Add Append and Plus for Result about fsharpplus HOT 8 CLOSED

I've experimented with making tuples instead of only choosing one over the other, however that gives some other weirdness.

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With tuples, I guess it will not "type check" the expected signatures.

I think we can use a new wrapper type, like Validation to achieve the other behavior. I mean validations are the opposite, between a Good and a Bad they prefer the Bad and they accumulate the bad ones. So if we go with the Plus I proposed above, validation will be exactly its dual.

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I was thinking, if we keep the notion of the Alternative as executing alternatives we can also make Async an instance of Append. It will run the first async, then do an Async.Catch and then if it fails will run the second.
This could be useful now with the coming redesign of generic Computation Expressions, where the user will have a separate builder for MonadPlus (or Alternative) computations.

Unless there is a more interesting Append for async?

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OK, I have two proposals:

1. https://github.com/gusty/FSharpPlus/tree/alt-for-result
2. https://github.com/gusty/FSharpPlus/tree/consistent-alt-for-result

They differ in case no alternative is choosen (all failures) 1 returns the last failure whereas 2 requires that failures form a Semigroup and so it returns all of them.

Regarding the failures for plus 1 returns the first one and 2 all of them (requires semigroup).

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I like the 2) approach.

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I also sympathize with approach 2.
In contrast, here's an explanation of how IO works with what seems to be approach 1: https://stackoverflow.com/questions/48450826/what-is-the-purpose-of-instance-alternative-io

cc: @drvink

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I would also go with the second approach. I'm familiar with the instance for Alternative IO described in the link, though it's never really sat well with me!

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Deciding over the second option, closing by ce2f8bb

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