Encourage to use == instead of === for writing unit tests.

[HyukjinKwon]

FunSuite provides === for comparison. I see == and === are used in a mixed way across unit tests. It might be great if this is encouraged documented.

See http://www.scalatest.org/getting_started_with_fun_suite

ScalaTest lets you use Scala's assertion syntax, but defines a triple equals operator (===) to give you better error messages. The following code would give you an error indicating only that an assertion failed:

assert(1 == 2)
Using triple equals instead would give you the more informative error message, "1 did not equal 2":

assert(1 === 2)

[aarondav]

As of I think Scala 2.10, ScalaTest's assert is a macro which does print nice error messages. If you're working in a 2.10+ codebase, I would recommend double equals, as triple equals can be hard to remember, especially when it's unnecessary.

[HyukjinKwon]

@aarondav Thank you so much for your comment!
I feel like it would be great if it is documented regardless of which one is preferred because apparently they are being used in a mixed way in Spark and it seems some prefer == whereas other do === including Spark committers.

[HyukjinKwon]

If it does not matter, then I would like to close this anyway.

[aarondav]

Feel free to add a section recommending == -- I think having a guideline here is very reasonable, especially since there's some history behind it.

[HyukjinKwon]

Hi, @aarondav,
I just simply ran the tests as below (my scala version is 2.11.6):

• assert(1 == 2)

  org.scalatest.exceptions.TestFailedException was thrown.
  

• assert(1 === 2)

  1 did not equal 2
  

I ran this within my local and then also tried this with unit tests in Spark.

Am I missing something?

[HyukjinKwon]

But it seems apparently okay with other comparisons for example as below (not using constant) although I don't know why:

val a = 1
val b = 2
assert(a == b)

produces

1 did not equal 2

So, If you think it is fine, I will submit a PR with saying things such as ...

"... == is encouraged because triple equals can be hard to remember, especially when it's unnecessary and this might confuse newbies of Scala developers ...".

[HyukjinKwon]

Hm.. cc @rxin too. Do you have any preference or think it is something we should not bother for now?

[rxin]

I don't have a strong preference. It doesn't really hurt my eye ...

[HyukjinKwon]

Let me close this for now. I will reopen when I happen to get a better reason.