[Functional Programming] liftA2 and converge

Sometimes I am confused with 'liftA2' and 'converge' functions. 

 

Main difference between those is that:

　　liftA2 takes applicative functor as second and third arguements, for example, array

　　converge takes functions as second and third arguements

// liftA2 :: Applicative m => (a -> b -> c) -> m a -> m b -> m c
// converge :: (b -> c -> d) -> (a -> b) -> (a -> c) -> a -> d

 

// liftA2 :: Applicative m => (a -> b -> c) -> m a -> m b -> m c
// converge :: (b -> c -> d) -> (a -> b) -> (a -> c) -> a -> d
const { option, curry, liftA2, converge } = require("crocks");
const { getState } = require("../../helper");

// getColors :: () -> State AppState [String]
const getColors = () => getState("colors").map(option([]));

// getShapes :: () -> State AppState [String]
const getShapes = () => getState("shapes").map(option([]));

// buildCard :: String -> String -> Card
const buildCard = curry((color, shape) => ({
  id: `${color}-${shape}`,
  color,
  shape
}));

// buildCards :: [String] -> [String] -> [Card]
const buildCards = liftA2(buildCard);

// generateCards :: () -> State AppState [ Card ]
const generateCards = converge(liftA2(buildCards), getColors, getShapes);

module.exports = {
  generateCards
};