Comment by jstimpfle

I think you've misunderstood what I said. For example here: https://golang.org/src/go/parser/parser.go you can clearly see how e.g. token.MAP is a special case.

    func (p *parser) tryIdentOrType() ast.Expr {
        ....
        case token.MAP:
            return p.parseMapType()

There is a special-cased function parseMapType() function in the Go parser. This isn't generic at all. I mean even the syntax (as in func foo(bar map[String]int) ...) does not seem to be generic at all.

Yes, you can use any type as keys and values. But that's a far cry from an abstract system that lets the user define any parameterizable data type. And that's for good reason: By hardcoding just the most important parameterizable data types, the problems that an abstract system would bring can be avoided.