Re: Possible inconsistent error for "+=" shorthand while using immutable set

Hey Andrew,

Thanks for the clear explanation! The behavior makes sense to me now :)

This might be crazy, but I wonder if it makes sense for immutable set to 
have a "+=" member (along with any other assignment operator) that throws a 
reassignment error.

Thanks again,
Chris

On Sunday, March 27, 2016 at 2:03:14 PM UTC-4, Andrew Phillips wrote:
>
> Hi Chris
>
> The behaviour you're seeing is indeed consistent with the language spec, 
> although I can imagine that it appears odd at first sight. In short, the 
> rewriting of a += b to a = a + b can only take place if a is something 
> "that can be assigned". Since that's not true for a val, the rewriting does 
> *not* take place, and so the compiler is forced to look for a member "+=" 
> on a instead. Since a in your example is an immutable set, it can't find 
> one (mutable sets *do* have a "+=" method [1]).
>
> Section 6.12.4 of the language spec [2] describes this in detail; here an 
> excerpt:
>
>
> Let’s consider an assignment operator such as += in an infix operation l 
> += r ,
> where l, r are expressions. This operation can be re-interpreted as an 
> operation
> which corresponds to the assignment
>
> l = l + r
>
> [...] if the following two conditions are fulfilled.
>
> [...]
>
> 2. The assignment l = l + r is type-correct. In particular this implies 
> that l
> refers to a variable or object that can be assigned to, and that is 
> convertible
> to a value with a member named +.
>
>
> Hope that helps!
>
> ap
>
> [1] 
> http://www.scala-lang.org/api/current/index.html#scala.collection.mutable.Set@+=(elem:A):SetLike.this.type
> [2] 
> http://www.scala-lang.org/files/archive/spec/2.11/06-expressions.html#assignment-operators
>
> On Sunday, 27 March 2016 18:11:00 UTC+1, Christopher Lee wrote:
>>
>> Hello,
>>
>> I'm just starting to learn Scala, and I came across the following example 
>> in the "Programming in Scala" book.
>>
>> var jetSet = Set("Boeing", "Airbus")
>> jetSet += "Lear"
>> println(jetSet.contains("Cessna"))
>>
>> One of the lesson points is that the default Set is immutable. So, I 
>> wanted to test that line 2 in the above actually reassigns a new immutable 
>> set with the element added to the jetSet variable.
>>
>> I then *changed the var to a val* and expected to see a reassignment 
>> error. Instead I got the below error.
>>
>> immutableSet.scala:2: error: value += is not a member of scala.collection
>> .immutable.Set[String]
>> jetSet += "Lear"
>>        ^
>>
>> While the error message is true, it's strange that the compiler didn't 
>> complain about "+=" before. "+=" is shorthand for `jetSet = jetSet + 
>> "Lear"`, so I would expect to see a reassignment error.
>>
>> E.g. If I were to run the program with the long form, I get the expected 
>> error message.
>>
>> val jetSet = Set("Boeing", "Airbus")
>> jetSet = jetSet + "Lear"
>> println(jetSet.contains("Cessna"))
>>
>> immutableSet.scala:2: error: reassignment to val
>> jetSet = jetSet + "Lear"
>>        ^
>>
>> Am I missing something (very likely since I'm a newb) or does this seem 
>> inconsistent?
>>
>> Thanks for reading!
>> Chris
>>
>

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