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This question is about HISTORY (not your current opinions on the matter).

While reading post about dropping support for increment/decrement operators for Swift I read such text "Swift already deviates from C in that the =, += and other assignment-like operations returns Void (for a number of reasons)".

So at some time in the past developers consciously decided to evaluate assignments to void for some reasons.

And I am looking for those historical (now) reasons. Pretty much as this thread is about historical reasons for Scala.

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greenoldman
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  • Could you provide a link to the article about dropping support for increment/decrement please? I'm interested in reading it. – FreeNickname Dec 09 '15 at 07:37
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    @FreeNickname, sure, take a look: https://github.com/apple/swift-evolution/blob/master/proposals/0004-remove-pre-post-inc-decrement.md – greenoldman Dec 09 '15 at 07:39
  • in C you can use something like if i=1 {}. the result of an assignment returns value which you can use. in swift such statements will not compile and the reason (i think so) is to avoid unwanted side effect. i can agree with this idea, on the opposite side i really don't see, why ++i, i++. --i, i-- should be removed from the language. i read the article at least twice and it is still not very clear to me. – user3441734 Dec 09 '15 at 07:49
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    @user3441734, it might be the reason, however I don't think it is. Because in Swift only boolean expressions are accepted in `if`, if I'm not mistaken. So even if `i = 1` returned `if`, `if i = 1 {}` wouldn't compile, since in Swift `1` is not a boolean value. @greenoldman, thank you for the link) – FreeNickname Dec 09 '15 at 08:33
  • I can't talk specifically about Swift, but some people consider using an expression for both its value and its side-effect a bad practice, because such code is hard to understand. It can also lead to code like `i = ++i + i++;`, which has undefined behavior in C and C++. – svick Dec 09 '15 at 13:53

1 Answers1

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At least one reason is to be safer in comparison operations. When writing in C, Objective-C, etc., how many times have you written this:

if (x = 2)

instead of 

if (x == 2)

Newer versions of compilers have introduced specific warnings for the above case, but wow has that one missing equal sign caused hard-to-identify bugs in my code over the years.

With the Swift type system, this would be less of a problem, since the returned value would most likely not comply to the BooleanType protocol, but if it did (if x = false), you might still hit these bugs. A lot of Swift is designed to eliminate common causes of bugs that people have encountered, including this one.

This is stated in the Swift Programming Language book, under "Basic Operators":

Unlike the assignment operator in C and Objective-C, the assignment operator in Swift does not itself return a value. The following statement is not valid:

if x = y {
    // this is not valid, because x = y does not return a value
}

This feature prevents the assignment operator (=) from being used by accident when the equal to operator (==) is actually intended. By making if x = y invalid, Swift helps you to avoid these kinds of errors in your code.

Brad Larson
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  • I never in my life (30+ years as developer) made this error, but I wrote a lot of x=y=2 like code. I feel punished for the errors of others :-) – woens Sep 25 '19 at 12:11