Callback solution to the Async Problem

async-problem-callbacks-solution.js

//    pipe :: Array (Any -> Any) -> Any -> Any
const pipe = fs => x => fs.reduce ((y, f) => f (y), x)

//    lmap :: (a -> b) -> Array a -> Array b
const lmap = f => xs => xs.map (f)

//    append :: a -> Array a -> Array a
const append = x => xs => [...xs, x]



//    pure :: b -> Future a b
const pure = x => l => r => r (x)

//    then :: (b -> Future a c) -> Future a b -> Future a c
const then = f => m => l => r => m (l) (x => f (x) (l) (r))

//    fmap :: (b -> c) -> Future a b -> Future a c
const fmap = f => then (x => pure (f (x)))

//    all :: Array (Future a b) -> Future a (Array b)
//        -- Note: This implementation resolves things in sequence for brevity.
const all = ms => ms.reduce
  ((mxs, mx) => then (x => fmap (append (x)) (mxs)) (mx), pure ([]))



const filesystem = require ('fs')
const path = require ('path')

//    readFile :: String -> String -> Future Error String
const readFile = encoding => filename => l => r => {
  filesystem.readFile (filename, encoding, (e, contents) => {
    if (e) l (e)
    else r (contents)
  })
}

//    readText :: String -> Future Error String
const readText = readFile ('utf8')

//    lines :: String -> Array String
const lines = s => s.split ('\n')

//    unlines :: Array String -> String
const unlines = ss => ss.join ('\n')

//concatFiles :: (String -> String) -> Future Error String
const concatFiles = path =>
  pipe ([ path
        , readText
        , fmap (lines)
        , fmap (lmap (path))
        , fmap (lmap (readText))
        , then (all)
        , fmap (unlines) ])
       ('index.txt')


const main = () => {
  concatFiles (x => path.resolve (process.argv[2], x))
              (e => { process.stderr.write(e.message); process.exit(1) })
              (x => { process.stdout.write(x); process.exit(0) })
}

main()

For the process leading up to this solution, read this article.