Disjunctive or, but with weights

[lvh]

Sometimes, to generate realistic specs, some specs should be more probable than others.

Suggested implementation:

(defmacro weighted-or
  "Like [[s/or]], but with weights."
  [& weighted-options]
  (let [[names specs ws] (apply map vector (partition 3 weighted-options))
        sums (reductions + ws)]
    `(s/with-gen
       (s/or ~@(interleave names specs))
       (fn []
         (gen/fmap
          (fn [count#] (condp < count# ~@(interleave sums specs)))
          (gen/choose 1 ~(last sums)))))))

[lvh]

There's a bug in that implementation, and I'm not sure where it is yet. Firstly, the cond should probably be >=, otherwise you get a 1/n for the missing clause. However, when I do that, I get:

 ExceptionInfo Couldn't satisfy such-that predicate after 100 tries.  clojure.core/ex-info (core.clj:4724)

Which I don't quite understand because this doesn't appear to have a such-that clause? I suppose the fn isn't even returning a generator, so I'm not sure why that appears to work at all.

[lvh]

Found it; got confused with another macro I was writing that also used fmap.

(defmacro weighted-or
  "Like [[s/or]], but with weight."
  [& weighted-options]
  (let [[names specs ws] (apply map vector (partition 3 weighted-options))
        sums (reductions + ws)]
    `(s/with-gen
       (s/or ~@(interleave names specs))
       (fn []
         (gen/fmap
          (fn [count#]
            (let [schema# (condp >= count# ~@(interleave sums specs))]
              (gen/generate (s/gen schema#))))
          (gen/choose 1 ~(last sums)))))))

[gfredericks]

gen/generate is not intended to be used as part of a generator (you lose determinism & shrinking when you use it) -- I think gen/frequency would work fine here.

A PR would be nice.

[lvh]

I had previously used generate because no runtime test.check dependency; I guess that doesn't matter for schpec :)