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

On Sun, Mar 27, 2016, 9:24 PM Christopher Lee <[email protected]>
wrote:

> 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.
>

So you wouldn't know about the issue until/if that line of code is run?


> 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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