Chained assignment in Scala

[Eugen Labun]

Just in case someone always been missing chained assignment in Scala, here is an equivalent of ´a =
b = value´:

scala> val a@b = new Object
a: java.lang.Object = java.lang.Object@80b973
b: java.lang.Object = java.lang.Object@80b973

Notice that the variables above get initialized with the same value, as opposed to:

scala> val a,b = new Object
a: java.lang.Object = java.lang.Object@d1223d
b: java.lang.Object = java.lang.Object@1ee8c1

Also possible with ´var´s and with more than two names (but not with ´def´s):

scala> var a@(b@(c@d)) = new Object
a: java.lang.Object = java.lang.Object@cbbdf3
b: java.lang.Object = java.lang.Object@cbbdf3
c: java.lang.Object = java.lang.Object@cbbdf3
d: java.lang.Object = java.lang.Object@cbbdf3

Enjoy :)

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EL

[Vlad Patryshev]

Thanks a lot! Please let me use it in my kittens (scala snippets)

-Vlad

[Eugen Labun]

On 2012-02-13 05:32, Vlad Patryshev wrote:
> Thanks a lot! Please let me use it in my kittens (scala snippets)

Of course! (Just in case it was not a purely ritorical polite phrase)

[Dave]

Cool feature.
Only too bad that parentheses are needed in the last case (i.e. with 3
or more chained var assignments)
Or is this a bug?

scala> var a@b@c = new Object
<console>:1: error: '=' expected but '@' found.
var a@b@c = new Object
^

[Eugen Labun]

On 2012-02-14 14:31, Dave wrote:
> Cool feature.
> Only too bad that parentheses are needed in the last case (i.e. with 3
> or more chained var assignments)
> Or is this a bug?

Probably, not. IIUC the grammar doesn't allow for chained bindings.

[Eugen Labun]

On 2012-02-14 14:31, Dave wrote:

> Cool feature.
> Only too bad that parentheses are needed in the last case (i.e. with 3
> or more chained var assignments)
> Or is this a bug?

I looked again into the grammar, and still see no way how to come from a ´Pattern3´ (in the
´Pattern2´ rule) to a chain ´id @ ...´ (or ´varid @ ...´) to make possible a definition without
parentheses.

Here is the relevant snippet:

PatVarDef ::= ‘val’ PatDef
| ‘var’ VarDef

PatDef ::= Pattern2 {‘,’ Pattern2} [‘:’ Type] ‘=’ Expr
VarDef ::= PatDef
| ids ‘:’ Type ‘=’ ‘_’

Pattern2 ::= varid [‘@’ Pattern3]
| Pattern3
Pattern3 ::= SimplePattern
| SimplePattern { id [nl] SimplePattern }
SimplePattern ::= ‘_’
| varid
| Literal
| StableId
| StableId ‘(’ [Patterns] ‘)’
| StableId ‘(’ [Patterns ‘,’] [varid ‘@’] ‘_’ ‘*’ ‘)’
| ‘(’ [Patterns] ‘)’
| XmlPattern

varid ::= lower idrest
Path ::= StableId
| [id ‘.’] ‘this’
StableId ::= id
| Path ‘.’ id
| [id ’.’] ‘super’ [ClassQualifier] ‘.’ id

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EL