View source for Monads/Writer monad

{{task}}
[[Category:Monads]]

The Writer monad is a programming design pattern which makes it possible to compose functions which return their result values paired with a log string. The final result of a composed function yields both a value, and a concatenation of the logs from each component function application.

Demonstrate in your programming language the following:

# Construct a Writer monad by writing the 'bind' function and the 'unit' (sometimes known as 'return') function for that monad (or just use what the language already provides)
# Write three simple functions: root, addOne, and half
# Derive Writer monad versions of each of these functions
# Apply a composition of the Writer versions of root, addOne, and half to the integer 5, deriving both a value for the Golden Ratio φ, and a concatenated log of the function applications (starting with the initial value, and followed by the application of root, etc.)

=={{header|ALGOL 68}}==
{{Trans|Go}}
<lang algol68>BEGIN
    MODE MWRITER = STRUCT( LONG REAL value
                         , STRING    log
                         );
    PRIO BIND = 9;
    OP   BIND = ( MWRITER m, PROC( LONG REAL )MWRITER f )MWRITER:
    (    MWRITER n := f( value OF m );
         log OF n  := log OF m + log OF n;
         n
    );

    OP   LEN     = ( STRING s )INT: ( UPB s + 1 ) - LWB s;
    PRIO PAD     = 9;
    OP   PAD     = ( STRING s, INT width )STRING: IF LEN s >= width THEN s ELSE s + ( width - LEN s ) * " " FI;

    PROC unit    = ( LONG REAL v, STRING s )MWRITER: ( v, "  " + s PAD 17 + ":" + fixed( v, -19, 15 ) + REPR 10 );
 
    PROC root    = ( LONG REAL v )MWRITER: unit( long sqrt( v ), "Took square root" );
    PROC add one = ( LONG REAL v )MWRITER: unit( v+1, "Added one" );
    PROC half    = ( LONG REAL v )MWRITER: unit( v/2, "Divided by two" );
 
    MWRITER mw2 := unit( 5, "Initial value" ) BIND root BIND add one BIND half;
    print( ( "The Golden Ratio is", fixed( value OF mw2, -19, 15 ), newline ) );
    print( ( newline, "This was derived as follows:-", newline ) );
    print( ( log OF mw2 ) )
END</lang>
{{out}}
<pre>
The Golden Ratio is  1.618033988749895

This was derived as follows:-
  Initial value    :  5.000000000000000
  Took square root :  2.236067977499790
  Added one        :  3.236067977499790
  Divided by two   :  1.618033988749895
</pre>

=={{header|AppleScript}}==


{{trans|JavaScript}}


More than a light-weight scripting language is really likely to need, but a way of stretching it a bit, and understanding its relationship to other languages. What AppleScript mainly lacks (apart from a well-developed library, and introspective records/dictionaries which know what keys/fields they have), is a coherent type of first class (and potentially anonymous) function. To get first class objects, we have to wrap 2nd class handlers in 1st class scripts.

<lang AppleScript>-- WRITER MONAD FOR APPLESCRIPT

-- How can we compose functions which take simple values as arguments
-- but return an output value which is paired with a log string ?

-- We can prevent functions which expect simple values from choking 
-- on log-wrapped output (from nested functions)
-- by writing Unit/Return() and Bind() for the Writer monad in AppleScript

on run {}
    
    -- Derive logging versions of three simple functions, pairing
    -- each function with a particular comment string
    
    -- (a -> b) -> (a -> (b, String))
    set wRoot to writerVersion(root, "obtained square root")
    set wSucc to writerVersion(succ, "added one")
    set wHalf to writerVersion(half, "divided by two")
    
    loggingHalfOfRootPlusOne(5)

    --> value + log string
end run


-- THREE SIMPLE FUNCTIONS
on root(x)
    x ^ (1 / 2)
end root

on succ(x)
    x + 1
end succ

on half(x)
    x / 2
end half

-- DERIVE A LOGGING VERSION OF A FUNCTION  BY COMBINING IT WITH A 
-- LOG STRING FOR THAT FUNCTION
-- (SEE 'on run()' handler at top of script)
-- (a -> b) -> String -> (a -> (b, String))
on writerVersion(f, strComment)
    script
        on call(x)
            {value:sReturn(f)'s call(x), comment:strComment}
        end call
    end script
end writerVersion


