both keywordp and macrop then eval from a functionp

Question

'Hope it won't bother my using defmacros named in the style of keywords (colon-prefixed, e.g., :set-this or :get-that): it apparently tricks ELisp from otherwise requiring quoted symbols (') or function symbols (#').

For example:

(foo :set-this value-that) ; line 0

Nevermind that: the problem goes as follows.

Say there are two "keywordp"-like defmacros with one parameter, and given a cl-flet* context, there should be two similar forms (as in line #0), but with two different macros and arguments.

An MWE is the following.

(defmacro :m (x) `(length ,x)) ; line 1
    ⇒ :m
(and (keywordp :m) (macrop :m)) ; line 2
    ⇒ t

(:m `(3)) ; line 3
    ⇒ 1

(defmacro :w (x) `(length (nreverse ,x))) ; line 4
    ⇒ :w
(and (keywordp :w) (macrop :w)) ; line 5
    ⇒ t

(:w `(6)) ; line 6
    ⇒ 1

(defun x (y z) (eval `(,y ,z))) ; line 7
    ⇒ x

(x :m `(8)) ; line 8
    ⇒ Invalid function: 8

(x :w '(9)) ; line 9
    ⇒ Invalid function: 8

(- (eval `(:m '(8)) (eval `(:w `(9))) ; line 10
    ⇒ 0

The faulty code is in line #7.

In these simple cases, it don't matter if the argument-list is backquoted `(8), quoted '(9), or returned as (list 8).

Lines #8 and #9 show what is wrong.

Yet, line #10 shows that evaluating as it "should be" is possible.

Now, if instead of defun there were a defmacro:

(defmacro x (y z) `(eval (,y ,z))) ; line 11
    ⇒ x

then lines #8 or #9 would be equivalent to line 10, but this is impossible within a cl-flet*.

Any hints? Thank s.

Answer 1

Try this for line 7:

(defun x (y z) (eval `(,y ',z)))

That is, ensure that the second argument is quoted in the expansion. When you call (x :m `(8)), in your version the body of x ends up as:

(eval '(:m (8)))

which obviously implies a call to the function 8. With the extra quote in the definition of x, it would try this instead:

(eval '(:m '(8)))

which passes the list (8) to the :m macro without evaluating it.