Scala-“ val a = b = c”的赋值关联性

Question

As I read from book, scala operator associativity comes from left to right except operator ends with ":" char.

Given the val a = b = c , it becomes val a = (b = c) and makes a is initialized as Unit.

But why does not it become (val a = b) = c , and it cause that compile error because to use a Unit(returned from a=b ) to receive c ?

And after I really types (val a = b) = c , the compiler complains illeagal start of simple expression and pointers to val . Why are these two assignment operators not grouped from left to right?

Answer 1

The important thing to see here is that val a = b = c is a declaration and not an expression, but precedence and associativity are notions applying to expressions.

Looking at the Scala 2.11 Language Specification, 4.1 Value Declarations and Definitions , you can see that

val a = b = c

is a basic declaration falling into the PatVarDef case, so it can be read as (simplifying the Pattern2 part to the specific case of varid here):

'val' varid '=' Expr

Thus, here we have

• the variable identifier varid is a ,
• the expression Expr is b = c ,
• a is assigned the value b = c evaluates to, which happens to be Unit .

For the later see What is the motivation for Scala assignment evaluating to Unit rather than the value assigned?

Note that the above assumes that b is a var , eg,

val c = 17
var b = 3
val a = b = c

as b is reassigned in the third line (else you would get error: reassignment to val ). If you want to assign the same value to two val for some reason, you can use

val c = 17
val a, b = c