-- DEFINE A COMPOSITION OF THE SAFE VERSIONS
on loggingHalfOfRootPlusOne(x)
    logCompose([my wHalf, my wSucc, my wRoot], x)
end loggingHalfOfRootPlusOne


-- Monadic UNIT/RETURN and BIND functions for the writer monad
on writerUnit(a)
    try
        set strValue to ": " & a as string
    on error
        set strValue to ""
    end try
    {value:a, comment:"Initial value" & strValue}
end writerUnit

on writerBind(recWriter, wf)
    set recB to wf's call(value of recWriter)
    set v to value of recB
    
    try
        set strV to " -> " & (v as string)
    on error
        set strV to ""
    end try
    
    {value:v, comment:(comment of recWriter) & linefeed & (comment of recB) & strV}
end writerBind

-- THE TWO HIGHER ORDER FUNCTIONS ABOVE ENABLE COMPOSITION OF 
-- THE LOGGING VERSIONS OF EACH FUNCTION
on logCompose(lstFunctions, varValue)
    reduceRight(lstFunctions, writerBind, writerUnit(varValue))
end logCompose

-- xs: list, f: function, a: initial accumulator value
-- the arguments available to the function f(a, x, i, l) are
-- v: current accumulator value
-- x: current item in list
-- i: [ 1-based index in list ] optional
-- l: [ a reference to the list itself ] optional
on reduceRight(xs, f, a)
    set sf to sReturn(f)
    
    repeat with i from length of xs to 1 by -1
        set a to sf's call(a, item i of xs, i, xs)
    end repeat
end reduceRight

-- Unit/Return and bind for composing handlers in script wrappers
-- lift 2nd class function into 1st class wrapper 
-- handler function --> first class script object
on sReturn(f)
    script
        property call : f
    end script
end sReturn

-- return a new script in which function g is composed
-- with the f (call()) of the Mf script
-- Mf -> (f -> Mg) -> Mg
on sBind(mf, g)
    script
        on call(x)
            sReturn(g)'s call(mf's call(x))
        end call
    end script
end sBind</lang>

{{Out}}

<pre>{
    value:1.61803398875,
    comment:"Initial value: 5\n
             obtained square root -> 2.2360679775\n
             added one -> 3.2360679775\n
             divided by two -> 1.61803398875"
}</pre>

=={{header|C++}}==
<lang cpp>#include <cmath>
#include <iostream>
#include <string>

using namespace std;

// Use a struct as the monad
struct LoggingMonad
{
    double Value;
    string Log;
};

// Use the >> operator as the bind function
auto operator>>(const LoggingMonad& monad, auto f)
{
    auto result = f(monad.Value);
    return LoggingMonad{result.Value, monad.Log + "\n" + result.Log};
}

// Define the three simple functions
auto Root = [](double x){ return sqrt(x); };
auto AddOne = [](double x){ return x + 1; };
auto Half = [](double x){ return x / 2.0; };

// Define a function to create writer monads from the simple functions
auto MakeWriter = [](auto f, string message)
{
    return [=](double x){return LoggingMonad(f(x), message);};
};

// Derive writer versions of the simple functions
auto writerRoot = MakeWriter(Root, "Taking square root");
auto writerAddOne = MakeWriter(AddOne, "Adding 1");
auto writerHalf = MakeWriter(Half, "Dividing by 2");


int main()
{
    // Compose the writers to compute the golden ratio
    auto result = LoggingMonad{5, "Starting with 5"} >> writerRoot >> writerAddOne >> writerHalf;
    cout << result.Log << "\nResult: " << result.Value;
}
</lang>
{{out}}
<pre>
Starting with 5
Taking square root
Adding 1
Dividing by 2
Result: 1.61803
</pre>

=={{header|EchoLisp}}==
Our monadic Writer elements will be pairs (string . value), where string is the log string.

