Clojure宏（如C的#define）中的文本替换功能

Question

I want to write a macro

(defmacro params [] 'a 'b 'c)

that will be used in places like this

;; without macro
(fnc a b c)

;; with macro
(fnc params) => (fnc a b c)

If you see this could be easily done by C's #define which was just a text replacement

But this just returns a 'c

Answer 1

The point of Lisp style macros is that they operate on code , not on text (or more specific, on the abstract syntax tree instead of the token sequence ), and this makes them immensely more useful than the C preprocessor.

What you actually seem to intend is to say that the function is to be called with three arguments that happen to be the values of variables of the same name at each call site. That's quite a few assumptions. Let's see how that would look in practice:

(def-magic-splat-macro params    ; whatever, doesn't exist
  a b c)

(defn foo [a]
  (let [b (frob a)
        c (blah)]
    (func params)))  ; What the hell

I do not see how this can be useful except to obfuscate.

If you want to define standard parameters:

(def standard-params [1 "quux" :never])  ; values!

(defn foo [bar]
  …
  (apply func standard-params))

Answer 2

I'm not quite sure why you want to do this, but in clojure one would typically use the following idom:

(defmacro with-params
  [params fncall]
  `~(concat fncall params))

(defn myfn [x y z]
  (spyx :myfn [x y z]))

(let [a 1
      b 2
      c 3]
  (println :with-params)
  (with-params [a b c]
    (myfn))

with result:

:with-params
:myfn [x y z] => [1 2 3]

If, however, you want to hard-code the code to always use params a, b, and c, you could do this:

(defmacro with-params-abc
  [fncall]
  `~(concat fncall '[a b c]))

(println :with-params-abc)
(with-params-abc
  (myfn))

with result:

:with-params-abc
:myfn [x y z] => [1 2 3]