[Functional Programming] Write a simple version of Maybe

Maybe has two types: Just / Nothing. Just() will just return the value that passed in. Nothing returns nothing...

Just/ Nothing are both functors, they should have .map() method:

const Just = x => ({
    map: f => Just(f(x)),
    inspect: () => `Just ${x}`,
})

const Nothing = x => ({
    map: f => Nothing('Nothing'),
    inspect: () => `${x}`,
})

const Maybe = {
    Just,
    Nothing
}

We added 'inspect' method so that in REPL we can log out understandable message. for example, 'Just 4'  instead of '{ map: [Function: map] }'.... or whatever...

Currently, the code below return 'Just 5'

const inc = n => n + ;

const input = Just()
const result = input.map(inc)

But we don't need 'Just' as a result, we want just 5; in order to achieve that, we add 'option()' method:

const Just = x => ({
    map: f => Just(f(x)),
    inspect: () => `Just ${x}`,
    option: (_) => x,
})

const Nothing = x => ({
    map: f => Nothing('Nothing'),
    inspect: () => `${x}`,
    option: defVal => defVal
})

For Just, it return whatever the current value 'x', ignore the default value we passed in; Nothing it will return the defautl vlaue back.

Now, we got:

const input = Just(4)
const result = input.map(inc).option() //

const input = Nothing()
const result = input.map(in) // Nothing
const result = input.map(inc).option() //

Since we don't know it should be Just or Nothing, we can use some predict function as helper:

const safeNum = num => typeof num === 'number' ? Maybe.Just(num): Maybe.Nothing()
const input = safeNum()
const result = input.map(inc).option() //

const input = safeNum('')
const result = input.map(inc).option() //

---------------

const Just = x => ({
    map: f => Just(f(x)),
    inspect: () => `Just ${x}`,
    option: (_) => x,
})

const Nothing = x => ({
    map: f => Nothing('Nothing'),
    inspect: () => `${x}`,
    option: defVal => defVal
})

const safeNum = num => typeof num === 'number' ? Maybe.Just(num): Maybe.Nothing()

const Maybe = {
    Just,
    Nothing
}

const inc = n => n + ;

const input = safeNum()
const result = input.map(inc).option()

console.log(result)