<lang scheme>
(define (Writer.unit x (log #f)) 
	(if log (cons log x)
		(cons (format "init → %d" x) x)))

;; f is a lisp function
;; (Writer.lift f) returns a  Writer function which returns a Writer element

(define (Writer.lift f name)     
	(lambda(elem) 
             (Writer.unit 
             	(f (rest elem)) 
                (format "%a \n %a  → %a" (first elem) name (f (rest elem))))))
                            
;; lifts and applies
(define (Writer.bind f elem) ((Writer.lift f (string f)) elem))

(define (Writer.print elem) (writeln 'result (rest elem)) (writeln (first elem)))
		
;; Writer monad versions
(define w-root  (Writer.lift sqrt "root"))
(define w-half  (Writer.lift (lambda(x) (// x 2)) "half"))
(define w-inc  ( Writer.lift add1 "add-one"))


;; no binding required, as we use Writer lifted functions
(->  5 Writer.unit w-root w-inc w-half Writer.print)

result 1.618033988749895    
init → 5
root → 2.23606797749979
add-one → 3.23606797749979
half → 1.618033988749895    

;; binding
(->>  0 Writer.unit (Writer.bind sin) (Writer.bind cos)  w-inc w-half Writer.print)

result 1    
init → 0
sin → 0
cos → 1
add-one → 2
half → 1    
</lang>

=={{header|Factor}}==
Factor comes with an implementation of Haskell-style monads in the <code>monads</code> vocabulary.
{{works with|Factor|0.99 2019-10-06}}
<lang factor>USING: kernel math math.functions monads prettyprint ;
FROM: monads => do ;

{
    [ 5 "Started with five, " <writer> ]
    [ sqrt "took square root, " <writer> ]
    [ 1 + "added one, " <writer> ]
    [ 2 / "divided by two." <writer> ]
} do .</lang>
{{out}}
<pre>
T{ writer
    { value 1.618033988749895 }
    { log
        "Started with five, took square root, added one, divided by two."
    }
}
</pre>

=={{header|Go}}==
{{trans|Kotlin}}
<lang go>package main

import (
    "fmt"
    "math"
)

type mwriter struct {
    value float64
    log   string
}

func (m mwriter) bind(f func(v float64) mwriter) mwriter {
    n := f(m.value)
    n.log = m.log + n.log
    return n
}

func unit(v float64, s string) mwriter {
    return mwriter{v, fmt.Sprintf("  %-17s: %g\n", s, v)}
}

func root(v float64) mwriter {
    return unit(math.Sqrt(v), "Took square root")
}

func addOne(v float64) mwriter {
    return unit(v+1, "Added one")
}

func half(v float64) mwriter {
    return unit(v/2, "Divided by two")
}

func main() {
    mw1 := unit(5, "Initial value")
    mw2 := mw1.bind(root).bind(addOne).bind(half)
    fmt.Println("The Golden Ratio is", mw2.value)
    fmt.Println("\nThis was derived as follows:-")
    fmt.Println(mw2.log)
}</lang>

{{out}}
<pre>
The Golden Ratio is 1.618033988749895

This was derived as follows:-
  Initial value    : 5
  Took square root : 2.23606797749979
  Added one        : 3.23606797749979
  Divided by two   : 1.618033988749895
</pre>

=={{header|Haskell}}==

Haskell has the built-in <code>Monad</code> type class, and a built-in <code>Writer</code> monad (as well as the more general <code>WriterT</code> monad transformer that can make a writer monad with an underlying computation that is also a monad) already conforms to the <code>Monad</code> type class.

Making a logging version of functions (unfortunately, if we use the built-in writer monad we cannot get the values into the logs when binding):
<lang haskell>import Control.Monad.Trans.Writer
import Control.Monad ((>=>))

loggingVersion :: (a -> b) -> c -> a -> Writer c b
loggingVersion f log x = writer (f x, log)

logRoot = loggingVersion sqrt "obtained square root, "
logAddOne = loggingVersion (+1) "added 1, "
logHalf = loggingVersion (/2) "divided by 2, "

halfOfAddOneOfRoot = logRoot >=> logAddOne >=> logHalf

main = print $ runWriter (halfOfAddOneOfRoot 5)</lang>

{{Out}}

<pre>
(1.618033988749895,"obtained square root, added 1, divided by 2, ")
</pre>

=={{header|J}}==

Based on javascript implementation:

