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Question

gen/for macro for alternate combinator syntax

asked Apr 2, 2014 in test.check by jira

I think the syntax of {{clojure.core/for}} would be a good fit for test.check's combinators. For example:

(defn gen-even-subset
  "Returns a generator that generates an even-cardinality
   subset of the given elements"
  [elements]
  (gen/for [bools (apply gen/tuple (repeat (count elements) gen/boolean))
            :let [true-count (->> bools (filter identity) (count))]
            :when (even? true-count)]
    (->> (map list bools elements)
         (filter first)
         (map second)
         (set))))

This combines the capabilities of {{fmap}}, {{bind}}, and {{such-that}} into a familiar syntax.

One downside here is the temptation to use multiple clauses for independent generators, resulting in a use of {{gen/bind}} when {{gen/tuple}} would be simpler and presumably shrink easier. An approach to this is an additional supported clause, perhaps called {{:parallel}}, that uses the syntax of {{:let}} to provide the functionality of {{gen/tuple}}:

(gen/for [:parallel [n1 gen/nat
                     n2 gen/nat]
          :let [sum (+ n1 n2)]]
  {:nums [n1 n2] :sum sum})

Compared to {{gen/tuple}}, this has the advantage of placing generators syntactically next to names, rather than segregating the generators from the names.

The {{:parallel}} feature has not been added to the current patches.

10 Answers

jira · Answer 1 · 2014-04-05T21:23:25+0000

_Comment made by: gfredericks_

I think there might be some design ambiguity around the meaning of {{:when}}. In particular, in the following contrived example:

(for [n nat
      v (vec (return n))
      :let [sum (reduce + v)]
      :when (pos? sum)]
  v)

In my default design this can hang, for the same reason that this code can hang:

(bind nat
      (fn [n]
        (such-that
          (fn [v] (pos? (reduce + v)))
          (vector (return n)))))

But it could just as well have been written:

(such-that
  (fn [v] (pos? (reduce + v)))
  (bind nat (fn [n] (vector (return n)))))

So the issue is whether a {{:when}} filter is applied to just the previous generator or to all of the previous generators. I have some hazy notion that the latter would be less efficient in some cases, but I'm not sure what. So I think our options are:

# Decide to always do it one way or the other
# Provide a third keyword ({{:when-all}}?) with different behavior
# Don't write this macro at all because it's too difficult to understand

My gut is to do option 1 and just apply :when to the previous generator.

jira · Answer 2 · 2014-04-09T03:24:26+0000