[% setvar title Variable interpolation on demand. %]

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Variable interpolation on demand.


  Maintainer: Glenn Linderman <glenn@linderman.com>
  Date: 14 Sep 2000
  Last Modified: 28 Sep 2000
  Mailing List: perl6-language@perl.org
  Number: 229
  Version: 2
  Status: Frozen


Make Perl's powerful string interpolation facilities are available to variables, in addition to literals.


Version 2 adds more description to the IMPLEMENTATION section.

Notes on Freeze: not much discussion resulted on the list. Someone suggested the implementation now mentioned in the implementation section, but it isn't really general enough due to the need to choose a delimiter character, although it works for some strings some of the time.



$foo = 'def'; $bar = 'ghi'; $x = "abc$foo$bar"; $y = 'abc$foo$bar';

There is no way to turn obtain the value of $x from the value of $y. In other words, while $foo and $bar were interpolated into $x, they were not interpolated into $y. It would be nice to get Success! from:

$z = interpolate ( $y ); print 'Success!" if $z eq $x;

However, there is no direct language facility for this, because interpolation only happens one level deep (which is good), so attempts such as

$z = qq/$y/;

will interpolate $y, but not $foo and $bar, so $z eq $y, instead of $z eq $x.

Shown above is a functional syntax. However, due to scoping rules and lexical variables, it is unlikely that this functionality could be implemented as a function, more likely it would have to be a keyword. An alternate syntax might be:

$z = qd/$y/;

implying a double interpolation pass. Or

$z = q2/$y/;

which means 2 interpolation passes, and which could be extended to q3, q4, q5 ??? Or

$z = qq/$y/2;

which likewise would mean two passes, and could be extended to 3, 4, 5, or even to

$z = qq/$y/$n;

a variable number of passes. The same could be achieved with the keyword technique via an additional parameter:

interpolate ( $y, $n );

Compatibility considerations

This is a new feature, so name conflict is the only issue.

This seems compatible with other extensions to string interpolation... whatever extensions get implemented should work here too.


Just a re-write rule.

interpolate ( $y )

could become

eval "qq\000".$y."\0";

if you make the assumption that null characters would never exist within $y. Of course, that is an invalid assumption in Perl. Avoiding that assumption would either require that you scan $y and discover a character not contained within it that can be used for the delimiter, or instead of the eval, make a direct call to the interpolation function inside Perl. This latter is probably the better solution, and explains why this should be in core.