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; a multi w/ 2 parameters, defined for a mix of int and float
>>> , :add ( :int :int ) [ __int+__ ] defmulti
>>> , :add ( :float :float ) [ __float+__ ] defmulti
>>> , :add ( :int :float ) [ 'int->float dip add ] defmulti
>>> , :add ( :float :int ) [ int->float add ] defmulti
>>> 1 2 add
3
>>> 1.0 add
4.0
To provide a "fallback"/"default" for a multi, use a signature
consisting only of :_ keywords (the number matching the arity).
>>> , :add ( :_ :_ ) [ nil ] defmulti
>>> "foo" () add
nil
NB: defmulti always creates or extends a multi in the current
module; to extend a multi from e.g. the prelude, alias it first.
NB: multis are "global state": extending e.g. a prelude multi will affect all code using that multi.
NB: the language specification prohibits "redefining" a multi for the same signature but this is currently not enforced by the implementations.