I am often writing a procedure to implement a function and realize before long that my elegant poem of a procedure has become something akin to an assembly manual for discount fiberboard furniture translated through three languages. The source of this entropy more often than not is the in-creeping of code to control not what value I want to produce but how the value should be produced.
For example, you may want to map a million URLs to objects representing that results of performing a GET request on them. But you live in the real world, so you don't want to naively initiate a million asynchronous tasks, because your pipe is only so wide and the machine(s) you're pounding will tolerate only so much abuse, so you want to limit the concurrency. Oh, and the Internet, it's not reliable, so you want to re-try each request a few times before giving up. And you do want to give up eventually, because not all of those URLs are going to be valid--or point to data.
And on and on. Pretty soon what was a single line of code has become a stomach-turning mess. Wouldn't it be nice if someone wrote a combinator library that allows you to apply various useful behaviors to a function, letting you concentrate on writing elegant code-haiku you can send to your parents when they ask, "What exactly do you do for a living?"
I aspire to write such a library for you. And myself, of course.
I distinguish between the terms procedure and function as if I were a pedantic Scheme programmer. There's an essay ruminating to be written on how these two terms may productively be applied to Clojure.
To use this library, add the following to your Leiningen project's
:dependencies section:
Full API documentation is available.
An example wherein a function that waits two seconds before printing
its argument is decorated with the spawn combinator, allowing it to
run asynchronously meanwhile the main thread of control waits for the
function to complete at which point a message is printed:
(let [done? (atom false)
f (fn [x]
((stall 2000 prn) x)
(swap! done? (constantly true)))
g (-> f spawn)]
(g 42)
((upon done? identity prn) :done))An example that returns true:
((nilf (complement identity) identity) nil)An example that tries moderately hard to make nil truthy, patiently waiting half a second between attempts. It does not end well:
((retry 3 500 identity) nil)An example to return a map containing a channel with slurp results and the number of items:
(defn example
[]
(let [results (a/chan)
n (atom 0)
urls #{"http://example.com/" "http://poseur.com/"
"https://google.com/" "http://clojure.org/"}
f (->> slurp
nile
argment
(retry 3)
(pinch 2)
(deposit results)
spawn
(tally n))]
(doseq [url urls] (f url))
{:n @n :ch results}))
(let [{:keys [n ch]} (example)]
(into {} (repeatedly n #(a/<!! ch))))Copyright © 2014 Edwin Watkeys.
Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.