What is the reason behind the `=>` in a self type?

Question

A self type looks like the following example:

trait A { self: String => }

This says, that trait A (or a subtype of it) must inherit the class String.

The keyword self is followed by : analogue to a variable in var s: String, where the type comes after :.

But what does the => state for in a self type ? What is the reason behind this ?

Answer 1

Just guess work... You need some specific delimiter of the self-type declaration. Imagine the => was just omitted. this: String would be a syntactically valid statement (although the type checker will complain).

So which should be the delimiter? You wouldn't want nested braces like trait A { self: String { ... }}. So which non-paired existing delimiters exist? => is the only one that I can think of.

Where is => used elsewhere? As sugar for function types (A => B) and for function bodies (i: Int => i + 1). Also for call-by-name arguments, and for the cases of a pattern match. This last usage is somewhat coherent with the self-type. It's like matching this to be of a particular type, and then defining the body depending on this 'match'. I don't know, maybe this is a silly analogy.