> On Oct 19, 2016, at 5:50 AM, David Goodine via swift-evolution
> <[email protected]> wrote:
>
> Hey all,
>
> I don’t know if this is really an ‘evolution’ topic per se, and I’m not on
> Swift Dev, but thought someone here could shed some light on this.
>
> Often when developing code, if I need create mode switches (constant Bools)
> so that I can move back and forth between different features/implementations
> for testing, developing experimental features, migration, etc. Currently if
> I write the following:
>
> let useFoo = true
>
> if useFoo {
> // Foo code
> } else {
> // Non-Foo code
> }
>
> I will get compiler warnings either way that there’s unreachable code based
> on the value of useFoo. That makes sense in most cases, but in some cases I
> need to leave the code as is for a while, and I hate leaving long-standing
> compiler warnings in production code. (If it’s yellow, you need to pay
> attention to it.) So I poked around and discovered that if I use the
> following declaration of useFoo, the warnings go away.
>
> let useFoo = { return true }()
>
> So, the question is, is this intentional? The compiler could certainly
> ascertain that useFoo will always be true and carry out the same dead code
> detection as for the first declaration. If this isn’t intentional, and the
> compiler may at some point optimize away the closure and trigger dead code
> warnings, I might come up with a proposal that there is some idiom that
> allows one to do the above without the warnings. I’m really not a fan of
> #define (because it’s just butt-ugly in such a beautiful language), but its
> existence is unavoidable for some cases.
>
> And, as I write this, I realized I haven’t tried declaring useFoo as var, but
> I expect that would trigger a warning that useFoo is not modified and should
> be defined by let. I think this is a use case that, for practical purposes,
> should be covered to allow this type of evolution of code without generating
> warnings. If the current behavior is intentional and going to stay, then
> that’s probably the best solution.
>
> -d
You should use Active Compilation Conditions to create flags for conditional
compilation you can check with #if. If you’re not yet using Xcode 8, you can
manually define the flags under the Other Swift Flags build setting (e.g.,
-DUSEFOO).
Preston
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