Use a Javascript Array to Construct a Maybe

<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css" />
  </head>

  <body>
    <script src="logger.js"></script>
    <script src="helpers.js"></script>
    <script src="Maybe.js"></script>
    <script src="script.js"></script>
  </body>

</html>

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  globalName:"log",
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  selector:"body",
  fontSize:"28px",
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    nil:"green",
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(function(root) {
  
  // compose : ((b -> c), (a -> b)) -> a -> c
  const compose =
    (f, g) => x => f(g(x))
  
  // isValid : a -> Boolean
  const isValid =
    Number.isFinite
  
  // stringify : a -> String
  const stringify =
    JSON.stringify
 
  root.helpers = {
    compose,
    isValid,
    stringify
  }
})(window)

const { compose, isValid, stringify } = helpers
const { maybe, mapMaybe, optionMaybe } = Maybe

// maybeValid : * -> Maybe Number
const maybeValid =
  maybe(isValid)

// add10 : Number -> Number
const add10 =
  num =>  num + 10

// square : Number -> Number
const square =
  num => Math.pow(num, 2)

// doMath : Number -> Number
const doMath =
  compose(square, add10)

// safeMath : a -> Maybe Number
const safeMath =
  compose(mapMaybe(doMath), maybeValid)

// safeStringMath : a -> Maybe String
const safeStringMath =
  compose(mapMaybe(stringify), safeMath)

// stringMath : a -> String
const stringMath =
  compose(optionMaybe('None'), safeStringMath)

log('doMath with a Number', doMath(0))
log('doMath with a String', doMath('kitty cat'))

log('maybeValid with a Number (Just)', maybeValid(0))
log('maybeValid with a NaN (Nothing)', maybeValid(NaN))

log('safeMath with a Number (Just)', safeMath(0))
log('safeMath with a NaN (Nothing)', safeMath(NaN))

log('safeStringMath with a Number (Just)', safeStringMath(0))
log('safeStringMath with a NaN (Nothing)', safeStringMath(NaN))

log('stringMath with a Number (Just)', stringMath(0))
log('stringMath with a NaN (Nothing)', stringMath(NaN))

(function(root) {
  // maybe : (* -> Boolean) -> * -> Maybe a
  const maybe =
    pred => x => pred(x) ? [ x ] : []
  
  // mapMaybe : (a -> b) -> Maybe a -> Maybe b
  const mapMaybe =
    fn => m => m.map(fn)
  
  // optionMaybe : a -> Maybe a -> a
  const optionMaybe =
    def => m => m.length ? m[0] : def

  root.Maybe = {
    mapMaybe,
    maybe,
    optionMaybe
  }
})(window)