<lang J>root=: %:
incr=: >:
half=: -:

tostr=: ,@":

loggingVersion=: conjunction define
  n;~u
)

Lroot=: root loggingVersion 'obtained square root'
Lincr=: incr loggingVersion 'added 1'
Lhalf=: half loggingVersion 'divided by 2'

loggingUnit=: verb define
  y;'Initial value: ',tostr y
)

loggingBind=: adverb define
  r=. u 0{::y
  v=. 0{:: r
  v;(1{::y),LF,(1{::r),' -> ',tostr v 
)

loggingCompose=: dyad define
  ;(dyad def '<x`:6 loggingBind;y')/x,<loggingUnit y
)</lang>

Task example:

<lang J>   0{::Lhalf`Lincr`Lroot loggingCompose 5
1.61803
   1{::Lhalf`Lincr`Lroot loggingCompose 5
Initial value: 5
obtained square root -> 2.23607
added 1 -> 3.23607
divided by 2 -> 1.61803</lang>

=={{header|JavaScript}}==

===ES5===

<lang JavaScript>(function () {
    'use strict';

    // START WITH THREE SIMPLE FUNCTIONS

    // Square root of a number more than 0
    function root(x) {
        return Math.sqrt(x);
    }

    // Add 1
    function addOne(x) {
        return x + 1;
    }

    // Divide by 2
    function half(x) {
        return x / 2;
    }


    // DERIVE LOGGING VERSIONS OF EACH FUNCTION

    function loggingVersion(f, strLog) {
        return function (v) {
            return {
                value: f(v),
                log: strLog
            };
        }
    }

    var log_root = loggingVersion(root, "obtained square root"),

        log_addOne = loggingVersion(addOne, "added 1"),

        log_half = loggingVersion(half, "divided by 2");


    // UNIT/RETURN and BIND for the the WRITER MONAD

    // The Unit / Return function for the Writer monad:
    // 'Lifts' a raw value into the wrapped form
    // a -> Writer a
    function writerUnit(a) {
        return {
            value: a,
            log: "Initial value: " + JSON.stringify(a)
        };
    }

    // The Bind function for the Writer monad:
    // applies a logging version of a function
    // to the contents of a wrapped value
    // and return a wrapped result (with extended log)

    // Writer a -> (a -> Writer b) -> Writer b
    function writerBind(w, f) {
        var writerB = f(w.value),
            v = writerB.value;

        return {
            value: v,
            log: w.log + '\n' + writerB.log + ' -> ' + JSON.stringify(v)
        };
    }

    // USING UNIT AND BIND TO COMPOSE LOGGING FUNCTIONS

    // We can compose a chain of Writer functions (of any length) with a simple foldr/reduceRight
    // which starts by 'lifting' the initial value into a Writer wrapping,
    // and then nests function applications (working from right to left)
    function logCompose(lstFunctions, value) {
        return lstFunctions.reduceRight(
            writerBind,
            writerUnit(value)
        );
    }

    var half_of_addOne_of_root = function (v) {
        return logCompose(
            [log_half, log_addOne, log_root], v
        );
    };

    return half_of_addOne_of_root(5);
})();</lang>

{{Out}}

<pre>{
    "value":1.618033988749895,
    "log":"Initial value: 5\n
           obtained square root -> 2.23606797749979\n
           added 1 -> 3.23606797749979\n
           divided by 2 -> 1.618033988749895"
}</pre>

=={{header|Jsish}}==
From Javascript ES5 entry.

<lang javascript>'use strict';

/* writer monad, in Jsish */
function writerMonad() {
 
    // START WITH THREE SIMPLE FUNCTIONS
 
    // Square root of a number more than 0
    function root(x) {
        return Math.sqrt(x);
    }
 
    // Add 1
    function addOne(x) {
        return x + 1;
    }
 
    // Divide by 2
    function half(x) {
        return x / 2;
    }
 
 
    // DERIVE LOGGING VERSIONS OF EACH FUNCTION
 
    function loggingVersion(f, strLog) {
        return function (v) {
            return {
                value: f(v),
                log: strLog
            };
        };
    }
 
    var log_root = loggingVersion(root, "obtained square root"),
 
        log_addOne = loggingVersion(addOne, "added 1"),
 
        log_half = loggingVersion(half, "divided by 2");
 
