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Marc Chantreux


open my $db, "/etc/passwd"; my $max = 5; my $index = 0; my @top5;

while ( $index <= $boundary ) { last unless defined (my $line = <$db> ); chomp; last if $line =~ /zsh$/; $top5[$index++] = $line; }

open my $db, "/etc/passwd"; my @places = 0..5; my @top5;

open my $db, "/etc/passwd"; my @top5 = ( grep {chomp; /zsh$/ } <$db> )[0..4];

use Perlude;

my @top5 = fold take 5, filter {/zsh$/} lines "/etc/passwd";

        use Perlude;
        now {say} take 5, lines "/etc/passwd";


Perlude stole some keywords from the Haskell Prelude (mainly) to make iterators easy to combine and consume.

If you're used to a unix shell,windows Powershell or any langage comming with the notion of streams, perl could be frustrating as functions like map and grep only works with arrays.

        map say, grep /foo/, <STDIN>;

Perlude provides "streamed" counterpart where a stream is a set (whole or partial) of results an iterator can return.

So to define the basic concepts of Perlude. the functions provided are in the next section.

an iterator

is a function reference that can produce a list of at least one element at each calls. an exhausted iterator returns an empty list.

an iteration

one call of an iterator

a stream

the list of all elements an iterator can produce (it may be infinite).

a generator

is a function that retuns an iterator.

a filter (think unix shell)

is a function that take an iterator as argument and returns an iterator, applying a behavior to the elements of the stream.

such behavior can be removing or adding elements of the stream, exhaust it or applying a function in the elements of it.

a consumer

filters are abount combining things nothing is computed as long as you don't use the stream. consumers actually starts to iterate on them (think python3 list() or the perl6 &eager).

to sumarize

A stream is a list finished by an empty list (which makes sense if you come from a functional langage).


A an iterator is a function that can return the elements of an iterator one by one. A generator is a function that retuns the iterator

        sub from_to { # the generator
                my ( $from, $to ) = @_;
                sub { # the iterator
                        return () if $from > $to;
                        my $r = $from;
                        return $r

note that perlude authors are used to implicit notations so we're used to write more like

        sub {
                return if $from > $to;
                (my $r, $from) = ( $from, $from + 2 );

(see the code of the &lines generator)

More examples

A math example: every elements of fibo below 1000 (1 element a time in memory)

    use Perlude;
    use strict;
    use warnings;

    sub fibo {
        my @seed = @_;
        sub {
            push @seed, $seed[0] + $seed[1];
            shift @seed

    now {say} takeWhile { $_ < 1000 } fibo 1,1;

Used to shell?

    # in sh:
    # yes "happy birthday" | sed 5q

    sub yes ($msg) { sub { $msg } }

    now {say} take 5, yes "happy birthday"

A sysop example: throw your shellscripts away

    use Perlude;
    use strictures;
    use 5.10.0;

    # iterator on a glob matches stolen from Perlude::Sh module
    sub ls {
        my $glob = glob shift;
        my $match;
        sub {
            return $match while $match = <$glob>;

    # show every txt files in /tmp
    now {say} ls "/tmp/*txt

    # remove empty files from tmp

    now { unlink if -f && ! -s } ls "/tmp/*"

    # something more reusable/readable ?

    sub is_empty_file { -f && ! -s }
    sub empty_files_of { filter {is_empty_file} shift }
    sub rm { now {unlink} shift }

    rm empty_files_of ls "/tmp/*./txt";

Function composition

When relevant, i used the Haskell Prelude documentation descriptions and examples. for example, the take documentation comes from http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#v:take



range $begin, [ $end, [ $step ] ]

A range of numbers from $begin to $end (infinity if $end isn't set) $step by $step.

    range 5     # from 5 to infinity
    range 5,9   # 5, 6, 7, 8, 9
    range 5,9,2 # 5, 7, 9

cycle @set

infinitly loop on a set of values

    cycle 1,4,7

    # 1,4,7,1,4,7,1,4,7,1,4,7,1,4,7,...

records $ref

given any kind of ref that implements the "<>" iterator, returns a Perlude compliant iterator.

    now {print if /data/} records do {
        open my $fh,"foo";


just easier (yet safer?) to use wrapper on the sub described in perldoc -f open (also "open" in perlfunc).

the goal is to have an wrapper on open does a coercion (just return @_ if nothing to do). so

  • don't carre about prototype (so you can call it with an array, not only a list)

  • return a FILEHANDLE instead of having a side effect on the first variable

  • just return a FILEHANDLE passed as argument (so it's a coercion from @_ to an open handler).

        open FILEHANDLE
        open EXPR
        open MODE,EXPR
        open MODE,EXPR,LIST
        open MODE,EXPR,REF


    now {say if /data/} records do {
        open my $fh,"foo";


filters are composition functions that take a stream and returns a modified stream.



