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.

You are encouraged to solve this task according to the task description, using any language you may know.

Demonstrate in your programming language the following:

1. 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)
2. Write three simple functions: root, addOne, and half
3. Derive Writer monad versions of each of these functions
4. 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.)

## ALGOL 68

Translation of: Go
```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;
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```
Output:
```The Golden Ratio is  1.618033988749895

This was derived as follows:-
Initial value    :  5.000000000000000
Took square root :  2.236067977499790
Divided by two   :  1.618033988749895
```

## AppleScript

Translation of: 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.

```-- 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

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
```
Output:
```{
value:1.61803398875,
comment:"Initial value: 5\n
obtained square root -> 2.2360679775\n
divided by two -> 1.61803398875"
}```

## ATS

The entry for Haskell inspired me to do such things as add a `>=>` operator, and to use `return` as a name for the unit operation. But notice that I write `return<double>`. The template system, for whatever reason, could not infer the type, if I left out the template parameter. It did not signal an error, but instead produced C code that could not be compiled. This kind of behavior is common with the Postiats implementation of ATS, and must be gotten used to.

(Footnote: Sometimes the template system will produce C code that can be compiled but does not function correctly! No doubt the template system can be made more shipshape, but it is what it is. In any case, one then goes through the code and fills in elided template parameters, until the code works.)

```#include "share/atspre_staload.hats"

%{^
#include <math.h>
%}

#define NIL list_nil ()
#define ::  list_cons

(* The log is a list of strings. For efficiency, it is ordered
most-recent-first. The static value "n" represents the number of
entries in the log. (It exists and is used only during the
typechecking phase.) *)
datatype Writer (a : t@ype+, n : int) =
| Writer of (a, list (string, n))
typedef Writer (a : t@ype+) = [n : int] Writer (a, n)

prfn
lemma_Writer_param {a : t@ype}
{n : int}
(m : Writer (a, n))
:<prf> [0 <= n] void =
let
val+ Writer (_, log) = m
in
lemma_list_param log
end

fn {a : t@ype}
unit_Writer (x : a) : Writer (a, 1) =
let
val msg = string_append ("unit_Writer (",
tostring_val<a> x, ")")
val msg = strptr2string msg
in
Writer (x, msg :: NIL)
end

fn {a, b : t@ype}
bind_Writer {n : int}
(m : Writer (a, n),
f : a -<cloref1> Writer b)
: [n1 : int | n <= n1] Writer (b, n1) =
let
val+ Writer (x, log) = m
val y = f (x)
prval () = lemma_Writer_param y
val+ Writer (y, entries) = y
in
Writer (y, list_append (entries, log))
end

infixl 0 >>=

fn {a, b, c : t@ype}
compose_Writer (f : a -<cloref1> Writer b,
g : b -<cloref1> Writer c)
: a -<cloref1> Writer c =
lam m => f m >>= g

infixl 0 >=>

(* "make_Writer_closure_from_fun" wraps an ordinary function from a to
b, resulting in a closure that will produce exactly one log
entry. *)
fn {a, b : t@ype}
make_Writer_closure_from_fun (func     : a -> b,
make_msg : (a, b) -<cloref1> string)
: a -<cloref1> Writer (b, 1) =
lam x =>
let
val y = func x
in
Writer (y, make_msg (x, y) :: NIL)
end

(* A note regarding "root": interfaces to the C math library are
available, even within the Postiats distribution, but I shall
simply make a foreign function call to sqrt(3). The Postiats
prelude itself provides no (or very little) interface to libm. *)
fn root (x : double) : double = \$extfcall (double, "sqrt", x)
fn addOne (x : double) : double = succ x
fn half (x : double) : double = 0.5 * x

fn {a, b : t@ype}
make_logging (func     : a -> b,
notation : string)
: a -<cloref1> Writer (b, 1) =
let
fn
make_msg (x : a, y : b) :<cloref1> string =
let
val msg = string_append ("(", tostring_val<a> x,
" |> ", notation, ") --> ",
tostring_val<b> y)
in
strptr2string msg
end
in
make_Writer_closure<a,b> (func, make_msg)
end

val logging_root = make_logging<double,double> (root, "sqrt")
val logging_half = make_logging<double,double> (half, "(0.5 *)")

val the_big_whatchamacallit =

fn
print_log (log : List string) : void =
let
fun
loop (lst : List0 string) : void =
case+ lst of
| NIL => ()
| hd :: tl =>
begin
println! ("  ", hd);
loop tl
end

prval () = lemma_list_param log
in
loop (list_vt2t (list_reverse log))
end

implement
main0 () =
let
val x = 5.0
val m = return<double> x
val+ Writer (y, log) = m >>= the_big_whatchamacallit
in
println! ("(1 + sqrt(", x : double, "))/2 = ", y : double);
println! ("log:");
print_log log
end```
Output:
```\$ patscc -std=gnu2x -g -O2 -DATS_MEMALLOC_GCBDW writer_monad_ats.dats -lgc -lm && ./a.out
(1 + sqrt(5.000000))/2 = 1.618034
log:
unit_Writer (5.000000)
(5.000000 |> sqrt) --> 2.236068
(2.236068 |> (+ 1.0)) --> 3.236068
(3.236068 |> (0.5 *)) --> 1.618034```

