Promises in a ClojureScript REPL

Roman wrote a nice post on working inside ClojureScript REPLs, also touching on how to deal with promises. If you're unfamiliar, the problem is that in Javascript many operations return promises and unlike in Clojure you cannot block until the promise is resolved. Instead you have to handle the resulting value asynchronously. So if you for instance use fetch that could look something like this:

(.then (js/fetch "") prn)

This will use prn to print the value of the resolved promise. Sometimes you don't just want to print things though, the real power of a REPL lies in reusing values and successively building up just the shape of data you need.

One nice trick I learned from Sean Grove years ago is that you can just use def. This isn't something you'd do in production code but it's zero-ceremony and very handy to capture values.

(.then (js/fetch "") #(def -r %))

After this you can evaluate the -r symbol in your REPL and it will give you the value of the fetch promise. Alternatively to def we could also use an atom to store the return value.

(def s (atom nil))
(.then (js/fetch "") #(reset! s %))


Now that we know how we can access the resulting value of a promise, let's make it convenient. For convenience I basically want two things:

  • Make it easy to wrap any promise-returning form to capture it's return value
  • Make it easy to access the return values of multiple promises

What I came up with is a function I just named t which can be used like this:

(let [s (atom {})]
  (defn t
    ([kw] (get @s kw))
    ([p kw] (.then p (fn [r] (swap! s assoc kw r) r)))))
(-> (js/fetch "")
    (t :jsonip))

When t receives two arguments it will consider the first argument a promise storing the resulting value in an atom under the key provided as the second argument, :jsonip in this case.

This API is particularly nice when you consider that most editor integrations provide the ability to evaluate the form around your cursor. If I place my cursor within (t :jsonip) and evaluate this form I can look at the value the promise returned without changing any of the code. I can also just continue chaining with then since t returns the original promise.

Another nice feature is that I can reuse the values for future REPL evaluations by referring to them using forms like (t :jsonip).

Obviously this is just one way but I liked how that simple 4 line function made working with promises in a REPL a lot more enjoyable.