Typeclass instances for Reader. #249
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dangerousben
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Jul 31, 2020
dangerousben
commented
- Alternative
- Defer
- Monad
- Monoid
- Eq
| def defer[A](fa: => Reader[A]): Reader[A] = Reader.flatten(Reader.value(() => fa).map(_())) | ||
| } | ||
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| private[util] trait ReaderMonoidK extends MonoidK[Reader] { |
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Just to confirm, this and the two above are separate traits just for the sake of code organisation?
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Mainly - I have a vague idea about trying to implement some of the cats-effect typeclasses for Reader (not actually sure if it's possible), and if so I'd want to be able to mix Defer and Monad in without MonoidK.
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- Alternative - Defer - Monad - Monoid - Eq
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There seems to be an infinite recursion for Reader somewhere. Is this reproducible locally or only on CI (I haven't checked yet)? |
It's reproducible (although somehow it didn't show up before I submitted the PR). IIRC the problem is that it's impossible to implement a valid I think the other typeclass instances are (at least from a practical point of view) valid, but the laws can't be tested without a working |
It might of course be possible to implement an invalid one that works well enough for the tests, but I haven't figured out how so far. |
For testing you could maybe drain the streams into local variables, compare them and reset the streams with the elements you drained. That is, of course, not a good idea in production, but maybe it's enough in the tests (with documentation why we have to do this). edit: Some streaming implementation also allow copying/broadcasting a stream without mutating the "original" instance. I don't know if the Twitter ones have this, though. |
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