## C++

```#include <cmath>
#include <iostream>
#include <string>

using namespace std;

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

// Use the >> operator as the bind function
{
}

// 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)
{
};

// Derive writer versions of the simple functions
auto writerRoot = MakeWriter(Root, "Taking square root");
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;
}
```
Output:
```Starting with 5
Taking square root
Dividing by 2
Result: 1.61803
```

## EchoLisp

Our monadic Writer elements will be pairs (string . value), where string is the log string.

```(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)))

(define w-root  (Writer.lift sqrt "root"))
(define w-half  (Writer.lift (lambda(x) (// x 2)) "half"))

;; 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
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
half → 1
```

## FreeBASIC

Translation of: Go
```Type mwriter
value As Double
log_ As String
End Type

Function Unit(v As Double, s As String) As mwriter
Dim As mwriter mw
mw.value = v
mw.log_ = "  " & s & ": " & Str(v) & Chr(10)
Return mw
End Function

Function Root(mw As mwriter) As mwriter
mw.value = Sqr(mw.value)
mw.log_ = mw.log_ & "  Took square Root: " & Str(mw.value) & Chr(10)
Return mw
End Function

Function addOne(mw As mwriter) As mwriter
mw.value = mw.value + 1
mw.log_ = mw.log_ & "  Added one       : " & Str(mw.value) & Chr(10)
Return mw
End Function

Function Half(mw As mwriter) As mwriter
mw.value = mw.value / 2
mw.log_ = mw.log_ & "  Divided by two  : " & Str(mw.value) & Chr(10)
Return mw
End Function

Dim As mwriter mw1
mw1 = Unit(5, "Initial value   ")
mw1 = Root(mw1)
mw1 = Half(mw1)
Print "The Golden Ratio is "; mw1.value
Print !"\nThis was derived as follows:-"
Print mw1.log_

Sleep
```
Output:
```The Golden Ratio is  1.618033988749895

This was derived as follows:-
Initial value   : 5
Took square Root: 2.23606797749979
Divided by two  : 1.618033988749895```

## F#

```// Monads/Writer monad . Nigel Galloway: July 20th., 2022
type Riter<'n>=Riter of 'n * List<string>
let eval=function |Riter(n,g)->(n,g)
let compose f=function |Riter(n,g)->let n,l=eval(f n) in Riter(n,List.append g l)
let initV n=Riter(n,[sprintf "Initial Value %f" n])
let sqrt n=Riter(sqrt n,["Took square root"])
let div n g=Riter(n/g,[sprintf "Divided by %f" n])
log|>List.iter(printfn "%s")
printfn "Final value = %f" result
```
Output:
```Initial Value 5.000000
Took square root
Divided by 2.000000
Final value = 0.618034
```

## Factor

Factor comes with an implementation of Haskell-style monads in the `monads` vocabulary.

Works with: Factor version 0.99 2019-10-06
```USING: kernel math math.functions monads prettyprint ;

