s/inst-in and s/int-in generators should have uniform distribution not biased towards min value

Question

The s/inst-in and s/int-in generators are based on gen/large-integer* which grows from 0.

`
(require '[clojure.spec.alpha :as s] '[clojure.spec.gen.alpha :as gen])
(gen/sample (s/gen (s/int-in 0 100)))
;;=> (1 0 1 1 1 0 1 1 72 1)

(gen/sample (s/gen (s/inst-in #inst "2001-01-01" #inst "2001-12-31")))
;;=> (#inst "2001-01-01T00:00:00.000-00:00" #inst "2001-01-01T00:00:00.000-00:00" #inst "2001-01-01T00:00:00.001-00:00" #inst "2001-01-01T00:00:00.001-00:00" ...)
`

Proposed: Instead, s/inst-in should use a uniform distribution generator:

After on same:

`
(26 16 65 96 63 37 31 4 94 9)

(#inst "2001-03-03T04:51:43.702-00:00"
#inst "2001-07-25T07:13:03.224-00:00"
#inst "2001-03-31T18:28:41.625-00:00"
#inst "2001-04-17T19:33:14.176-00:00"
#inst "2001-01-14T07:03:08.521-00:00"
#inst "2001-06-06T09:52:03.421-00:00" ...)
`

Patch: clj-2179.patch

Answer 1

Comment made by: gfredericks

What problem is this trying to solve?

Answer 2

Comment made by: alexmiller

Typically I find having the values biased towards the min value of the range (particularly in the inst case where values have to grow a lot to seem different) to not be what I expect as a user.

But I think the question is what the intended behavior should be for range specs. Whether or not Rich and Stu agree, I don't know yet. :)

Answer 3

Comment made by: gfredericks

For inst, I'd recommend at least generating the components of the timestamp separately (year, month, day, hour, etc.) and combining them with {{gen/fmap}}. That makes it shrink more naturally, and makes it easier to specify whatever strategy you like for biasing toward the present.

W.r.t. int ranges, I will only point out that one of test.check's features is to start tests with "small" values, so to whatever extent you impose uniform distributions, you neutralize that feature.

Answer 4

_Comment made by: gfredericks_

if you'd like a generator that starts out at both the min *and* the max and can shrink to either, something like this should work:


(defn bi-biased-int-range
  [min max]
  (let [g (gen/large-integer* {:min min, :max max})
        g' (gen/let [x g] (- max (- x min)))]
    (gen/one-of [g g'])))

Answer 5

Comment made by: stu

Maybe these two cases need separate consideration. I like Gary's idea for timestamp and think that int generators should be left as-is.

Answer 6

Reference: https://clojure.atlassian.net/browse/CLJ-2179 (reported by alexmiller)