Data commit

Task/Monads-Writer-monad/Perl/monads-writer-monad.pl

@@ -0,0 +1,28 @@
+# 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];