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

## Go

Translation of: Kotlin
```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 half(v float64) mwriter {
return unit(v/2, "Divided by two")
}

func main() {
mw1 := unit(5, "Initial value")
fmt.Println("The Golden Ratio is", mw2.value)
fmt.Println("\nThis was derived as follows:-")
fmt.Println(mw2.log)
}
```
Output:
```The Golden Ratio is 1.618033988749895

This was derived as follows:-
Initial value    : 5
Took square root : 2.23606797749979
Divided by two   : 1.618033988749895
```

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

```import Control.Monad.Trans.Writer

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

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

main = print \$ runWriter (halfOfAddOneOfRoot 5)
```
Output:
```(1.618033988749895,"obtained square root, added 1, divided by 2, ")
```

## J

Based on javascript implementation:

```root=: %:
incr=: >:
half=: -:

tostr=: ,@":

loggingVersion=: conjunction define
n;~u
)

Lroot=: root loggingVersion 'obtained square root'
Lhalf=: half loggingVersion 'divided by 2'

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

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

)
```

```   0{::Lhalf`Lincr`Lroot loggingCompose 5
1.61803
1{::Lhalf`Lincr`Lroot loggingCompose 5
Initial value: 5
obtained square root -> 2.23607
divided by 2 -> 1.61803
```

## Java

```import java.util.function.Function;

public static void main(String[] aArgs) {
System.out.println("The Golden Ratio is " + result.getValue() + System.lineSeparator());
System.out.println("This was derived as follows:"  + System.lineSeparator() + result.getText());
}

}

}

}

}

public static <T> Monad<T> unit(T aValue, String aText) {
}

}

public T getValue() {
return value;
}

public String getText() {
return text;
}

private Monad(T aValue, String aText) {
value = aValue;
text = String.format("%-21s%s%n", "    " + aText, ": " + aValue);
}

private T value;
private String text;

}
```
Output:
```The Golden Ratio is 1.618033988749895

This was derived as follows:
Initial value    : 5.0
Took square root : 2.23606797749979
Divided by two   : 1.618033988749895
```

## JavaScript

### ES5

```(function () {
'use strict';

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

})();
```
Output:
```{
"value":1.618033988749895,
"log":"Initial value: 5\n
obtained square root -> 2.23606797749979\n
divided by 2 -> 1.618033988749895"
}```

## Jsish

From Javascript ES5 entry.

```'use strict';

/* writer monad, in Jsish */

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

}

;writer.value;
;writer.log;

/*
=!EXPECTSTART!=
writer.value ==> 1.61803398874989
writer.log ==> Initial value: 5
obtained square root -> 2.23606797749979
divided by 2 -> 1.61803398874989
=!EXPECTEND!=
*/
```
Output:
```prompt\$ jsish -u writerMonad.jsi

## 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"))))
```
Output:
```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
```

## Kotlin

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

fun main(args: Array<String>) {
val iv = Writer.unit(5.0, "Initial value")
println("The Golden Ratio is \${fv.value}")
println("\nThis was derived as follows:-\n\${fv.log}")
}
```
Output:
```The Golden Ratio is 1.618033988749895

This was derived as follows:-
Initial value    : 5.0
Took square root : 2.23606797749979
Divided by two   : 1.618033988749895
```

## 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, "
logHalf = ((x: float) => x/2'f).bindWith "divided by 2, "

```
Output:
```(1.618033988749895, "obtained square root, added 1, divided by 2, ")
```

## Perl

Translation of: Raku
```# 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 half { Unit \$_[0]/2, "Divided by two" }

```
Output:
```Initial value    : 5.000000000000
Took square root : 2.236067977500
Divided by two   : 1.618033988750
```

## Phix

```with javascript_semantics
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

