Add repeat for parametrized tests and fuzz test examples

[lydell]

This PR adds a small convenience function called repeat (and one called testWith, which is test but shaped to fit repeat). This does not unlock anything that wasn’t already possible in user land – it’s all about convenience and teaching.

Me and @Janiczek were chatting, and Martin mentioned that when a fuzz test failed, he was missing a convenient way of adding that case as a regression test. repeat to the rescue!

    repeat "String.words never returns an empty list"
        [ testWith " \n\t" "only whitespace" -- regression test
        , fuzz Fuzz.string "fuzz"
        ]
    <|
        \string ->
            String.words string
                |> Expect.notEqual []

What I really like about this solution (and that I haven’t seen in ad-hoc implementations of this in test suites):

• You can use skip and only. No commenting out stuff in the list!
• IDE:s have a chance to find a location for each sub test. Ad-hoc implementations typically take a list of data – instead of using test functions in the list, like above – which means that there isn’t an “anchor” for each test that IDE:s can use. Also, by having this in the package, IDE:s have blessed names to look for (in addition to test, fuzz, describe etc.), whereas user land functions don’t really have that possibility.

[jfmengels]

Very interesting!

One thing I would change is the order of arguments for testWith: I would put the title first, that way even if the data is large, the title is at the top like all tests.

-- Currently
testWith
  """Multi
       line
       piece
       of
       data
  """
  "only whitespace"

-- Suggested
testWith "only whitespace"
  """Multi
       line
       piece
       of
       data
  """

[lydell]

Yep, I would also like it the other way. But I followed the fuzz order. (Which is wrong, too, IMO. The title should always come first.)

Maybe it’s better to do testWith “correctly” rather than “consistently”?

[jfmengels]

I agree about the arguments for the fuzz function being out of order. That said, the title remains near the top in the usual (non-repeat) usage. Maybe that's a possible breaking change to suggest for the next major version?

For testWith, I think it would be better the way I suggested.

[lydell]

That said, the title remains near the top in the usual (non-repeat) usage.

I’m not quite following here. Do you have an example?

[jfmengels]

What I meant is that when defining a fuzz test such as

fuzz
  myFuzzer
  "title"
  <|
    \data -> ...

the title is still "near" the top of the test definition. Not as high as it could be (fuzz "title" myFuzzer) though.

[lydell]

Btw, is it confusing that testWith and fuzzWith aren’t really the same? testWith is testWithValue while fuzzWith is fuzzWithOptions.

[jfmengels]

Right, that's a good point. testWithValue could be a better name indeed.

[lydell]

Maybe that's a possible breaking change to suggest for the next major version

Btw, I found an issue about this: #97