Interpolation - Arbitrary string interpolation semantics


  use Interpolation name => \&function, ...;
  print "la la la la $name{blah blah blah}";

  # This is like doing:
  $VAR = &function(blah blah blah);
  print "la la la la $VAR";


Beginners always want to write this:

  print "The sum of three and four is: 3+4";

And they want the 3+4 part to be evaluated, so that it prints this:

  The sum of three and four is: 7

Of course, it's a double-quoted string, so it's not evaluated. The only things that are evaluated in double-quoted strings are variable references.

There are solutions to this, but most of them are ugly. This module is less ugly. It lets you define arbitrary interpolation semantics.

For example, you can say

   use Interpolation money => \&commify_with_dollar_sign,
                     E     => 'eval',
                     placename => 'ucwords', 

And then you can write these:

   print "3 + 4 = $E{3+4}";
   # Prints  ``3 + 4 = 7''

   $SALARY = 57500;
   print "The salary is $money{$SALARY}";
   # Prints  ``The salary is $57,500.00''

   $PLACE2 = 'n.y. state';
   print "$placename{$PLACE1} is not near $placename{$PLACE2}";
   # Prints  ``San Bernadino High School is not near N.Y. State";


The arguments to the use call should be name-function pairs. If the pair is ($n, $f), then $n will be the name for the semantics provided by $f. $f must either be a reference to a function that you supply, or it can be the name of one of the built-in formatting functions provided by this package. Interpolation will take over the %n hash in your package, and tie it so that acessing $n{X} calls f(X) and yields its return value.

If for some reason you want to, you can add new semantics at run time by using

  import Interpolation name => function, ...

You can remove them again with

  unimport Interpolation 'name', ...


Interpolation provides a few useful built-in formatting functions; you can refer to these by name in the use or import line. They are:

      eval     Evaluate argument
      null     Same as eval
      identity Also the same as eval
      ucwords  Capitalize Input String Like This
      commify  1428571 => 1,428,571.00
      reverse  reverse string
      sprintf  makes "$S{'%.2f %03d'}{37.5,42}" turn into "37.50 042".
      sprintf1 makes "$S{'%.2f %03d', 37.5,42}" turn into "37.50 042".


It's easy to forget that the index to a $hash{...} is an arbitrary expression, unless it looks like an identifier. There are two gotchas here.

Trap 1.
  print "$X{localtime}";

Here the X formatter is used to format the literal string localtime; the localtime built-in function is not invoked. If you really want the current time, use one of these:

  print "$X{+localtime}";
  print "$X{localtime()}";

Trap 2.
  print "$X{What ho?}";

This won't compile---you get `search pattern not terminated'. Why? Because Perl sees the ? and interprets it as the beginning of a pattern match operator, similar to /. (Ah, you forgot that ? could be a pattern match delimiter even without a leading m, didn't you?) You really need

  print "$X{'What ho?'}";

The rule is simple: That thing in the braces that looks like a hash key really is a hash key, and so you need to put it in quotes under the same circumstances that you need to put any other hash key in quotes. You probably wouldn't expect this to work either:

  $V = $X{What ho?};


Mark-Jason Dominus (, Plover Systems co.

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