I'm working on modernizing Rosetta Code's infrastructure. Starting with communications. Please accept this time-limited open invite to RC's Slack.. --Michael Mol (talk) 20:59, 30 May 2020 (UTC)

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

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:

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;    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`
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 stringend run  -- THREE SIMPLE FUNCTIONSon root(x)    x ^ (1 / 2)end root on succ(x)    x + 1end succ on half(x)    x / 2end 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 scriptend writerVersion  -- DEFINE A COMPOSITION OF THE SAFE VERSIONSon loggingHalfOfRootPlusOne(x)    logCompose([my wHalf, my wSucc, my wRoot], x)end loggingHalfOfRootPlusOne  -- Monadic UNIT/RETURN and BIND functions for the writer monadon 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 FUNCTIONon 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 ] optionalon 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 repeatend reduceRight -- Unit/Return and bind for composing handlers in script wrappers-- lift 2nd class function into 1st class wrapper -- handler function --> first class script objecton sReturn(f)    script        property call : f    end scriptend sReturn -- return a new script in which function g is composed-- with the f (call()) of the Mf script-- Mf -> (f -> Mg) -> Mgon sBind(mf, g)    script        on call(x)            sReturn(g)'s call(mf's call(x))        end call    end scriptend sBind`
Output:
```{
value:1.61803398875,
comment:"Initial value: 5\n
obtained square root -> 2.2360679775\n
divided by two -> 1.61803398875"
}```

## C++

`#include <cmath>#include <iostream>#include <string> using namespace std; // Use a struct as the monadstruct LoggingMonad{    double Value;    string Log;}; // Use the >> operator as the bind functionauto operator>>(const LoggingMonad& monad, auto f){    auto result = f(monad.Value);    return LoggingMonad{result.Value, monad.Log + "\n" + result.Log};} // Define the three simple functionsauto 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 functionsauto MakeWriter = [](auto f, string message){    return [=](double x){return LoggingMonad(f(x), message);};}; // Derive writer versions of the simple functionsauto 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;} `
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))) ;; 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 → 5root → 2.23606797749979add-one → 3.23606797749979half → 1.618033988749895     ;; binding(->>  0 Writer.unit (Writer.bind sin) (Writer.bind cos)  w-inc w-half Writer.print) result 1    init → 0sin → 0cos → 1add-one → 2half → 1     `

## 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 ;FROM: monads => do ; {    [ 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 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)}`
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.Writerimport Control.Monad ((>=>)) loggingVersion :: (a -> b) -> c -> a -> Writer c bloggingVersion 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)`
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'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)`

`   0{::Lhalf`Lincr`Lroot loggingCompose 51.61803   1{::Lhalf`Lincr`Lroot loggingCompose 5Initial value: 5obtained square root -> 2.23607added 1 -> 3.23607divided by 2 -> 1.61803`

## JavaScript

### ES5

`(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);})();`
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 */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 ==> 1.61803398874989writer.log ==> Initial value: 5obtained square root -> 2.23606797749979added 1 -> 3.23606797749979divided 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) = 7xf2(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 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}")}`
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 sqrtfrom 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), a & 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 `
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, \$log.@\$n );} sub Unit { Writer->new(\$_, sprintf("%-17s: %.12f\n",\$_,\$_)) } sub root { Unit sqrt(\$_), "Took square root" } sub addOne { Unit \$_+1, "Added one" } sub half { Unit \$_/2, "Divided by two" } print Unit(5, "Initial value")->Bind(\&root)->Bind(\&addOne)->Bind(\&half)->; `
Output:
```Initial value    : 5.000000000000
Took square root : 2.236067977500
Divided by two   : 1.618033988750
```

## Phix

`function bind(object m, integer f)    return f(m)end function function unit(object m)    return mend 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))`
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 {		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());`
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 functoolsimport mathimport os from typing import Anyfrom typing import Callablefrom typing import Genericfrom typing import Listfrom typing import TypeVarfrom 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) `
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 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
Divided by two   : 1.618033988750
```

## 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")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)`
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
```