为什么“val a = -1”在scala中不起作用？

Question

I found val a = -1 works well in scala REPL, but if I skip the space around the = like val a=-1 , the expression doesn't return the result.

Does anyone have ideas about this? Why the space arount the = is necessary here?

Answer 1

=- is a legitimate method name in Scala; the following will work:

class A {
  def =-(i: Int) = i
}
val a = new A
a=-1

So the parser can't distinguish your val a=-1 from this case.

Answer 2

val is used in 2 cases:

1) value declaration:

val a = 2
> a: Int = 2

2) pattern definition:

val Some(x) = Some(2)
> x: Int = 2

when you write val a=-1 , it clearly fails to match the "value declaration" syntax, so the compiler attempts "pattern definition" syntax.

To see this is the case, let's put a semi-colon in the end of the line.

 val a=-1 ;
 > <console>:1: error: '=' expected but ';' found.

Indeed, the compiler is looking for the right hand side of pattern definition.

Now notice that =- is a valid identifier name. So if it is a case class (or a normal class with unapply method), it can be used in pattern match syntax.

Let's see if this actually works:

case class =- (i: Int, j: Int)
> defined class $eq$minus

val a =- b = =-(2, 3)  // infix syntax for pattern match
> a: Int = 2
  b: Int = 3

// Yes. it works!

// This is same as:
val =-(a, b) = =-(2, 3)