Monads/Writer monad: Difference between revisions

</pre>

=={{header|Perl}}==

{{trans|Raku}}

<lang Perl># 20200704 added Perl programming solution

package Writer;

use strict;

use warnings;

sub new {

my ($class, $value, $log) = @_;

return bless [ $value => $log ], $class;

}

sub Bind {

my ($self, $code) = @_;

my ($value, $log) = @$self;

my $n = $code->($value);

return Writer->new( @$n[0], $log.@$n[1] );

}

sub Unit { Writer->new($_[0], sprintf("%-17s: %.12f\n",$_[1],$_[0])) }

sub root { Unit sqrt($_[0]), "Took square root" }

sub addOne { Unit $_[0]+1, "Added one" }

sub half { Unit $_[0]/2, "Divided by two" }

print Unit(5, "Initial value")->Bind(\&root)->Bind(\&addOne)->Bind(\&half)->[1];

</lang>

{{out}}

<pre>

Initial value : 5.000000000000

Took square root : 2.236067977500

Added one : 3.236067977500

Divided by two : 1.618033988750