 
    // UNIT/RETURN and BIND for the the WRITER MONAD
 
    // The Unit / Return function for the Writer monad:
    // 'Lifts' a raw value into the wrapped form
    // a -> Writer a
    function writerUnit(a) {
        return {
            value: a,
            log: "Initial value: " + JSON.stringify(a)
        };
    }
 
    // The Bind function for the Writer monad:
    // applies a logging version of a function
    // to the contents of a wrapped value
    // and return a wrapped result (with extended log)
 
    // Writer a -> (a -> Writer b) -> Writer b
    function writerBind(w, f) {
        var writerB = f(w.value),
            v = writerB.value;
 
        return {
            value: v,
            log: w.log + '\n' + writerB.log + ' -> ' + JSON.stringify(v)
        };
    }
 
    // USING UNIT AND BIND TO COMPOSE LOGGING FUNCTIONS
 
    // We can compose a chain of Writer functions (of any length) with a simple foldr/reduceRight
    // which starts by 'lifting' the initial value into a Writer wrapping,
    // and then nests function applications (working from right to left)
    function logCompose(lstFunctions, value) {
        return lstFunctions.reduceRight(
            writerBind,
            writerUnit(value)
        );
    }

    var half_of_addOne_of_root = function (v) {
        return logCompose(
            [log_half, log_addOne, log_root], v
        );
    };

    return half_of_addOne_of_root(5);
}

var writer = writerMonad();
;writer.value;
;writer.log;

/*
=!EXPECTSTART!=
writer.value ==> 1.61803398874989
writer.log ==> Initial value: 5
obtained square root -> 2.23606797749979
added 1 -> 3.23606797749979
divided by 2 -> 1.61803398874989
=!EXPECTEND!=
*/</lang>

{{out}}
<pre>prompt$ jsish -u writerMonad.jsi
[PASS] writerMonad.jsi</pre>

=={{header|Julia}}==
<lang julia>struct Writer x::Real; msg::String; end

Base.show(io::IO, w::Writer) = print(io, w.msg, ": ", w.x)

unit(x, logmsg) = Writer(x, logmsg)

bind(f, fmsg, w) = unit(f(w.x), w.msg * ", " * fmsg)

f1(x) = 7x
f2(x) = x + 8

a = unit(3, "after intialization")
b = bind(f1, "after times 7 ", a)
c = bind(f2, "after plus 8", b)

println("$a => $b => $c")
println(bind(f2, "after plus 8", bind(f1, "after times 7", unit(3, "after intialization"))))
</lang>{{out}}
<pre>
after intialization: 3 => after intialization, after times 7: 21 => after intialization, after times 7, after plus 8: 29
after intialization, after times 7, after plus 8: 29
</pre>

=={{header|Kotlin}}==
<lang scala>// version 1.2.10

import kotlin.math.sqrt

class Writer<T : Any> private constructor(val value: T, s: String) {
    var log = "  ${s.padEnd(17)}: $value\n"
        private set

    fun bind(f: (T) -> Writer<T>): Writer<T> {
        val new = f(this.value)
        new.log = this.log + new.log
        return new
    }

    companion object {
        fun <T : Any> unit(t: T, s: String) = Writer<T>(t, s)
    }
}

fun root(d: Double) = Writer.unit(sqrt(d), "Took square root")

fun addOne(d: Double) = Writer.unit(d + 1.0, "Added one")

fun half(d: Double) = Writer.unit(d / 2.0, "Divided by two")

fun main(args: Array<String>) {
    val iv = Writer.unit(5.0, "Initial value")
    val fv = iv.bind(::root).bind(::addOne).bind(::half)
    println("The Golden Ratio is ${fv.value}")
    println("\nThis was derived as follows:-\n${fv.log}")
}</lang>

{{out}}
<pre>
The Golden Ratio is 1.618033988749895

This was derived as follows:-
  Initial value    : 5.0
  Took square root : 2.23606797749979
  Added one        : 3.23606797749979
  Divided by two   : 1.618033988749895
</pre>

