Mini HowTo: How to port Perl 5 modules to Perl 6

Having ported 20 Perl 5 CPAN modules to Perl 6 in the last few days, these are some things I noticed:

Perl 6 is sexy! :)
Before you start porting a module, make sure you understand the class hierarchy of that module. It helps if you've actually used that module in Perl 5 :)
Port a module even if it depends on some other (not yet ported) modules -- the dependencies can be ported later on.
Often, the translation P5 → P6 is quite mechanic:
- $array[idx] →
  @array[idx]
- a ? b : c →
  a ?? b :: c
- $self->method(...) →
  .method(...)
- sub { my ($self, $a, $b) = @_; ... } →
  method($a, $b) { ... }
- $x =~ s/.../.../g →
  $x ~~ s:g/.../.../
- $self->{foo} →
  $.foo
- $foo = "bar" unless defined $foo →
  $foo //= "bar" # (//) and (//=) will be in 5.9, too, IIRC
- if($foo eq "a" or $foo eq "b" or $foo eq "c") {...} →
  if $foo eq "a"|"b"|"c" {...}
- foreach my $foo (@baz) {...} →
  for @baz -> $foo {...}
- Regular expressions:
  - [abc] → <[abc]>
  - [^abc] → <-[abc]>
  - (?:...) → [...]
Often, you can remove all that Perl 5 argument parsing and simply substitute it by a nice subroutine|method|whatever signature.

# This Perl 5 exporting code...
require Exporter;
our @ISA    = qw< Exporter >;
our @EXPORT = qw< foo >;
sub foo {...}

# ...becomes this in Perl 6:
sub foo(...) is export {...}
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return map {.4.} sort {.3.} grep {.2.} map {.1.} →
map {.1.} ==> grep {.2.} ==> sort {.3.} ==> map {.4.} ==> return

Especially Perl 6's translation of Perl 5's getter/setter idiom is cool:

# Perl 5
sub get_foo {
  my $self = shift;

  my $ret = $self->{foo};
  return lc $ret; # always normalize
} 

sub set_foo {
   my ($self, $to) = @_;

   $to =~ s/\s+$//; # strip whitespace at the end
   $self->{foo} = $to;
}

# Perl 6:
has $:foo;
sub foo() is rw {
  return new Proxy:
    FETCH => { lc $:foo },
    STORE => -> $to is copy {
      $to ~~ s/\s+$//;
      $:foo = $to;
    };
}
# And then:
say $obj.foo;
$obj.foo = "...";
# Notice: Standard assignment syntax!
# Assignments should look line assingments, not like method calls
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If you trust the user to give correct data to accessors, you can also use:

has $.foo is rw;
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Alternatively, if you don't trust The Evil Users:

subtype OddInt of Int where { $^n % 2 == 1 }
has OddInt $.foo is rw;
# And then:
$obj.foo = 12; # will die (at compile-time!)
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You can find a perhaps more recent version of this document in the Pugs SVN repository.

--Ingo

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