Swift idiomatic way to guard both aren't nil

Question

Lets say I have two variables, both optionals:

var a:Int? = 42
var b:Int? = 13

I have a condition where it's OK to proceed as long as these are not BOTH currently nil. I thoughtlessly put together something like:

guard let _ = a, let _ = b else { return }

I was absentmindedly thinking the conditions would be OR'ed, rather than AND'ed. Obviously that was wrong. The question then becomes, is there an idiomatic/preferred way to test that? Or do I just regress to the basics:

if a == nil && b == nil { return }

Aside

If I use the message extensions added by this post, then I might happily write something like

guard a.notNil || b.notNil else { return }

Which is about is close as I can come to "make certain (guard) that a is not nil or b is not nil"

this isn't an answer, but I think going back to an `if` statement is EXACTLY what you need. — Dan Rosenstark, Dec 18 '15 at 17:59
Just use explicit nil check. If you dont want to unwrap, don't use `let` — Sulthan, Dec 18 '15 at 18:00
I would probably prefer the explicit `if`. – If both optionals are of the same type, you can do `guard case .Some = a ?? b else { return }` — Martin R, Dec 18 '15 at 18:30
I keep forgetting about `??`, thanks @MartinR! Also: you should make that into an answer. It's valid and quite concise and readable, I think — Dan Rosenstark, Dec 18 '15 at 20:54

score 3 · Accepted Answer · answered Dec 18 '15 at 20:53

A guard is an if, really, so you can do this the same way. This is clear and uses guard, which seems to be part of the fun. I present OR and AND options so you can pick one.

func doItOr(a: Int?, b:Int?) {
    guard (a != nil || b != nil) else { return }
    print("either A and B is not nil");
}

func doItAnd(a: Int?, b:Int?) {
    guard (a != nil && b != nil) else { return }
    print("both A and B are not nil");
}


doItOr(nil, b: nil)
doItOr(nil, b: 5)
doItOr(4, b: 5) // prints

doItAnd(nil, b: nil)
doItAnd(nil, b: 5)
doItAnd(nil, b: nil)
doItAnd(4, b: 5) // prints

matt · Answer 2 · 2015-12-18T21:04:33.290

1

it's OK to proceed as long as these are not BOTH currently nil

Your question poses two quite different possible conditions, so I will assume, for purposes of discussion, that you mean this one, namely "not both currently nil". Okay, then: "Not" is !. "Both" is &&. So, like this:

guard !(a == nil && b == nil) else {return}

edited Dec 18 '15 at 21:04

answered Dec 18 '15 at 17:59

matt

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You're right and this answers the question, but an `if` statement is preferable in all ways, I think. Except for the word `guard` it's easier to read in every way. – Dan Rosenstark Dec 18 '15 at 18:02
1

@Yar I don't agree and in any case that isn't what the OP asked (as far as I can tell). – matt Dec 18 '15 at 18:13

score 0 · Answer 3 · answered Dec 18 '15 at 18:01

If you really don't need to bind a or b as non-optionals or discern which of the two is non-nil, I believe the idiomatic approach would still be to use the if statement.

I generally use guard to narrow the specificity of my parameters by binding optionals, downcasting, etc.

You don't seem to be interested in doing that here. You just want to check them, so if seems to appropriately express that.

Swift idiomatic way to guard both aren't nil

3 Answers3