=={{header|Nim}}==
<lang Nim>from math import sqrt
from sugar import `=>`, `->`

type
  WriterUnit = (float, string)
  WriterBind = proc(a: WriterUnit): WriterUnit

proc bindWith(f: (x: float) -> float; log: string): WriterBind =
  result = (a: WriterUnit) => (f(a[0]), a[1] & log)

func doneWith(x: int): WriterUnit =
  (x.float, "")

var
  logRoot = sqrt.bindWith "obtained square root, "
  logAddOne = ((x: float) => x+1'f).bindWith "added 1, "
  logHalf = ((x: float) => x/2'f).bindWith "divided by 2, "

echo 5.doneWith.logRoot.logAddOne.logHalf
</lang>
{{out}}
<pre>
(1.618033988749895, "obtained square root, added 1, divided by 2, ")
</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
</pre>

=={{header|Phix}}==
<lang Phix>function bind(object m, integer f)
    return f(m)
end function
 
function unit(object m)
    return m
end function
 
function root(sequence al)
    {atom a, string lg} = al
    atom res = sqrt(a)
    return {res,lg&sprintf("took root: %f -> %f\n",{a,res})}
end function
 
function addOne(sequence al)
    {atom a, string lg} = al
    atom res = a + 1
    return {res,lg&sprintf("added one: %f -> %f\n",{a,res})}
end function

function half(sequence al)
    {atom a, string lg} = al
    atom res = a / 2
    return {res,lg&sprintf("halved it: %f -> %f\n",{a,res})}
end function
 
printf(1,"%f obtained by\n%s", bind(bind(bind({5,""},root),addOne),half))</lang>
{{out}}
<pre>
1.618034 obtained by
took root: 5.000000 -> 2.236068
added one: 2.236068 -> 3.236068
halved it: 3.236068 -> 1.618034
</pre>

=={{header|PHP}}==
<lang php>class WriterMonad {

	/** @var mixed */
	private $value;
	/** @var string[] */
	private $logs;

	private function __construct($value, array $logs = []) {
		$this->value = $value;
		$this->logs = $logs;
	}

	public static function unit($value, string $log): WriterMonad {
		return new WriterMonad($value, ["{$log}: {$value}"]);
	}

	public function bind(callable $mapper): WriterMonad  {
		$mapped = $mapper($this->value);
		assert($mapped instanceof WriterMonad);
		return new WriterMonad($mapped->value, [...$this->logs, ...$mapped->logs]);
	}

	public function value() {
		return $this->value;
	}

	public function logs(): array {
		return $this->logs;
	}
}

$root = fn(float $i): float => sqrt($i);
$addOne = fn(float $i): float => $i + 1;
$half = fn(float $i): float => $i / 2;

$m = fn (callable $callback, string $log): callable => fn ($value): WriterMonad => WriterMonad::unit($callback($value), $log);

$result = WriterMonad::unit(5, "Initial value")
	->bind($m($root, "square root"))
	->bind($m($addOne, "add one"))
	->bind($m($half, "half"));

print "The Golden Ratio is: {$result->value()}\n";
print join("\n", $result->logs());</lang>

{{out}}
<pre>
The Golden Ratio is: 1.6180339887499
Initial value: 5
square root: 2.2360679774998
add one: 3.2360679774998
half: 1.6180339887499
</pre>

=={{header|Python}}==

<lang python>"""A Writer Monad. Requires Python >= 3.7 for type hints."""
from __future__ import annotations

import functools
import math
import os

from typing import Any
from typing import Callable
from typing import Generic
from typing import List
from typing import TypeVar
from typing import Union


T = TypeVar("T")


class Writer(Generic[T]):
    def __init__(self, value: Union[T, Writer[T]], *msgs: str):
        if isinstance(value, Writer):
            self.value: T = value.value
            self.msgs: List[str] = value.msgs + list(msgs)
        else:
            self.value = value
            self.msgs = list(f"{msg}: {self.value}" for msg in msgs)

    def bind(self, func: Callable[[T], Writer[Any]]) -> Writer[Any]:
        writer = func(self.value)
        return Writer(writer, *self.msgs)

    def __rshift__(self, func: Callable[[T], Writer[Any]]) -> Writer[Any]:
        return self.bind(func)

    def __str__(self):
        return f"{self.value}\n{os.linesep.join(reversed(self.msgs))}"

    def __repr__(self):
        return f"Writer({self.value}, \"{', '.join(reversed(self.msgs))}\")"


def lift(func: Callable, msg: str) -> Callable[[Any], Writer[Any]]:
    """Return a writer monad version of the simple function `func`."""