take $n, $xs

take $n, applied to a list $xs, returns the prefix of $xs of length $n, or $xs itself if $n > length $xs:

    sub top10 { take 10, shift }

    take 5, range 1, 10
    # 1, 2, 3, 4, 5, ()

    take 5, range 1, 3
    # 1, 2, 3, ()

takeWhile $predicate, $xs

takeWhile, applied to a predicate $p and a list $xs, returns the longest prefix (possibly empty) of $xs of elements that satisfy $p

    takeWhile { 10 > ($_*2) } range 1,5
    # 1, 2, 3, 4

drop $n, $xs

drop $n $xs returns the suffix of $xs after the first $n elements, or () if $n > length $xs:

    drop 3, range 1,5
    # 4 , 5

    drop 3, range 1,2
    # ()

dropWhile $predicate, $xs

dropWhile $predicate, $xs returns the suffix remaining after dropWhile $predicate, $xs

     dropWhile { $_ < 3 } unfold [1,2,3,4,5,1,2,3] # [3,4,5,1,2,3]
     dropWhile { $_ < 9 } unfold [1,2,3]           # []
     dropWhile { $_ < 0 } unfold [1,2,3]           # [1,2,3]


unfold $array

unfold returns an iterator on the $array ref so that every Perlude goodies can be applied. there is no side effect on the referenced array.

    my @lower = fold takeWhile {/data/} unfold $abstract

see also fold

pairs $hash

returns an iterator on the pairs of $hash stored in a 2 items array ref.

    now {
        my ( $k, $v ) = @$_;
        say "$k : $v";
    } pairs {qw< a A b B >};

aims to be equivalent to

    my $hash = {qw< a A b B >};
    while ( my ( $k, $v ) = each %$hash ) {
        say "$k : $v";

except that:

pairs can use an anonymous hash
can be used in streams
i hate the while syntax


now {actions} $xs

read the $xs stream and execute the {actions} block with the returned element as $_ until the $xs stream exhausts. it also returns the last transformed element so that it can be used to foldl.

(compare it to perl6 "eager" or haskell foldl)

fold $xs

returns the array of all the elements computed by $xs

    say join ',',      take 5, sub { state $x=-2; $x+=2 } # CODE(0x180bad8)
    say join ',', fold take 5, sub { state $x=-2; $x+=2 } # 0,2,4,6,8

see also unfold

nth $xs

returns the nth element of a stream

    say fold nth 5, sub { state $x=1; $x++ }
    # 5


non destructive splice alike (maybe best named as "traverse"? haskell name?). you can traverse an array by a group of copies of elements

    say "@$_" for fold chunksOf 3, ['a'..'f'];
    # a b c
    # d e f


concat @streams

concat takes a list of streams and returns them as a unique one:

    concat map { unfold [split //] } split /\s*/;

streams every chars of the words of the text

concatC $stream_of_streams

takes a stream of streams $stream_of_streams and expose them as a single one. A stream of streams is a steam that returns streams.

    concatC { take 3, range $_ } lines $fh

take 3 elements from the range started by the values of $fh, so if $fh contains (5,10), the stream is (5,6,7,10,11,12)

concatM $apply, $stream

applying $apply on each iterations of $stream must return a new stream. concatM expose them as a single stream.

    # ls is a generator for a glob

    sub cat { concatM {lines} ls shift }
    cat "/tmp/*.conf"


feedbacks are very welcome, todo list:

    * Improve the doc




  • Philippe Bruhat (BooK)

  • Marc Chantreux (eiro)

  • Olivier Mengué (dolmen)


Burak Gürsoy (cpanization)


  • Thanks to Nicolas Pouillard and Valentin (#haskell-fr), i leanrt a lot about streams, lazyness, lists and so on. Lazyness.pm was my first attempt.

  • The name "Perlude" is an idea from Germain Maurice, the amazing sysop of http://linkfluence.com back to early 2010.

  • Former versions of Perlude used undef as stream terminator. After my talk at the French Perl Workshop 2011, dolmen suggested to use () as stream terminator, which makes sense not only because undef is a value but also because () is the perfect semantic to end a stream. So Book, Dolmen and myself rewrote the entire module from scratch in the hall of the hotel with a bottle of chartreuse and Cognominal.

    We also tried some experiments about real lazyness, memoization and so on. it becomes clear now that this is hell to implement correctly: use perl6 instead :)

    I was drunk and and mispelled Perlude as "Perl dude" so Cognominal collected some quotes of "The Big Lebowski" and we called ourselves "the Perl Dudes". This is way my best remember of peer programming and one of the best moment i shared with my friends mongueurs.