Comments on: The Need for More Lack of Understanding

By: mb

mb — Sun, 17 May 2009 14:57:00 +0000

I’m also not a big fan of code walking. In general it bears a lot of (subtle) problems. And I would feel much more comfortable if something like quasiquote was in core complementing quote and syntax-quote. But Rich is not very keen on including quasiquote…

I saw a lot of its usage in scsh, eg. the shell and regex DSLs. And it worked there quite nicely. But Scheme brings it as a language construct. That is much more robust than a handcrafted hack. :]

By: Christophe Grand

Christophe Grand — Sun, 17 May 2009 11:08:00 +0000

Hi Meikel,
I have been bitten by code-walking (the previous Iteration of Enlive used unquote), I think I’ll avoid it for some time (until Clojure compiler is rewritten in Clojure). But I agree ‘unquote is maybe a better choice than ‘do to denote user-code in my current approach.

By: mb

mb — Sun, 17 May 2009 09:50:00 +0000

I use the following approach. It follows a bit Scheme quasiquote.
The implementation is basically a modified syntax-quote from
Clojure's compiler. The user can provide anything and verbatim
code is simply prefix with ~. As in a normal macro.

Eg:
(let [x 5]
(quasiquote (+ ~x 6))

will give

(+ 5 6)

Here is the implementation:

(defn- unquote?
"Tests whether the given form is of the form (unquote ...)."
[form]
(and (seq? form) (= (first form) `unquote)))

(defn- quasiquote*
"Worker for quasiquote macro. See docstring there. For use in macros."
[form]
(cond
(self-eval? form) form
(unquote? form) (second form)
(symbol? form) (list 'quote form)
(vector? form) (vec (map quasiquote* form))
(map? form) (apply hash-map (map quasiquote* (flatten-map form)))
(set? form) (apply hash-set (map quasiquote* form))
(seq? form) (list* `list (map quasiquote* form))
:else (list 'quote form)))

(defmacro quasiquote
"Quote the supplied form as quote does, but evaluate unquoted parts.

Example: (let [x 5] (quasiquote (+ ~x 6))) => (+ 5 6)"
[form]
(quasiquote* form))