Scala: what do ":=" and "::=" operator do?

I'm pretty new with 开发者_如何学编程scala. I skimmed through the book and stumbled these two operators in code. What do they do ?

Syntactic Sugar

There is some syntactic sugar that applies when using operators in Scala.

Consider an operator *. When compiler encounters a *= b, it will check if method *= is defined on a, and call a.*=(b) if possible. Otherwise the expression will expand into a = a.*(b).

However, any operator that ends with a : will have the right and left arguments swapped when converting to method invocation. So a :: b becomes b.::(a). On the other hand a ::= b becomes a = a.::(b) which could be counter-intuitive due to the lack of the order reversal.

Because of the special meaning, it is not possible to define an operator :. So : is used in conjunction with other symbols, for example :=.

Meaning of Operators

Operators in Scala are defined by the library writers, so they can mean different things.

:: operator is usually used for list concatenation, and a ::= b means take a, prepend b to it, and assign the result to a.

a := b usually means set the value of a to the value of b, as opposed to a = b which will cause the reference a to point to object b.

This calls the method : or :: on the object at the left hand side, with the object at the right hand side as argument, and assigns the result to the variable at the left hand side.

foo ::= bar

Is equivalent to

foo = foo.::(bar)

See the documentation for the : or :: method of the object's type.

(For collections, the :: method appends an element to the beginning of the list.)