Add Eq docs #1788
Add Eq docs #1788
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| "Hello" =!= "World" | ||
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| Implementing `Eq` instances yourself for every data type might seem like huge drawback compared to only slight gains of typesafety. |
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we could mention fromUniversalEquals which should work fine for case classes.
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not sure this should go in the documentation, but I recently found this useful:
implicit def productEq[A <: Product]: Eq[A] = Eq.fromUniversalEqualsunless I'm missing something (and if so, please let me know!) fromUniversalEqual should work fine with any Product. This makes it incredibly easy for the users to provide a reasonable Eq instance for every case class.
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That's pretty interesting, seems like it would work for all case classes! Any known downsides?
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not that I know. We're using it in a project since a few weeks and so far so good.
That said, I'm far from being an expert :)
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I agree it's worth mentioning in the documentation.
docs/src/main/tut/typeclasses/eq.md
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| Foo(10, "") === Foo(10, "") | ||
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| You can even go one step further and make use of the fact, that every case class will extend `scala.Product`, by creating an `Eq` instance for it. |
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I think this mechanism is probably good enough for all Product, so it doesn't limit to case classes. How about:
You can even go one step further by making this implicitly available for all scala.Product (which include case classes).
implicit def eqProduct[A <: Product]: Eq[A] = Eq.fromUniversalEquals
docs/src/main/tut/typeclasses/eq.md
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| # Eq | ||
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| Show is an alternative to the standard Java `equals` method. |
docs/src/main/tut/typeclasses/eq.md
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| case class Bar(a: Double, b: Int) | ||
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| implicit def eqProduct[A <: Product]: Eq[A] = Eq.fromUniversalEquals |
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Sorry to disagree with a couple of other commenters, but personally I wouldn't be too inclined to include this example in the docs. A couple of significant difference between this and kittens derivation is that kittens will only derive the Eq instance when all types within the case class have Eq instances, and it will be consistent with those individual Eq instances, while this relies on universal equality so it won't necessarily.
I like the fromUniversalEquals method and use it on occasion, but I think it's a big leap to add this instance into implicit scope for all Product types. I don't want to be a stick in the mud though, so if others want to go forward with this in the docs, that's okay.
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that's a really good point. It didn't come to my mind that fromUniversalEquals wont recursively use Eq for fields. Agree we should remove this example usage.
Adds some basic documentation for
Eq. I'm not sure if this is enough, or if more would complicate things and turn off users, would love to hear some feedback :)