问题
How would I express the following java code in scala?
a = b = c;
By the way, I'm re-assigning variables (not declaring).
回答1:
The closest shortcut syntax in Scala can only be used when you declare a var or val.
scala> val c = 1
c: Int = 1
scala> val a, b = c
a: Int = 1
b: Int = 1
From the Scala Reference, Section 4.1
A value declaration val x1 , ... , xn: T is a shorthand for the sequence of value declarations val x1: T ; ...; val xn: T. A value definition val p1, ..., pn = e is a shorthand for the sequence of value definitions val p1 = e ; ...; val pn = e . A value definition val p1, ... , pn : T = e is a shorthand for the sequence of value definitions val p1 : T = e ; ...; val pn: T = e .
This doesn't work for re-assignement to a var. The C/Java style doesn't work for reasons explained here: What is the Motivation for Scala Assignment Evaluating to Unit
回答2:
Using the fact that the left-hand-side of an assignment is syntactically a pattern. (See PatVarDef > PatDef > Pattern2 in SLS.)
a = b = 5
scala> val a@b = 5
a: Int = 5
b: Int = 5
x = y = z = new Object
scala> var x@(y@z) = new Object
x: java.lang.Object = java.lang.Object@205144
y: java.lang.Object = java.lang.Object@205144
z: java.lang.Object = java.lang.Object@205144
Note that the expression on the right-hand-site is evaluated only once.
Unfortunately, this syntax doesn't work for reassigning (so for x = y = value you still have to do x = value; y = x).
See also [scala-language] Chained assignment in Scala
回答3:
The "return type" of the expression (assignment) b = c is Unit, I'm afraid, which means this syntax is not valid.
回答4:
b = c; a = b
Awkward, I know. That's Scala pretty much telling you "don't do that". Consider it the Scala version of Python's space identation for block delimitation.
回答5:
val b = c
val a = b
You can't write
val a = b = c
since that defines an expression
来源：https://stackoverflow.com/questions/2442318/how-would-i-express-a-chained-assignment-in-scala