Well, a "list" is a "scanner". So your "not a scanner" doesn't make sense, at least from the point of view of Arc-F.

However if you mean "list" as in sequence of cons cells:

(def works-on-cons-cells-and-bools (x)
(err "this works only on cons cells and bools!"))
(defm works-on-cons-cells-and-bools ((t x cons))
(work-on-cons-cells x))
(defm works-on-cons-cells-and-bools ((t x bool))
(work-on-bool x))

Note that you can even define a unifying "type class" function which ensures that the given data is a cons cell or a bool, or is convertible to one (i.e. an analog to 'scanner). For example, you might want a "hooper" type class:

(def hooper (x)
(err "Not convertible to a bool or cons cell" x))
(defm hooper ((t x cons))
x)
(defm hooper ((t x bool))
x)

Then you can convert works-on-cons-cells-and-bools with the type class:

Then, someone can make a type which supports the "hooper" type class by either overloading hooper (and returning a true hooper), or overloading hooper and works-on-cons-cells-and-bools:

choice one:

(defm hooper ((t x my-type))
(convert-my-type-to-cons x))

choice two:

(defm hooper ((t x my-type))
x)
(defm works-on-cons-cells-and-bools ((t x my-type))
(work-on-my-type x))