonstrate the usage of "Devel::Declare" with a motivating
    example: a new "method" keyword, which acts like the builtin "sub", but
    automatically unpacks $self and the other arguments.

      package My::Methods;
      use Devel::Declare;

  Creating a declarator with "setup_for"
    You will typically create

      sub import {
        my $class = shift;
        my $caller = caller;

            { method => { const => \&parser } }
        no strict 'refs';
        *{$caller.'::method'} = sub (&) {};

    Starting from the end of this import routine, you'll see that we're
    creating a subroutine called "method" in the caller's namespace. Yes,
    that's just a normal subroutine, and it does nothing at all (yet!) Note
    the prototype "(&)" which means that the caller would call it like so:

        method {
            my ($self, $arg1, $arg2) = @_;

    However we want to be able to call it like this

        method foo ($arg1, $arg2) {

    That's why we call "setup_for" above, to register the declarator
    'method' with a custom parser, as per the next section. It acts on an
    optype, usually 'const' as above. (Other valid values are 'check' and

    For a simpler way to install new methods, see also

  Writing a parser subroutine
    This subroutine is called at *compilation* time, and allows you to read
    the custom syntaxes that we want (in a syntax that may or may not be
    valid core Perl 5) and munge it so that the result will be parsed by the
    "perl" compiler.

    For this example, we're defining some globals for convenience:

        our ($Declarator, $Offset);

    Then we define a parser subroutine to handle our declarator. We'll look
    at this in a few chunks.

        sub parser {
          local ($Declarator, $Offset) = @_;

    "Devel::Declare" provides some very low level utility methods to parse
    character strings. We'll define some useful higher level routines below
    for convenience, and we can use these to parse the various elements in
    our new syntax.

    Notice how our parser subroutine is invoked at compile time, when the
    "perl" parser is pointed just *before* the declarator name.

          skip_declarator;          # step past 'method'
          my $name = strip_name;    # strip out the name 'foo', if present
          my $proto = strip_proto;  # strip out the prototype '($arg1, $arg2)', if present

    Now we can prepare some code to 'inject' into the new subroutine. For
    example we might want the method as above to have "my ($self, $arg1,
    $arg2) = @_" injected at the beginning of it. We also do some clever
    stuff with scopes that we'll look at shortly.

          my $inject = make_proto_unwrap($proto);
          if (defined $name) {
            $inject = scope_injector_call().$inject;

    We've now managed to change "method ($arg1, $arg2) { ... }" into "method
    { injected_code; ... }". This will compile... but we've lost the name of
    the method!

    In a cute (or horrifying, depending on your perspective) trick, we
    temporarily change the definition of the subroutine "method" itself, to
    specialise it with the $name we stripped, so that it assigns the code
    block to that name.

    Even though the *next* time "method" is compiled, it will be redefined
    again, "perl" caches these definitions in its parse tree, so we'll
    always get the right one!

    Note that we also handle the case where there was no name, allowing an
    anonymous method analogous to an anonymous subroutine.

          if (defined $name) {
            $name = join('::', Devel::Declare::get_curstash_name(), $name)
              unless ($name =~ /::/);
            shadow(sub (&) { no strict 'refs'; *{$name} = shift; });
          } else {
            shadow(sub (&) { shift });

  Parser utilities in detail
    For simplicity, we're using global variables like $Offset in these
    examples. You may prefer to look at Devel::Declare::Context::Simple,
    which encapsulates the context much more cleanly.

    This simple parser just moves across a 'token'. The common case is to
    skip the declarator, i.e. to move to the end of the string 'method' and
    before the prototype and code block.

        sub skip_declarator {
          $Offset += Devel::Declare::toke_move_past_token($Offset);

    This builtin parser simply moves past a 'token' (matching
    "/[a-zA-Z_]\w*/") It takes an offset into the source document, and skips
    past the token. It returns the number of characters skipped.

    This parser skips any whitespace, then scans the next word (again
    matching a 'token'). We can then analyse the current line, and
    manipulate it (using pure Perl). In this case we take the name of the
    method out, and return it.

        sub strip_name {
          if (my $len = Devel::Declare::toke_scan_word($Offset, 1)) {
            my $linestr = Devel::Declare::get_linestr();
            my $name = substr($linestr, $Offset, $len);
            substr($linestr, $Offset, $len) = '';
            return $name;

    This builtin parser, given an offset into the source document, matches a
    'token' as above but does not skip. It returns the length of the token
    matched, if any.

    This builtin returns the full text of the current line of the source

    This builtin sets the full text of the current line of the source
    document. Beware that injecting a newline into the middle of the line is
    likely to fail in surprising ways. Generally, Perl's parser can rely on
    the `current line' actually being only a single line. Use other kinds of
    whitespace instead, in the code that you inject.

    This parser skips whitsepace.

        sub skipspace {
          $Offset += Devel::Declare::toke_skipspace($Offset);

    This builtin parser, given an offset into the source document, skips
    over any whitespace, and returns the number of characters skipped.

