Maximum call stack size exceeded when comparing long strings

[nonpop]

This might be the same problem as #22 or rtfeldman/node-test-runner#290 but here with very simple data which seems to point to a problem with output string diffing.

So, the following:

sscce : Test
sscce =
    test "Different long strings" <|
        \_ ->
            String.repeat 1000 "a"
                |> Expect.equal (String.repeat 1000 "b")

causes

Tests.elm changed. Rebuilding!

elm-test 0.19.0-beta12
----------------------

Running 1 test. To reproduce these results, run: elm-test --fuzz 100 --seed 354267269012927 /home/nonpop/projects/elm/sscce/tests/Tests.elm

/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:579
function _Utils_eqHelp(x, y, depth, stack)
                      ^

RangeError: Maximum call stack size exceeded
    at _Utils_eqHelp (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:579:23)
    at _Utils_eq (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:570:35)
    at /home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:3452:9
    at Function.f (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:3493:5)
    at A5 (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:113:28)
    at /home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:3497:13
    at Function.f (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:3434:11)
    at A2 (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:104:28)
    at Function.f (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:3494:11)
    at A5 (/home/nonpop/projects/elm/sscce/elm-stuff/generated-code/elm-explorations/test/elmTestOutput.js:113:28)

Everything works if I make the strings the same by just changing "b" to "a".

[mgold]

The stack trace would indeed indicate output, which is weird because I would have suspected shrinking.

[drathier]

Are these stack overflows fixed by the optimiser in 0.19?

[nonpop]

The stack trace would indeed indicate output, which is weird because I would have suspected shrinking.

I know nothing about the internals of of elm-test, but it sounds weird that shrinking would even happen when there are no fuzzers involved?

Are these stack overflows fixed by the optimiser in 0.19?

How to test that?

[drathier]

If you have a test that always gives a stack overflow in 0.18, you could update it to 0.19 and see if it still stack overflows.

[nonpop]

I don't remember ever getting a stack overflow in 0.18. This example also works just fine with Elm 0.18 and elm-test 0.18.13-beta3, even if I make the strings longer (then it just takes forever).

[drathier]

My bad, thought this was the 0.18 repo. Carry on.

[nonpop]

With a longer stack trace it looks like the problem arises from Test.Runner.Node.Vendor.Diff.makeChangesHelp calling itself recursively through andThen. In 0.18 makeChangesHelp is tail recursive. Maybe I should file the issue in the node-test-runner repo instead?