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  1. #!/usr/bin/perl
  2.  
  3. # Idiom #319 generator functions
  4.  
  5. use feature 'say';
  6. use strict;
  7.  
  8. # predeclare _upto with prototype (&) so it expects an anonymous sub
  9. # without this, the iterator won't work inside a foreach loop (only while)
  10. # see https://p...content-available-to-author-only...l.org/perlsub#Prototypes
  11. sub _upto (&) { return $_[0]; }  
  12.  
  13. # define a closure over $start and $end
  14. # see https://p...content-available-to-author-only...l.org/perlfaq7#What%27s-a-closure?
  15. sub upto {
  16.     my ($start, $end) = @_;
  17.  
  18.     my $n = $start;
  19.  
  20.     return _upto {
  21.         return
  22.             # in an array context, return list $start..$end
  23.             # in scalar context"
  24.             #   if we've reached the end, reset $n to $start and return an empty list
  25.             #   (which in scalar context will be taken as false and stop the loop)
  26.             #   otherwise increment $n and return it
  27.             wantarray ? $start .. $end    :
  28.             $n > $end ? ($n = $start, ()) :
  29.             $n++
  30.             ;
  31.         };
  32. }
  33.  
  34. my $it = upto(3, 5);
  35.  
  36. print 'foreach:';
  37. foreach ( $it->() ) { print ' ' . $_ }
  38. print "\n";
  39.  
  40. $it = upto(2, 6);
  41.  
  42. print 'while:  ';
  43. while ( my $n = $it->() ) { print ' ' . $n }
  44. print "\n";
  45.  
  46. # In array context, upto can be used to generate a range.
  47. # For that it uses perl's .. operator to generate an actual
  48. # list of numbers,  So this use case isn't an example of an 
  49. # iterator.  The functionality is provided in upto just so it
  50. # would be a bit more generally useful.
  51. my $range = upto(7, 11);
  52. print 'list context: ';
  53. my @list = $range->();
  54. print join ' ', @list;
  55. print "\n";
  56.  
Success #stdin #stdout 0.01s 5436KB
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Standard input is empty
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foreach: 3 4 5
while:   2 3 4 5 6
list context: 7 8 9 10 11
https://ideone.com/SsdO0S
Idiom #319 generator functions
language:
Perl (perl 5.28.1)
created:
2 years ago
visibility:
public

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