Writer monad

Basic semantic borrowed from the Monads/List monad entry

# 20200508 Raku programming solution

class Writer { has Numeric $.value ; has Str $.log }

sub Bind (Writer \v, &code) {
   my \n = v.value.&code;
   Writer.new: value => n.value, log => v.log ~ n.log
};

sub Unit(\v, \s) { Writer.new: value=>v, log=>sprintf "%-17s: %.12f\n",s,v}

sub root(\v) { Unit v.sqrt, "Took square root" }

sub addOne(\v) { Unit v+1, "Added one" }

sub half(\v) { Unit v/2, "Divided by two" }

say Unit(5, "Initial value").&Bind(&root).&Bind(&addOne).&Bind(&half).log;

Output:

Initial value    : 5.000000000000
Took square root : 2.236067977500
Added one        : 3.236067977500
Divided by two   : 1.618033988750