I would like to generate code: (nth 0 x) (nth 1 x) ... (nth n x)
where x is just some variable name, and n is some number.
I'm trying do this in a following way:
(defmacro gen(n)
(loop for i from 1 to n do
`(nth i x))
)
Checking how it expands by:
(print (macroexpand-1 '(gen 5)))
Console output is: NIL. How to do it properly?
Consider what the value of the following code is:
(loop for i from 1 to 5
do `(nth ,i x))
Since there's no collection happening, the return value of the loop is nil. If we change do to collect:
(loop for i from 1 to 5
collect `(nth ,i x))
We see that we are getting somewhere. However, the resulting list is not actually valid Common Lisp code (and relies on there being a variable x in the environment where the macro is used).
It is not clear what you want to do with these (just run them? they're side-effect free, so just wrapping this in a progn feels somewhat useless), but you need to either cons a progn, list or similar to the front of the list of lists to make it into valid code.
(defmacro gen (n &key (var 'x) (accumulator 'list))
(cons accumulator
(loop for i from 1 to n
collect `(nth ,i ,var))))
This eventually gives us this macro that seems to actually do something approaching "valid".
