Clojure - How to make my macro expand before system macros?

Question

If I do, for example:

(defmacro qqq [] '(toString [this] "Qqq"))
(reify Object (qqq))

it fails because of reify sees (qqq) instead of (toString [this] "Qqq").

The usual solution is a macro that wraps "reify" call with my own thing, but it is longer and more intrusive.

How to make my macros stronger that usual macros to be expanded first?

Expecting something like:

(defmacro ^{:priority 100500} qqq [] '(toString [this] "Qqq"))
(reify Object (qqq))

or

(defmacro qqq [] '(toString [this] "Qqq"))
(expand-first #{qqq} (reify Object (qqq)))

@Brian Carper, But I suspect `expand-first` can be implemented externally (and later included in Clojure). Trying to do that. — Vi., Feb 13 '11 at 19:37

score 9 · Answer 1 · answered Mar 02 '11 at 18:37

9

There is a reader-macro to evaluate things at read-time (before macro-expansion time)..

(defn qqq [] '(toString [this] "Qqq"))
(reify Object #=(qqq))

I've never seen this done in "real" code, and I think most people would consider it a hack, but it's there if you need it.

answered Mar 02 '11 at 18:37

Chris Perkins

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Shortest version and no additional dependencies. Looks nice. What pitfalls / what `#=` is normally used for/ – Vi. Mar 03 '11 at 22:41

score 1 · Accepted Answer · answered Feb 13 '11 at 21:00

1

The macro that forces given user macros to expand first (requires clojure.walk):

(defmacro expand-first [the-set & code] 
 `(do ~@(prewalk 
  #(if (and (list? %) (contains? the-set (first %)))
  (macroexpand-all %)
  %) code)))

Who has ideas how to make it better?

answered Feb 13 '11 at 21:00

Vi.

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`prewalk` vs `postwalk`? `macroexpand-all` vs just `macroexpand`? – Vi. Feb 13 '11 at 22:15

Psyllo · Answer 3 · 2011-02-16T16:02:31.670

0

This will work:

(defn reifyx [x y]
  (eval `(reify ~x ~y)))

(defn qqq [] '(toString [this] "Qqq"))

(reifyx 'Object (qqq))

I found an apply-macro. I tried it, but it seems to be broken. The most important thing I gleaned from looking at apply-macro was that it solved the problem using eval just as I have.

edited Feb 16 '11 at 16:02

answered Feb 15 '11 at 07:59

Psyllo

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CompilerException Unable to resolve symbol: toString in this context – Vi. Feb 16 '11 at 01:07
Fixed code is `(eval \`(reify Object ~(macroexpand \`(qqq))))` All this "macroexpands" gets duplicated over and over if I use qqq often. Is there better fix? – Vi. Feb 16 '11 at 01:08
Oh, I'm sorry. I forgot to mention. `qqq` should be a function that returns a list rather than a macro. – Psyllo Feb 16 '11 at 06:51

Clojure - How to make my macro expand before system macros?

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