delayed-async-functor.js

// httpGET :: url -> HttpGET
const httpGET  = (url) => {
  let transform = x => x
  const GET =  {
    url: url,
    map: (f) => { 
      transform = x => f(transform(x)) 
      return GET
    },
    fork: (reject,resolve) => {
      request(url, (err,data) => err? reject(err) : resolve(transform(data)))
    },
    resolve: (val) => transform(val)
  }
  return GET
}

const getCatPictures = () => httpGET('http://example.com/cats/pics.json')
                                .map(JSON.parse)
                                .map(imagesToHtml)

//app usage

getCatPictures()
  .map(renderInEl('#gallery'))
  .fork(console.log, console.error)

//test usage

const testGET =  getCatPictures()
expect(testGET.url).to.be('http://example.com/cats/pics.json')
const fakeResponse = JSON.stringify({image1: 'cat1.jpg', image2: 'cat2.jpg'})
const result  = testGET.resolve(fakeResponse)
expect(result).to.be('<ul><li><img src=...')