Locked in a Box: Closures in Raku

Extending the basic idea of closures in Perl (see "Locked in a Box: Closures in Perl5") this looks at some of the extensions offered by Raku's types, defaults, and signatures. Taken together these provide a whole new level of flexibility and generality to applying closures for cleaner code.

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1. Locked Thinking Inside the Box: Closures in Raku Steven Lembark Workhorse Computing lembark@wrkhors.com
2. Raku closures are... Just closures. They close over a variable.
3. Raku closures are... Just closures. They close over a variable. Raku has some syntax differences. Can produce simpler, more declarative results. Multiple dispatch opens a brave new world.
4. Named-sub closures Subs in Raku are lexically scoped. do { my $verbose = $ENV{ VERBOSE }; sub message { say @_ if $verbose; } }; &message ‘Foobar’; # this fails
5. Named-sub closures Subs in Raku are lexically scoped. “our” makes it visible. do { my $verbose = $ENV{ VERBOSE }; our sub message { say @_ if $verbose; } }; &message ‘Foobar’;
6. Named-sub closures The ‘&’ sigil indicates a subref. Doesn’t require the ‘&’. my &message = do { my $verbose = $ENV{ VERBOSE }; sub { say @_ if $verbose; } }; message ‘Foobar’;
7. Input handler Signiture requires two inputs. loop & prompt are Raku builtins. sub gen_prompt ( $prompt, &sanity ) { sub() { my $input = ‘’; loop() { $input = prompt( $prompt ) // fail; sanity( $input ) and last; } return $input } }
8. Input handler Code object via ‘&’ . Cleaner syntax to execute. sub gen_prompt ( $prompt, &sanity ) { sub() { my $input = ‘’; loop() { $input = prompt( $prompt ) // fail; sanity( $input ) and last; } return $input } }
9. Input handler Why use only code? Raku smart matches work! sub gen_prompt ( $prompt, $sanity ) { sub() { my $input = ‘’; loop() { $input = prompt( $prompt ) // fail; $input ~~ $sanity and last; } return $input } }
10. Raku sanity with ~~ $sanity = sub block inline test lazy list $sanity = sub( $n ) { 10 <= $n <= 99 }; = { 10 <= $_ <= 99 }; = { 10 <= $^val <= 99 }; = 10 <= * <= 99 ; = 10 .. 99; = /^\d\d$/ gen_prompt( ‘Two digit number:’ => $sanity );
11. Sequence Generator Perl5 sub generate_counter { my $value = shift; my $final = shift // 2**32-1; looks_like_number $value or die ... ; looks_like_number $final or die ... ; $value <= $final or die ... ; sub { $value > $final and return; $value++ } }
12. Sequence Generator sub generate_counter ( int $value = 0 is copy , int $final = ∞ ) { $value > $final and fail; return sub { $value <= $final or fail; return value++ } } Raku
13. Sequence Generator sub generate_counter ( int $value = 0 is copy , int $final = ∞ ) { $value > $final and fail; return sub { $value <= $final or fail; return $value++ } } Raku Args default ro, “is copy” allows rw.
14. Sequence Generator ∞ avoids arbitrary limits. sub generate_counter ( int $value = 0 is copy , int $final = ∞ ) { $value > $final and fail; return sub { $value <= $final or fail; return $value++ } }
15. Sequence Generator ro args Saner failure message. $first not closed over. sub generate_counter ( int $first = 0 , int $final = ∞ ) { $first > $final and fail; my $value = $first; return sub { $value <= $final or fail; return $value++ } }
16. Sequence Generator “return sub” required for “return $value++” sub generate_counter ( int $first = 0 , int $final = ∞ ) { $first > $final and fail; my $value = $first; return sub { $value <= $final or fail; return $value++ } }
17. Sequence Generator Sans return, use a block. sub generate_counter ( int $first = 0 , int $final = ∞ ) { $first > $final and fail; my $value = $first; { $value <= $final or fail; $value++ } }
18. Sequence Generator Raku sequence generator. sub generate_counter ( int $first = 0 , int $final = ∞ ) { $first > $final and fail; my $value = $first; { $value <= $final or fail; $value++ } }
19. Multiple dispatch Raku sub’s have signitures. Same name, differing signiture is a “variant”. Selection at runtime based on the arguments.
20. Sequence variants Generate the correct type of sequences based on the argument types. multi gen_seq ( int $first, int $final = ∞ ) { $first <= $final or fail; my $value = $first; { $value <= $final or fail; ++value } } multi gen_seq( string $first, string $final = ∞ ) { $first le $final or fail; my $value = $first; { $value le $final or fail; ++value } }
21. Summary Raku closures look like Perl closures: Sub ‘closes over’ variables declared outside sub. Smart match simplifies wide range of validators. Multimethods simplify generation.
22. Bedside Reading First Class Functions: https://en.wikipedia.org/wiki/First-class_function Thanks to Damian Conway for the use of his Functional Programming in Perl & Raku class notes.