Monads/Writer monad: Difference between revisions

</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