{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

```
Output:
```1.618034 obtained by
took root: 5.000000 -> 2.236068
halved it: 3.236068 -> 1.618034
```

## 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 {
}

public function bind(callable \$mapper): WriterMonad  {
\$mapped = \$mapper(\$this->value);
}

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);

->bind(\$m(\$root, "square root"))
->bind(\$m(\$half, "half"));

print "The Golden Ratio is: {\$result->value()}\n";
print join("\n", \$result->logs());
```
Output:
```The Golden Ratio is: 1.6180339887499
Initial value: 5
square root: 2.2360679774998
half: 1.6180339887499
```

## Python

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

import functools
import math
import os

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

T = TypeVar("T")
U = TypeVar("U")

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[U]]) -> Writer[U]:
writer = func(self.value)
return Writer(writer, *self.msgs)

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

@functools.wraps(func)
def wrapped(value: T) -> Writer[U]:
return Writer(func(value), msg)

return wrapped

if __name__ == "__main__":
square_root = lift(math.sqrt, "square root")

add_one: Callable[[Union[int, float]], Writer[Union[int, float]]] = lift(
lambda x: x + 1, "add one"
)

half: Callable[[Union[int, float]], Writer[float]] = lift(
lambda x: x / 2, "div two"
)

print(Writer(5, "initial") >> square_root >> add_one >> half)
```
Output:
```1.618033988749895
initial: 5
square root: 2.23606797749979
div two: 1.618033988749895
```

## Raku

Translation of: Go
```# 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 half(\v) { Unit v/2, "Divided by two" }

```
Output:
```Initial value    : 5.000000000000
Took square root : 2.236067977500
Divided by two   : 1.618033988750
```

## Ruby

```# 20220720 Ruby programming solution
class Writer

def initialize(value, log = "New")
@value = value
if value.is_a? Proc
@log = log
else
@log = log + ": " + @value.to_s
end
end

def self.unit(value, log)
Writer.new(value, log)
end

def bind(mwriter)
new_value = mwriter.value.call(@value)
new_log = @log + "\n" + mwriter.log
self.class.new(new_value, new_log)
end
end

lam_sqrt = ->(number) { Math.sqrt(number) }
lam_add_one = ->(number) { number + 1 }
lam_half = ->(number) { number / 2.0 }

sqrt = Writer.unit( lam_sqrt, "Took square root")
half = Writer.unit( lam_half, "Divided by 2")

m1 = Writer.unit(5, "Initial value")

puts "The final value is #{m2.value}\n\n"
puts "This value was derived as follows:"
puts m2.log
```
Output:
```The final value is 1.618033988749895

This value was derived as follows:
Initial value: 5
Took square root: 2.23606797749979
Divided by 2: 1.618033988749895
```

## Scheme

Works with: Gauche Scheme version 0.9.12
Works with: CHICKEN Scheme version 5.3.0

The program is written in R7RS-small Scheme. For CHICKEN you will need the `r7rs` egg.

```(define-library (monad base)
>>= >=>)
(import (scheme base)
(scheme case-lambda))
(begin

(define >>=
(case-lambda
((m f) ((monad-bind m) m f))
((m f . g*) (apply >>= (cons (>>= m f) g*)))))

(define >=>
(case-lambda
((f g) (lambda (x) (>>= (f x) g)))
((f g . h*) (apply >=> (cons (>=> f g) h*)))))

)) ;; end library

(export perform)
(import (scheme base)
(begin

(define-syntax perform
;; "do" is already one of the loop syntaxes, so I call this
(syntax-rules (<-)
((perform (x <- action) clause clause* ...)
(>>= action (lambda (x) (perform clause clause* ...))))
((perform action)
action)
((perform action clause clause* ...)
(action (perform clause clause* ...)))))

