RFC 226 (v1) Selective interpolation in single quotish context.

This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=head1 TITLE

Selective interpolation in single quotish context.

=head1 VERSION

  Maintainer: Brad Hughes <[email protected]>
  Date: Sep. 14, 2000
  Mailing List: [email protected]
  Number: 226
  Version: 1
  Status: Developing

=head1 ABSTRACT

Perl6 should allow scalars and arrays to be tagged such that they are
interpolated in single quotish context.

=head1 DESCRIPTION

Single quotish context, such as C<q{}>, C<''>, and specified here-docs,
is especially useful for large blocks of text which may contain
the C<$> and C<@> characters.  Frequently, however, interpolation within
that large block of text is desired.  This RFC proposes that in single
quotish context scalars and arrays may be tagged to be interpolated.

As an example, on the VMS operating system the C<$> runs rampant
through VMS namespaces, not to mention that each line of VMS' equivalent
of a Unix shell script (called a .com file) starts with a C<$>, and execution
of VMS .com files is invoked with a C<@>, resulting in backslashitis:

	print F <<END;
	\$!
	\$! execute a.com, copy and purge
	\$!
	\$ \@sys\$login:a.com
	\$ copy $filename sys\$login:*.*
	\$ purge sys\$login:$filename
	\$!
	\$ exit
	END

With this proposal, the scalar C<$filename> can be tagged to be interpolated
(for the sake of argument, by doubling the C<$>) and the double quotish context
replaced by single quotish context resulting in the following:

	print F <<'END';
	$!
	$! execute a.com, copy and purge
	$!
	$ @sys$login:a.com
	$ copy $$filename sys$login:*.*
	$ purge sys$login:$$filename
	$!
	$ exit
	END

An array can be tagged by doubling the C<@>.

Jarkko's original suggestion was for a different single quotish context
with this behavior;  this is possible as well, but I see no danger in extending
current single quotish context to incorporate this behavior.  If a different
single quotish context with this behavior is desired, then a new member needs
to be added to the C<q(), qq(), qx(), qr()> family, with all the attendant
arguing about which letter to use.  I find this addition to be most useful for
single quotish context here-docs, though, so would argue that it apply to
all single quotish context forms.

=head1 IMPLEMENTATION

This wouldn't seem a difficult change to make.  The p526 translator can
backslash C<$$\w+>, resulting in C<\$$\w+>, which should give current behavior.

=head1 REFERENCES

Suggestion by Jarkko Hietaniemi <[email protected]> in
<[email protected]>, and seconded by Mark-Jason
Dominus <[email protected]> in <[email protected]>.