How would I express a chained assignment in Scala?

Question

How would I express the following java code in scala?

a = b = c;

By the way, I'm re-assigning variables (not declaring).

Answer 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 , ... , x_n: T is a shorthand for the sequence of value declarations val x₁: T ; ...; val x_n: T. A value definition val p₁, ..., p_n = e is a shorthand for the sequence of value definitions val p₁ = e ; ...; val p_n = e . A value definition val p₁, ... , p_n : T = e is a shorthand for the sequence of value definitions val p₁ : T = e ; ...; val p_n: 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

Answer 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

Answer 3

The "return type" of the expression (assignment) b = c is Unit, I'm afraid, which means this syntax is not valid.

Answer 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.

Answer 5

val b = c
val a = b

You can't write

val a = b = c

since that defines an expression