Add result extract

This extension returns n as the default if none: option f n https://github.com/fsprojects/FSharpPlus/blob/master/src/FSharpPlus/Operators.fs#L39

This seems like it’s equivalent to (map f >> Option.defaultValue n).

Can we add something similar for Result?

I’d like Result.defaultValue and Result.defaultWith for similarity to existing Option functions but could also be like Haskell fromLeft/fromRight… here as fromOk/fromError https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Either.html#v:fromLeft

WDYT?

[gusty]

In #232 I did a concrete proposal of all the above.

Some notes:

It’s important to note the difference between Choice<_,_> and Result<_,_> the former is like the tuple but for DUs, and it’s (still) used everywhere in F# to abstract the shape of a DU while the latter is biased towards a Result-or-Error abstraction.

Therefore, some of the suggested functions makes more sense in one but not in the the other type.

In order to facilitate the use for these functions in both types we can add a simple pair of functions from one type to the other.

• These functions are Result.toChoice and Result.ofChoice it makes more sense in the Result module as Choice is more generic, so it makes no sense to add lot of conversion in Choice to other types, it’s better for the other types to have their corresponding link to Choice.

• The extract function makes sense in Result which has a concrete meaning of representing a value in Ok with an effect in Error. This function is called getOk but I think it should be called simply get.

• The additional extracting functions are also available in Result for the same reason, they are called defaultValue and defaultWith.

• As for the partition functions, they make more sense in the Choice module as @wallymathieu suggested given the inherent symmetry and general purpose. If we want to partition on Result, we can do List.partitionMap Result.toChoice x. The name partitionMap is also used in Scala libs for the same function. There is no Seq implementation for the same reason there is no unzip for Seq which is the tricky way to implement a function that yields 2 sequences (lazy?, strict?, buffering?).

[gusty]

A bit off topic:

Other functions I would like to add are List.partitionOfResult and List.partitionOfChoice (also for Array and seq). See Haskell similar functions

I found them very useful in the past.