var someArray = ["Foo", "Bar"]

if let foo = someArray.popFirst() {
    print("Who cares, we never get here anyway.")
}

That yields, on the popFirst() method:

Cannot use mutating member on immutable value: 'someArray' is immutable.

No it’s not.  It’s simply not.

For whatever reason, if you instead call popFirst() on ArraySlice – ostensibly indistinguishable from a real Array – it works just fine.

var someArray = ["Foo", "Bar"][0...]

if let foo = someArray.popFirst() {
    print("Yet this works correctly.")
}

Sigh.

I presume it’s trying to tell me something stupidly obscure about Swift’s byzantine type system.  Good luck finding out what.  Good luck even finding the definition of the popFirst() method, since Xcode claims it doesn’t exist if you Command-Control-click on it.  But Xcode can’t find most things, so that in itself says very little.

let componentsOfPotentialInterest = [Calendar.Component: ((Int) -> String)](
 .day: { String($0 + 1) },
)

Push button.  Expect results (or at least bacon).  Get:

Foo.swift:76:21: error: expected ',' separator
 .day: { String($0 + 1) },
     ^
     ,
Foo.swift:76:21: error: expected expression in list of expressions
 .day: { String($0 + 1) },
     ^
Foo.swift:76:21: error: expected ',' separator
 .day: { String($0 + 1) },
     ^
     ,

Believe it or not, Swift, blindly repeating your obtuse error messages does not help.

What it’s trying but as usual failing miserably to tell me is that Dictionary doesn’t have an initialiser that takes key: value pairs (my mistake for writing straight-forward, Python-like code).  You have to use the dictionary literal syntax instead:

let componentsOfPotentialInterest: [Calendar.Component: ((Int) -> String)] = [
 .day: { String($0 + 1) },
]

Now it merely complains about the expression being too complex for its pathetic little brain, rather than having no fucking clue what you’re doing to begin with.  Pick your utterly useless poison, I suppose.

The fuck?

⤹ Me             Swift compiler ⤵︎

In today’s episode of “what the fuck do you want, compiler?”, we tackle:

foo.swift:186:39: error: ambiguous reference to member 'joined()'
       log.debug("\(thingies.joined(separator: ", "))")
                    ^~~~~~~~
Swift.BidirectionalCollection:27:17: note: found this candidate
 public func joined() -> FlattenBidirectionalCollection<Self>
             ^
Swift.Sequence:27:17: note: found this candidate
 public func joined() -> FlattenSequence<Self>
             ^
Swift.Sequence:18:17: note: found this candidate
 public func joined<Separator : Sequence where Separator.Iterator.Element == Iterator.Element.Iterator.Element>(separator: Separator) -> JoinedSequence<Self>
             ^
Swift.Sequence:16:17: note: found this candidate
 public func joined(separator: String = default) -> String
             ^
Swift.Collection:27:17: note: found this candidate
 public func joined() -> FlattenCollection<Self>
             ^

For context, ‘thingies’ is an array of a custom type.

What the compiler wishes it could say, if it weren’t incompetent, is that every one of Array‘s joined implementations are conditional.  The one that I want is is the second last one, but it is only defined for Array<String> specifically.  No other Array types.

Similarly every other one is conditional on the Element type within the Array being a specific type or protocol, none of which happen to apply to the types I’m using in my Array.

Now, my intuition is that since my type is CustomDebugStringConvertible, that Swift would know then how to convert my type to a String and then go from there.  For better or worse, however, it does not.  Instead you have to do it manually, e.g.:

log.debug("\(thingies.map({ String(describing: $0) }).joined(separator: ", "))")

And you can probably tell from that alone that I’m very used to Objective-C, where it’s very easy to write what you intend and get the results you intend.