[Functional Programming 101] runWIth, evalWith, execWith

Recentlly, I am learning crocks.js ADT libaray. In the beginning, it is hard to understand when to use 'runWith', 'evalWith', 'execWith'. Until I went though the course a thrid times... I finally have some feelings for it.

State has 'get, put, modify' methods for use, and you can build them by yourself as well:

// getState :: () -> State s
const getState = () => State(s => Pair(s, s))

//putState :: s -> State s ()
const putState = state => State(() => Pair(Unit(), state));

// modifyState :: (s -> s) -> State s ()
const modifyState = fn => State(s => Pair(Unit(), fn(s)));

As well can see, for those methods, they are all using 'Pair'.

Pair(a, s):

On the left part of Pair, is 'a': stand for variable; On the right part of Pair, 's' is the State.

Now rules for using 'runWith', 'evalWith', 'execWith':

1. If you want to get result as Pair(a, s), you should use 'runWith':

const bubble = {
    bubbles: 40
};
const add = x => y => x+ y;
console.log(
    modify(mapProps({bubbles: add(1)}))
        .runWith(bubble) // Pair( (), { bubbles: 41 } )
)

2. If you only instreaded in the variable 'a', you should use 'evalWith':

const bubble = {
    bubbles: 40
};
const add = x => y => x+ y;
console.log(
    modify(mapProps({bubbles: add(1)}))
        .evalWith(bubble) // () Unit
)

3. If you instested in State, which is on the right part of Pair, you should use 'execWith':

const bubble = {
    bubbles: 40
};
const add = x => y => x+ y;
console.log(
    modify(mapProps({bubbles: add(1)}))
        .execWith(bubble) // { bubbles: 41 }
)