Create mapping functions for Dictionary

[guykogus]

🚀

These versions of map and compactMap return a Dictionary, rather than an Array.
I'm a bit unsure of the function names because of the clash with the built-in map and compactMap functions.

---

map seems to prioritise the new function. E.g. in

let intToString = [0: "0", 1: "1", 2: "2"]
let stringToInt = intToString.map { (key, value) in
    return (String(describing: key), Int(value)!)
}

stringToInt is of type [String: Int]. If you want it to return [(String, Int)], as the original function would do, you need to define the type explicitly. I.e.

let stringToInt: [(String, Int)] = intToString.map { (key, value) in

---

compactMap actually won't compile in a similar situation because it prioritises the built-in function. E.g.

let stringToInt = intToString.compactMap { (key, value) in
    guard let intValue = Int(value) else { return nil }
    return (String(describing: key), intValue)
}

will throw a compilation error because it thinks that the the closure passed to compactMap should contain a single argument, it won't accept a tuple. You need to explicitly define it as

let stringToInt: [String: Int] = intToString.compactMap { (key, value) in
// or
let stringToInt = intToString.compactMap { (key, value) -> (String, Int)? in

in order for it to compile and use the new function.

To use the original function you again need to explicitly define the type. I.e.

let words: [(String, IntWord)] = strings.compactMap { (key, value) in

What do you guys think, should we rename the functions? What names sound good to you?

Checklist

• I checked the Contributing Guidelines before creating this request.
• New extensions are written in Swift 4.
• New extensions support iOS 8.0+ / tvOS 9.0+ / macOS 10.10+ / watchOS 2.0+.
• I have added tests for new extensions, and they passed.
• All extensions have a clear comments explaining their functionality, all parameters and return type in English.
• All extensions are declared as public.
• I have added a changelog entry describing my changes.

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[SD10]

@guykogus This is interesting but I don't like how we're overloading map and compactMap with specific functionality here. The fact that the overload returns a specific type -- dictionary -- is counterintuitive to the general concept of map

[codecov]

Codecov Report

Merging #546 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
+ Coverage      94%   94.01%   +<.01%     
==========================================
  Files          68       68              
  Lines        2871     2875       +4     
==========================================
+ Hits         2699     2703       +4     
  Misses        172      172

Flag	Coverage Δ
#ios	94.01% <100%> (ø)	⬆️
#osx	94.01% <100%> (+5.12%)	⬆️
#tvos	94.01% <100%> (+5.12%)	⬆️

Impacted Files	Coverage Δ
.../Extensions/SwiftStdlib/DictionaryExtensions.swift	100% <100%> (ø)	⬆️

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[guykogus]

For me it makes more sense for map to actually return the same type, but you're right that overloading is not a good idea (especially considering the issues I outlined). So my question remains, what names sound good? Dictionary has a function called mapValues that does something similar, so maybe we could use mapKeysAndValues? Thoughts?

[SD10]

@guykogus I'd be more accepting of mapKeysAndValues given your example of mapValues on Dictionary. It's just when I see something called map I expect it to be a polymorphic function

[guykogus]

OK, updated the names.
I'm confused as to what you mean about map being polymorphic. Are these functions not polymorphic? They convert a Dictionary<K1, V1> into Dictionary<K2, V2>.

[SD10]

@guykogus I guess they are over the generic key/value, but the return type is still bound to Dictionary

[omaralbeik]

@guykogus Interesting extension, and amazing that you added complexity as well 💯

[guykogus]

@omaralbeik I stole the documentation from Apple's docs 😄. The complexity of compactMap is declared to be O(m + n), so for the dictionary it would probably be more like O(m + (2 * n)), but since constants are ignored for big-O notation it's still technically correct.

[LucianoPAlmeida]

Sorry couldn't be on the discussion 😭
But cool extensions @guykogus :))

[guykogus]

I'm a big fan, especially for using dictionaries where the keys are enums that I eventually need to pass as the parameters to an HTTP call, so they need to be converted to their raw values. I feel like there'd be lots of useful scenarios for this...