`Error` typeclass about zio HOT 8 CLOSED

@emilypi ^^^

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@ktonga Why not mzero(Monoid zero) for MonadPlus?

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That makes it harder to distinguish the empty IO (from other failed) in plus to hold the identity laws. It's not impossible, the implementation will differ from the one in Plus and we have to add Monoid and Equal constrains to E. If the default error is a singleton it is simpler and clearer.

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Another useful method could be isRetryable. I'm not saying we should mark E with it everywhere, just add the constraint when it makes sense and we can provide more functionality. For instance there could be two flavors of maybeRetry one with the constraint and another accepting an E => Boolean

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@jdegoes check my comments about that. We can do it, but it will mean an object allocation, and requiring Equal[E] (as well as Monoid[E]), and we have to be careful about breaking coherence in the implicit priorities for the case where there is no Equal or Monoid.

https://github.com/scalaz/ioeffect/blob/master/ioeffect/src/main/scala/scalaz/ioeffect/IOInstances.scala#L48-L55

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(I'll code it up so we can see exactyl what I mean...)

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scalaz/ioeffect#61

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Monoidal E it is

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