/perl6-fp-notes

Notes on functional programing techniques with Perl 6

Notes on functional programming with Perl 6

Table of Contents

Prelude

Perl 6 is a multi-paradigm programming language that offers several features that make writing in a functional style simple, most of the time. Perl 6 is not a pure functional programming language and shouldn't be treated as such. This repository was created to document various FP solutions and approaches using Perl 6.

Note: This is not to be considered the one_true_way_to_write_perl_6 in any paradigm.

Notes

Immutability

Type Description
Str Perl string (finite sequence of Unicode characters)
Int Perl integer (allows Inf/NaN, arbitrary precision, etc.)
Num Perl number (approximate Real, generally via floating point)
Rat Perl rational (exact Real, limited denominator)
FatRat Perl rational (unlimited precision in both parts)
Complex Perl complex number
Bool Perl boolean
Exception Perl exception
Block Executable objects that have lexical scopes
Seq A list of values (can be generated lazily)
Range A pair of Ordered endpoints
Set Unordered collection of values that allows no duplicates
Bag Unordered collection of values that allows duplicates
Enum An immutable Pair
Map A mapping of Enums with no duplicate keys
Signature Function parameters (left-hand side of a binding)
Capture Function call arguments (right-hand side of a binding)
Blob An undifferentiated mass of ints, an immutable Buf
Instant A point on the continuous atomic timeline
Duration The difference between two Instants

  • Use the bind := infix for scalars, which defaults to 'ro':

    my $name := "Camelia";
$name = "Elaine";

Cannot assign to an immutable value
  in block <unit> at <unknown file> line 1

    Keep in mind, you can still re-bind using :=:

    my $name := "Camelia";
$name := "Elaine";
Elaine

  • Lists without containers are ro immutable, use Lists instead of Arrays (@) for listy things:

    my $list = (1, 2, 3);
push $list, 4;
Cannot call 'push' on an immutable 'List'
  in block <unit> at <unknown file> line 1

    A list is immutable, which means you cannot change the number of elements in a list. But if one of the elements happens to be a scalar container, you can still assign to it. [1]

    In this example $list[0] is a rw container, $x, with a value of 42. This will allow replacing the value inside the $x scalar container:

    my $x = 42;
my $list = ($x, 1, 2);
$list[0] = 23;
say $x;                 # 23

    Instead, bind the Int value to $x which which creates a ro container:

    my $x := 42;
my $list = ($x, 1, 2);
$list[0] = 23;     # Error
Cannot modify an immutable Int
  in block <unit> at <unknown file> line 1

    This example tries replacing the value of $list[1], which is an immutable Int:

    my $x = 42;
my $list = ($x, 1, 2);
$list[1] = 23;     # Error
Cannot modify an immutable Int
  in block <unit> at <unknown file> line 1

    Use Array (@) in subroutine signatures for FP-style list processing of slurpy subparams. Lists will not work here. The new @ Array is ro immutable within the scope of the subroutine:

    multi sub do-sum([$head, *@tail] ) { do-sum( $head, @tail ); };
multi sub do-sum($total, [$head, *@tail] ) { do-sum( $total + $head, @tail ); };
multi sub do-sum($total, [] ) { $total; };

my List $list = (1,2,3,4);
do-sum($list);

# Attempting to modify an subparameter Array inside its subrouting
sub edit-param(($head, *@tail)) {
  @tail[2] = 30;  # Error
  return ($head, @tail).flat;
};

say edit-param((1,2,3,4));
  Cannot modify an immutable Int

    Use Map for associative Arrays / Hash like behavior:

    my Map $list = (name => "Camelia", email => "camelia@perl6.tld").Map
Map.new((:email("camelia\@perl6.tld"),:name("Camelia")))
say $list<name>;
Camelia

# Try modifying $list<name>
$list<name> = "Steve";
Cannot modify an immutable Str
  in block <unit> at <unknown file> line 1

# Try adding a new key-value pair
$list<email> = "camelia@perl6.tld";
Cannot modify an immutable Mu
  in block <unit> at <unknown file> line 1

Getting Involved

When you submit a PR and it gets merged, you will be automatically added as a collaborator, but if you wouldn't like to be added, please mention it in your submission.

The larger Perl 6 community is very approachable and does a great job of answering questions and helping those new to Perl 6. If you're looking for other projects to contribute to please see the Perl 6 Most Wanted Modules.

Copying

License

Creative Commons License This work is licensed under a Creative Commons Attribution 3.0 Unported License

Attribution

https://github.com/scmorrison/perl6-fp-notes/graphs/contributors