Provide some way to create Producers of sum types. about elm-check

[mgold]

The defintion of a producer is intentionally exported. You need a random generator and a shrinker.

The generator you should be able to do with Random.Extra.frequency, combining generators for each tag. (Be sure to bump to version 1.0.1 of that library; we fixed a bug with that function.) The shrinker will likely involve doing a case analysis on each tag and saying how to shrink it. You'll need to look at the shrink library for that.

If you come up with a good reusable abstraction, let me know. But my hunch is that you have to make this producer yourself.

[avh4]

There should be something like

oneOf : List (Producer a) -> Producer a

that gives equal frequency to each option. choice might be another possible name for it.

[mgold]

That's a possibility, but I don't think it will help with union types. Each producer in the list will presumably include a generator for one tag, but the shrinker must shrink all tags. So you have a few choices:

• Define the shrinker once, and use it to create N producers. It seems simpler to create the generator and add the shrinker once at the end.
• With each producer, define a shrinker that shrinks only that tag. This means you repeat _ -> Shrink.noShrink N times, and N case statements aren't a small amount of code. It seems simpler to put all the branches next to each other in one function. But if you go this route, the implementation has two choices:
  • When shrinking, pick only one shrinker. Odds are 1/N that you'll pick the one that shrinks the tag you have. Ick.
  • Use all the shrinkers and concatenate their lists of values. Sensible, but not oneOf the producers -> use choice, but above objection still applies.

I think these issues would become obvious to anyone if they tried to actually code it up. That's why I asked to see code: abstractions should be extracted from something that works, not plucked out of thin air.

All that being said, oneOf isn't a bad idea for generators (even though it's less general than frequency), but shrinkers work in the opposite direction, so it falls apart.

[mgold]

Action item: document the above.