    @functools.wraps(func)
    def wrapped(value):
        return Writer(func(value), msg)

    return wrapped


if __name__ == "__main__":
    square_root = lift(math.sqrt, "square root")
    add_one = lift(lambda x: x + 1, "add one")
    half = lift(lambda x: x / 2, "div two")

    print(Writer(5, "initial") >> square_root >> add_one >> half)
</lang>

{{out}}
<pre>
1.618033988749895
initial: 5
square root: 2.23606797749979
add one: 3.23606797749979
div two: 1.618033988749895
</pre>

=={{header|Raku}}==
Basic semantic borrowed from the Monads/List monad entry
{{trans|Go}}
<lang perl6># 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;</lang>
{{out}}
<pre>Initial value    : 5.000000000000
Took square root : 2.236067977500
Added one        : 3.236067977500
Divided by two   : 1.618033988750
</pre>

=={{header|Wren}}==
{{trans|Go}}
{{libheader|Wren-fmt}}
<lang ecmascript>import "/fmt" for Fmt

class Mwriter {
    construct new(value, log) {
        _value = value
        _log = log
    }

    value { _value }
    log {_log}
    log=(value) { _log = value }

    bind(f) {
        var n = f.call(_value)
        n.log = _log + n.log
        return n
    }

    static unit(v, s) { Mwriter.new(v, "  %(Fmt.s(-17, s)): %(v)\n") }
}

var root   = Fn.new { |v| Mwriter.unit(v.sqrt, "Took square root") }
var addOne = Fn.new { |v| Mwriter.unit(v + 1,  "Added one") }
var half   = Fn.new { |v| Mwriter.unit( v / 2, "Divided by two") }

var mw1 = Mwriter.unit(5, "Initial value")
var mw2 = mw1.bind(root).bind(addOne).bind(half)
System.print("The Golden Ratio is %(mw2.value)")
System.print("\nThis was derived as follows:-")
System.print(mw2.log)</lang>

{{out}}
<pre>
The Golden Ratio is 1.6180339887499

This was derived as follows:-
  Initial value    : 5
  Took square root : 2.2360679774998
  Added one        : 3.2360679774998
  Divided by two   : 1.6180339887499
</pre>

=={{header|zkl}}==
{{trans|EchoLisp}}
<lang zkl>class Writer{
   fcn init(x){ var X=x, logText=Data(Void,"  init \U2192; ",x.toString()) }
   fcn unit(text)  { logText.append(text); self }
   fcn lift(f,name){ unit("\n  %s \U2192; %s".fmt(name,X=f(X))) }
   fcn bind(f,name){ lift.fp(f,name) }
   fcn toString{ "Result = %s\n%s".fmt(X,logText.text) }

   fcn root{ lift(fcn(x){ x.sqrt() },"root") }
   fcn half{ lift('/(2),"half") }
   fcn inc { lift('+(1),"inc") }
}</lang>
<lang zkl>Writer(5.0).root().inc().half().println();</lang>
{{out}}
<pre>
Result = 1.61803
  init → 5
  root → 2.23607
  inc → 3.23607
  half → 1.61803
</pre>
<lang zkl>w:=Writer(5.0);
Utils.Helpers.fcomp(w.half,w.inc,w.root)(w).println();  // half(inc(root(w)))</lang>
{{out}}
<pre>
Result = 1.61803
  init → 5
  root → 2.23607
  inc → 3.23607
  half → 1.61803
</pre>
Use bind to add functions to an existing Writer:
<lang zkl>w:=Writer(5.0); 
root,inc,half := w.bind(fcn(x){ x.sqrt() },"root"), w.bind('+(1),"+ 1"), w.bind('/(2),"/ 2");
root(); inc(); half(); w.println();</lang>
{{out}}
<pre>
Result = 1.61803
  init → 5
  root → 2.23607
  + 1 → 3.23607
  / 2 → 1.61803
</pre>