Functional Programming for JavaScript People examples in Elm

examples.elm

{- js
const add1 = (a) => a + 1
const times2 = (a) => a * 2
const compose = (a, b) => (c) => a(b(c))
const add1OfTimes2 = compose(add1, times2)
add1OfTimes2(5) // => 11
-}
add1 = (+) 1
times2 = (*) 2
add1OfTimes2 = add1 << times2

-- shorter ( Haskell / Elm is left-assocative [ a b c == ((a b) c) ])
add1OfTimes2 = (+) 1 << (*) 2

{- js
const pipe = (fns) => (x) => fns.reduce((v, f) => f(v), x)
const times2add1 = pipe([times2, add1])
times2add1(5) // => 11
-}

-- pipe example
times2add1 x =
  x
  |> (*) 2
  |> (+) 1
  
{- js
const formalGreeting = (name) => `Hello ${name}`
const casualGreeting = (name) => `Sup ${name}`
const male = (name) => `Mr. ${name}`
const female = (name) => `Mrs. ${name}`
const doctor = (name) => `Dr. ${name}`
const phd = (name) => `${name} PhD`
const md = (name) => `${name} M.D.`
formalGreeting(male(phd("Chet"))) // => "Hello Mr. Chet PhD"


const identity = (x) => x
const greet = (name, options) => {
  return pipe([
    // greeting    
    options.formal ? formalGreeting :
    casualGreeting,
    // prefix
    options.doctor ? doctor :
    options.male ? male :
    options.female ? female :
    identity,
    // suffix
    options.phd ? phd :
    options.md ?md :
    identity
  ])(name)
}
-}

type Greeting = Formal | Casual
type Title = Doctor | Male | Female
type Degree = Md | Phd | NoDegree

prefix a b = a ++ " " ++ b 
suffix a b = b ++ " " ++ a  

addGreeting greeting name =
    case greeting of
        Formal ->
            prefix "Hello" name

        Casual ->
            prefix "Sup" name            

addTitle title name =
    case title of
        Doctor ->
            prefix "Dr." name

        Male ->
            prefix "Mr." name

        Female ->
            prefix "Mrs." name

addDegree degree name =
    case degree of
        Md -> 
            suffix "M.D." name

        Phd ->
            suffix "PhD" name

        NoDegree ->
            name

greet greeting title degree name =
    name
    |> addTitle title
    |> addGreeting greeting
    |> addDegree degree