附录C：无点风格实用工具

本附录提供了书中描述的经典JavaScript函数的无点风格实现。在练习中，这些函数作为全局上下文的一部分似乎可直接调用。需要特别注意的是，这些实现可能在性能与效率方面并非最优方案，它们仅供教学使用。

如需寻找更适用于生产环境的具体实现，可参考以下成熟库：ramda、lodash或folktale。

请注意，这些函数会调用附录A中定义的curry及compose函数。

add

// add :: Number -> Number -> Number
const add = curry((a, b) => a + b);

append

// append :: String -> String -> String
const append = flip(concat);

chain

// chain :: Monad m => (a -> m b) -> m a -> m b
const chain = curry((fn, m) => m.chain(fn));

concat

// concat :: String -> String -> String
const concat = curry((a, b) => a.concat(b));

eq

// eq :: Eq a => a -> a -> Boolean
const eq = curry((a, b) => a === b);

filter

// filter :: (a -> Boolean) -> [a] -> [a]
const filter = curry((fn, xs) => xs.filter(fn));

flip

// flip :: (a -> b -> c) -> b -> a -> c
const flip = curry((fn, a, b) => fn(b, a));

forEach

// forEach :: (a -> ()) -> [a] -> ()
const forEach = curry((fn, xs) => xs.forEach(fn));

head

// head :: [a] -> a
const head = xs => xs[0];

intercalate

// intercalate :: String -> [String] -> String
const intercalate = curry((str, xs) => xs.join(str));

join

// join :: Monad m => m (m a) -> m a
const join = m => m.join();

last

// last :: [a] -> a
const last = xs => xs[xs.length - 1];

map

// map :: Functor f => (a -> b) -> f a -> f b
const map = curry((fn, f) => f.map(fn));

match

// match :: RegExp -> String -> Boolean
const match = curry((re, str) => re.test(str));

prop

// prop :: String -> Object -> a
const prop = curry((p, obj) => obj[p]);

reduce

// reduce :: (b -> a -> b) -> b -> [a] -> b
const reduce = curry((fn, zero, xs) => xs.reduce(fn, zero));

replace

// replace :: RegExp -> String -> String -> String
const replace = curry((re, rpl, str) => str.replace(re, rpl));

reverse

// reverse :: [a] -> [a]
const reverse = x => (Array.isArray(x) ? x.reverse() : x.split('').reverse().join(''));

safeHead

// safeHead :: [a] -> Maybe a
const safeHead = compose(Maybe.of, head);

safeLast

// safeLast :: [a] -> Maybe a
const safeLast = compose(Maybe.of, last);

safeProp

// safeProp :: String -> Object -> Maybe a
const safeProp = curry((p, obj) => compose(Maybe.of, prop(p))(obj));

sequence

// sequence :: (Applicative f, Traversable t) => (a -> f a) -> t (f a) -> f (t a)
const sequence = curry((of, f) => f.sequence(of));

sortBy

// sortBy :: Ord b => (a -> b) -> [a] -> [a]
const sortBy = curry((fn, xs) => xs.sort((a, b) => {
  if (fn(a) === fn(b)) {
    return 0;
  }

  return fn(a) > fn(b) ? 1 : -1;
}));

split

// split :: String -> String -> [String]
const split = curry((sep, str) => str.split(sep));

take

// take :: Number -> [a] -> [a]
const take = curry((n, xs) => xs.slice(0, n));

toLowerCase

// toLowerCase :: String -> String
const toLowerCase = s => s.toLowerCase();

toString

// toString :: a -> String
const toString = String;

toUpperCase

// toUpperCase :: String -> String
const toUpperCase = s => s.toUpperCase();

traverse

// traverse :: (Applicative f, Traversable t) => (a -> f a) -> (a -> f b) -> t a -> f (t b)
const traverse = curry((of, fn, f) => f.traverse(of, fn));

unsafePerformIO

// unsafePerformIO :: IO a -> a
const unsafePerformIO = io => io.unsafePerformIO();