The following Vala code works fine:

void main () {
    print (Test.ONE.pretty_print () + "\n");
}
enum Test {
    ONE;

    public string pretty_print () {
        return @"The value of this enum is $this";
    }
}

The following Genie code:

init
    print( Test.ONE.pretty_print() )
enum Test
    ONE
        def pretty_print():string
            return @"The value of this enum is $this"

produces the error:
test.gs:5.3-5.5: error: syntax error, expected end of line but got `def' with previous `tab indent'
		def pretty_print():string

---

Vala allows a function to be defined after the enum values are listed. If this was to be implemented in Genie then a similar scheme is probably best. It would mean adding some additional code to the end of parse_enum_declaration () n the Genie parser. Vala uses the semi-colon to denotes the end of the list of values, but it would be more Genie like to use a new block, i.e. an indent.

One test would be to convert https://git.gnome.org/browse/vala/tree/vala/valagenietokentype.vala to Genie, although this couldn't go in mainline because it would only build with the most recent versions of Vala.