[swift-evolution] ternary operator ?: suggestion

[possen p]

Another idea is put the conditional at the beginning:

let foo if conditional then = 10 else = 20

But that requires an additional keyword to separate out the conditional but might read better. 



> On Dec 6, 2015, at 2:14 AM, possen p <possen at gmail.com> wrote:
> 
> (I originally posted as possen p and since hopefully fixed that) 
> 
> So, As Kevin Ballard and Ole Bergman pointed out, making a regular “if” an expression has some major complications. Having the potential to return different object types in a strictly typed language will not be easy. Also what to do if the if does not have an else. The ternary operator does not allow the if/else results of different types. In Python doing an expression with if/else is not a problem because it will let you return different types, this is legal, (which makes sense for Python):
> 
> val = 10 if p == 5 else “abc"
> 
> it will not however let you omit the else part:
> 
> val = 10 if False  // produces an error. 
> 
> so it appears if you assign to an expression in Python it will require the else part. So it is different than a regular if. 
> 
> So I ask, is making the regular “if" an expression truly desired for Swift? I do see other languages doing it, but it does seem a bit odd to assign from any “if”. In my original proposal I was suggesting that assignment is done like this:
> 
> let val = 10 else 40 if p == 5 
> 
> In this it is very similar to the ternary expression in that else part is required and both types must match, and has improved readability. This is overloading the if/else keywords to provide essentially the same thing as ternary operators.. 
> 
> The following from Kevin Ballard's email:
> 
>> let foo = if condition {
>>     funcWithSideEffectsThatReturnsInt()
>> } else {
>>     funcWithSideEffectsThatReturnsString() 
>> }
> 
> 
> I find it is hard to tell that the two results are returning a value. To me it looks like it is either ignoring the return value or is returning void. 
> 
> Maybe emphasizing the assignment would help as below does not have return values in the method names: 
> 
> let foo if condition {
>     = dosomething()
> } else {
>     = dosomethingelse() 
> }
> 
> putting the = after the else shows that it is returning a value for the let clause. It also helps emphasize that the return types must match and I think looks better. 
> 
> Going back to my proposal with the = suggestion: 
> 
> let foo = dosomething() else = dosomethingelse() if condition
> 
> this really shows that the let variable is being assigned. 
> 
> Note that these are still chainable expressions: 
> 
> var foo = ((10 else = 40 if p == 5) else = (15 else = 14 if p == 10) if p == 4)
> 
> It may be good to make the space optional after the else:
> 
> var foo = ((10 else= 40 if p == 5) else= (15 else= 14 if p == 10) if p == 4)
> 
> - Paul 
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