Variable Mapping

[bastienFalcou]

I am suggesting this addition to Snail. This allows to map a Unique<T> into a Unique<U> that will receive the same notifications, with the value transformed from its initial type T to the expected new type U.

This is useful for me here when I use:

public enum ValidationField {
    case binding(Variable<String?>) // expects a Variable of type <String?>
    // ...
}

But in my case, I need to bind viewModel.showingAgent which is of type Unique<FullContact?>. I don't want to create a new enumeration member taking that type specifically. I prefer to reuse that existing enumeration member and map viewModel.showingAgent into a Unique<String?>.

I think this could be useful in multiple places as well along the way 👍

[bastienFalcou]

Note: another suggestion of enhancement could be to support the Swift error handling system by adding the following:

func map<U>(transform: @escaping (T) throws -> U) rethrows -> Unique<U>

This would allow to handle throwing from the map closure, and would be consistent with the native map signature implemented by Foundation on the Collection protocol:

func map<T>(_ transform: (Element) throws -> T) rethrows -> [T]

This change will require to make onNext, onError and onDone support throws. I gave it a quick try and it seems like it will be quite an extensive change. Will keep that for the future, in another Pull Request 👍

[codecov-io]

Codecov Report

Merging #95 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #95   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      7           
  Lines         160    164    +4     
=====================================
+ Hits          160    164    +4

Impacted Files	Coverage Δ
Snail/Variable.swift	100% <100%> (ø)	⬆️

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