generators/one-of does not seem to shrink across provided generators.

[cstorey]

I'm using simple-check to test some parser-combinators, using simple-check to generate some random parsers, and I'm using one-of to choose between a series of parsers, like so:

(def re-gen
  (gen/one-of
    [(gen/return (re/zero))
     (gen/return (re/epsilon))
     (gen/fmap re/literal alphabet)]))

So Ideally, if a check for a certain parser fails, I'd like it to re-try the same check with a "simpler" parser, so the shrinking order would be something like (re/literal :z) → (re/literal :a) → (re/epsilon) → (re/zero).

However, that doesn't seem to be the case. Looking at the rose-tree representations, one-of seems to choose a sub-generator and then delegate the shrinking entirely to that generator. So, for example:

user=> (pprint (take 10 (repeatedly #(gen/call-gen (gen/one-of [(gen/elements [:foo :bar]) gen/boolean ]) (java.util.Random.) 7))))
([true ([false ()])]
 [:foo ()]
 [true ([false ()])]
 [:foo ()]
 [false ()]
 [:foo ()]
 [:bar ([:foo ()])]
 [true ([false ()])]
 [true ([false ()])]
 [:foo ()])
nil
user=>

I'd hoped to see a shrinking order of something like: :bar → :foo → true → false here. Would that make sense, do you think?

[reiddraper]

Nice observation. This is something I thought about a bit, and (somewhat arbitrarily) chose not to do. But I think I'm changing my mind. Fortunately, I think the change is pretty simple, and you can play around with this function to get you going:

(defn one-of-shrink
  [coll]
  (gen/bind (gen/choose 0 (dec (count coll)))
                (partial nth coll)))

Let me know if this works out, and I'll probably replace the current one-of implementation with this.

[cstorey]

Thanks! That seems to do exactly what I want. I did make a vague attempt to figure out how to do it myself, but I sadly couldn't quite see the wood for the trees, as it were.

Also, I just wanted to say a more general thanks for your hard work on the project, the shrinking functionality is very much appreciated.

[reiddraper]

You're quite welcome, happy to hear you're finding it useful.

[reiddraper]

ps., this is now included in 0.5.1