Testing Either List about fpdart HOT 4 CLOSED

expect does not use ==, but has a more sophisticated matching method.
See https://github.com/dart-lang/matcher/blob/3a80b989c5c1071d32af11ea9d3fae55807a14c2/lib/src/equals_matcher.dart#L18 ,
and https://github.com/dart-lang/matcher/blob/3a80b989c5c1071d32af11ea9d3fae55807a14c2/lib/src/equals_matcher.dart#L107 in particular.

So, Iterables are compared deeply by expect, even if == would return false on them.

But you don't give an Iterable to expect, you give a Right([1, 2, 3, 4, 5]) to expect.

So expect eventually compares the Either instances by ==.
And Either compares the wrapped value by ==, which will return false, because List don't implement ==, and since these are two instances of Lists, == will return false.

@SandroMaglione suggested using Lists that do implement ==, for example fast_immutable_collections.

Your would use it like this: Right(IList([1, 2, 3, 4, 5]));
I would recommend doing this anyway since mutable collections don't play well with functional programming.

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Hi @Babwenbiber

The reason the test fails is that natively dart compares lists by identity. This means that dart will not compare the elements inside the list but it will check that the two lists are the same instance (which is not the case).

There are many solutions to this problem. Generally, you should override the == and hashCode methods (like fpdart does for Either).

Another solution would be to use Equatable package.

For lists specifically, you must either implement a custom equality method (that compares by equality and not by identity) or relay on some package that does this already:

• fast_immutable_collections
• kt_dart

Does this answer solve your doubts?

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Thanks a lot for the detailed answer.

I am still quite unsure about some points.

I have read many times, that == does compare identity, but expect([1, 2, 3, 4, 5], [1, 2, 3, 4, 5]); yields true. So how does this fit into the story?

Furthermore, I checked out the fast_.. package and couldn't understand how to apply this to either. Can you give me a mini example or best practice on how you write your tests?

Thanks

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Thanks.
I didn't know people use IList instead of List :)

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