)) ;; end library

(import (scheme base)
(begin

;; The messages are a list, most recent message first, of whatever
;; data f decides to log.
(define (bind m f)
(append new-messages old-messages)))))
(unless (or (null? messages) (pair? messages))
;;
;; I do not actually test whether the list is proper, because
;; to do so would be inefficient.
;;
;; The R7RS-small test for properness of a list is called
;; "list?" (and the report says something tendentious in
;; defense of this name, but really it is simply historical
;; usage). The SRFI-1 procedure, by constrast, is called
;; "proper-list?".
;;
(error "should be a proper list" messages))

)) ;; end library

(import (scheme base)
(scheme inexact)
(scheme write)

(define root sqrt)
(define (addOne x) (+ x 1))
(define (half x) (/ x 2))

(define-syntax make-logging
(syntax-rules ()
((_ proc)
(lambda (x)
(define (make-msg x y) (list x 'proc y))
(let ((y (proc x)))
(make-writer-monad y (list (make-msg x y))))))))

(define logging-root (make-logging root))
(define logging-half (make-logging half))

(define (display-messages messages)
(begin
(display "  messages:")
(newline)
(let loop ((lst (reverse messages)))
(when (pair? lst)
(display "    ")
(write (car lst))
(newline)
(loop (cdr lst)))))))

(display "---------------") (newline)
(display "Using just >>=") (newline)
(display "---------------") (newline)
(define result
(display "  (1 + sqrt(5))/2 = ")
(display-messages result)

(newline)

(display "------------------") (newline)
(display "Using >>= and >=>") (newline)
(display "------------------") (newline)
(define result
(display "  (1 + sqrt(5))/2 = ")
(display-messages result)

(newline)

(display "-----------------------") (newline)
(display "Using 'perform' syntax") (newline)
(display "-----------------------") (newline)
(define result
(x <- (logging-root x))
(logging-half x)))
(display "  (1 + sqrt(5))/2 = ")
(display-messages result)
```
Output:

Compile and run with

`gosh -r7 writer_monad_r7rs.scm`

or

`csc -O5 -X r7rs -R r7rs writer_monad_r7rs.scm && ./writer_monad_r7rs`

(I use the high optimization level `-O5` to check I have done nothing to impede such optimization.)

The result is computed in three different notations. The `perform` syntax is something that looks like Haskell's `do` syntax. (The name `do` is already used as the Scheme and Common Lisp name for a kind of for-loop.)

Notice that the `>>=` and `>->` are ordinary "prefix" procedures, rather than infix operators. One might think this would make them very difficult to write with, but a Scheme procedure can be made to recursively perform a chain of operations, so that you will need to write the procedure name only once. I have made `>>=` and `>->` work that way.

```---------------
Using just >>=
---------------
(1 + sqrt(5))/2 = 1.61803398874989
messages:
(5 root 2.23606797749979)
(3.23606797749979 half 1.61803398874989)

------------------
Using >>= and >=>
------------------
(1 + sqrt(5))/2 = 1.61803398874989
messages:
(5 root 2.23606797749979)
(3.23606797749979 half 1.61803398874989)

-----------------------
Using 'perform' syntax
-----------------------
(1 + sqrt(5))/2 = 1.61803398874989
messages:
(5 root 2.23606797749979)
(3.23606797749979 half 1.61803398874989)
```

## Wren

Translation of: Go
Library: Wren-fmt
```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")
System.print("The Golden Ratio is %(mw2.value)")
System.print("\nThis was derived as follows:-")
System.print(mw2.log)
```
Output:
```The Golden Ratio is 1.6180339887499

This was derived as follows:-
Initial value    : 5
Took square root : 2.2360679774998
Divided by two   : 1.6180339887499
```

## zkl

Translation of: EchoLisp
```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") }
}```
`Writer(5.0).root().inc().half().println();`
Output:
```Result = 1.61803
init → 5
root → 2.23607
inc → 3.23607
half → 1.61803
```
```w:=Writer(5.0);
Utils.Helpers.fcomp(w.half,w.inc,w.root)(w).println();  // half(inc(root(w)))```
Output:
```Result = 1.61803
init → 5
root → 2.23607
inc → 3.23607
half → 1.61803
```

Use bind to add functions to an existing Writer:

```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();```
Output:
```Result = 1.61803
init → 5
root → 2.23607
+ 1 → 3.23607
/ 2 → 1.61803
```