    This is a more complex parser that checks if it's found something that
    starts with '(' and returns everything till the matching ')'.

        sub strip_proto {

          my $linestr = Devel::Declare::get_linestr();
          if (substr($linestr, $Offset, 1) eq '(') {
            my $length = Devel::Declare::toke_scan_str($Offset);
            my $proto = Devel::Declare::get_lex_stuff();
            $linestr = Devel::Declare::get_linestr();
            substr($linestr, $Offset, $length) = '';
            return $proto;

    This builtin parser uses Perl's own parsing routines to match a
    "stringlike" expression. Handily, this includes bracketed expressions
    (just think about things like "q(this is a quote)").

    Also it Does The Right Thing with nested delimiters (like "q(this (is
    (a) quote))").

    It returns the effective length of the expression matched. Really, what
    it returns is the difference in position between where the string
    started, within the buffer, and where it finished. If the string
    extended across multiple lines then the contents of the buffer may have
    been completely replaced by the new lines, so this position difference
    is not the same thing as the actual length of the expression matched.
    However, because moving backward in the buffer causes problems, the
    function arranges for the effective length to always be positive,
    padding the start of the buffer if necessary.

    Use "get_lex_stuff" to get the actual matched text, the content of the
    string. Because of the behaviour around multiline strings, you can't
    reliably get this from the buffer. In fact, after the function returns,
    you can't rely on any content of the buffer preceding the end of the

    If the string being scanned is not well formed (has no closing
    delimiter), "toke_scan_str" returns "undef". In this case you cannot
    rely on the contents of the buffer.

    This builtin returns what was matched by "toke_scan_str". To avoid
    segfaults, you should call "clear_lex_stuff" immediately afterwards.

  Munging the subroutine
    Let's look at what we need to do in detail.

    We may have defined our method in different ways, which will result in a
    different value for our prototype, as parsed above. For example:

        method foo         {  # undefined
        method foo ()      {  # ''
        method foo ($arg1) {  # '$arg1'

    We deal with them as follows, and return the appropriate "my ($self,
    ...) = @_;" string.

        sub make_proto_unwrap {
          my ($proto) = @_;
          my $inject = 'my ($self';
          if (defined $proto) {
            $inject .= ", $proto" if length($proto);
            $inject .= ') = @_; ';
          } else {
            $inject .= ') = shift;';
          return $inject;

    Now we need to inject it after the opening '{' of the method body. We
    can do this with the building blocks we defined above like "skipspace"
    and "get_linestr".

        sub inject_if_block {
          my $inject = shift;
          my $linestr = Devel::Declare::get_linestr;
          if (substr($linestr, $Offset, 1) eq '{') {
            substr($linestr, $Offset+1, 0) = $inject;

    We want to be able to handle both named and anonymous methods. i.e.

        method foo () { ... }
        my $meth = method () { ... };

    These will then get rewritten as

        method { ... }
        my $meth = method { ... };

    where 'method' is a subroutine that takes a code block. Spot the
    problem? The first one doesn't have a semicolon at the end of it! Unlike
    'sub' which is a builtin, this is just a normal statement, so we need to
    terminate it. Luckily, using "B::Hooks::EndOfScope", we can do this!

      use B::Hooks::EndOfScope;

    We'll add this to what gets 'injected' at the beginning of the method

      sub scope_injector_call {
        return ' BEGIN { MethodHandlers::inject_scope }; ';

    So at the beginning of every method, we are passing a callback that will
    get invoked at the *end* of the method's compilation... i.e. exactly
    then the closing '}' is compiled.

      sub inject_scope {
        on_scope_end {
          my $linestr = Devel::Declare::get_linestr;
          my $offset = Devel::Declare::get_linestr_offset;
          substr($linestr, $offset, 0) = ';';

  Shadowing each method.
    We override the current definition of 'method' using "shadow".

        sub shadow {
          my $pack = Devel::Declare::get_curstash_name;
          Devel::Declare::shadow_sub("${pack}::${Declarator}", $_[0]);

    For a named method we invoked like this:

        shadow(sub (&) { no strict 'refs'; *{$name} = shift; });

    So in the case of a "method foo { ... }", this call would redefine
    "method" to be a subroutine that exports 'sub foo' as the (munged)
    contents of "{...}".

    The case of an anonymous method is also cute:

        shadow(sub (&) { shift });

    This means that

        my $meth = method () { ... };

    is rewritten with "method" taking the codeblock, and returning it as is
    to become the value of $meth.

    This returns the package name *currently being compiled*.

    Handles the details of redefining the subroutine.

    One of the best ways to learn "Devel::Declare" is still to look at
    modules that use it:


    Matt S Trout - <> - original author

    Company: Blog:

    Florian Ragwitz <> - maintainer

    osfameron <> - first draft of documentation

    This library is free software under the same terms as perl itself

    Copyright (c) 2007, 2008, 2009 Matt S Trout

    Copyright (c) 2008, 2009 Florian Ragwitz

    stolen_chunk_of_toke.c based on toke.c from the perl core, which is

    Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
    2000, 2001, 2002, 2003, 2004, 2005, 2006, by Larry Wall and others