I've read what he has to say about backquotes, but he really doesn't give as clear and concise a method for figuring out what's going on as Graham does.
To evaluate a backquoted expression, you remove the outermost backquote and each matching ~, and replace the expression following each matching ~ with its value. Evaluating an expression that begins with a ~ causes an error. A ~ matches a backquote if there are the same number of ~ as backquotes between them. This means that in a well-formed expression the outermost backquote matches the innermost ~s. THE ABOVE LAST STATEMENT IS CRUCIAL IN UNDERSTANDING HOW IT WORKS! Suppose that x evaluates to say user/a, which evaluates to 1; and that y evaluates to user/b, which evaluates to 2. You can set this up pretty easily like this: (def x `a) (def a 1) (def y `b) (def b 2) Evaluating the follwing expression ``(w ~x ~~y) gives: `(user/w ~user/x ~user/b) Actually, what the REPL will print out is: (clojure.core/seq (clojure.core/concat (clojure.core/list (quote user/ w)) (clojure.core/list user/x) (clojure.core/list user/b))) But it's the SAME THING! ;) Now evaluate again, and of course what you get is: (user/w user/a 2) A piece of cake ;) Rock On Aug 11, 11:07 pm, rob <r.p.l...@gmail.com> wrote: > Let Over Lambda by Hoyte contains a very lucidly well-written > discussion of quotation levels in macros. It also includes a pretty > useful technique for being explicit about variable capture. The code > is in Common Lisp, but will mostly only differ syntactically from the > Clojure code (for the basic example, I mean). There is one non- > trivial difference to implement for use in Clojure, which is that > Lisp-1 versus Lisp-2 requires being concerned about capture of > function names as well, but as indicated in the text this is a simple > modification to make. > > On Aug 11, 1:21 pm, Rock <rocco.ro...@gmail.com> wrote: > > > For nested backquotes, Paul Graham in ANSI Common Lisp gives the > > clearest explanation. He makes it look almost trivial. As a matter of > > fact, once you get the hang of it, it's not at all that complicated. > > > I've pretty much verified that what he says concerning CL backquotes > > applies perfectly to Clojure as well. The only difference is that you > > have to remember that in Clojure, backquotes can *resolve* unqualified > > symbols too (which is actually powerful and cool). So no problem > > there. > > > Rock > > > On Aug 11, 6:00 pm, Jonathan Smith <jonathansmith...@gmail.com> wrote: > > > > For the sake of contradicting myself, in this paper Alan Bawden > > > explains quasi-quote much better than I can. > > > >https://eprints.kfupm.edu.sa/60346/1/60346.pdf > > > > Note the explanation towards the end of defmacro and constructs such > > > as ',' (in clojure I believe it would be '~' ) > > > > Also note that Clojure has the added wrinkle of namespace > > > qualification. > > > > On Aug 11, 1:31 am, Jonathan Smith <jonathansmith...@gmail.com> wrote: > > > > > On Aug 10, 3:20 pm, Dragan Djuric <draga...@gmail.com> wrote: > > > > > > For example: > > > > > > (defmacro creator [param] > > > > > `(defmacro created [p] `(the code...)) ;; note the nested quote... > > > > > how to resolve that? any examples? > > > > > Although I wouldn't cite my own code as a necessarily *good* or easy > > > > to understand example, I'll pimp it anyway as it is written and > > > > already on the net... > > > > >http://github.com/JonathanSmith/CRIB/blob/89a3f4d9c797ef6f4e4f456001f... > > > > > A macro is exactly the same as a function, except it has special > > > > evaluation rules. > > > > You'll notice that in what I posted, I only use defmacro once, > > > > everything else is a function that returns a list > > > > > (I've found that this gives me extra flexibility later on when I want > > > > to continue extending the macro-metaphor... (or if I happen to want to > > > > do runtime definition of certain things, I can swap out the macro for > > > > a function call and an 'eval')). > > > > > You'll also notice that syntax quote qualifies every symbol that it is > > > > passed. > > > > Your particular example wouldn't work, because you can't define a name- > > > > space qualified symbol. There are two ways that you could circumvent > > > > this. > > > > 1.) use regular old list > > > > (list 'defmacro 'created ['p] `(code ...)) > > > > > 2a.) Escape and quote certain things: > > > > > `(defmacro ~'created [p#] `(code goes here..)) > > > > > or > > > > > 2b.) `(defmacro ~'created [p#] ~(fn-that-returns-your-code p# and > > > > probably some args and stuff)) > > > > > or > > > > > 2c.) > > > > (let [fn-that-generates-expansion > > > > (code-to-generate-a-fn-that-generates-an-expansion)] > > > > `(defmacro ~'created [p#] (~fn-that-generates-expansion p#))) > > > > > You don't actually need to use nested backquote* (For anything**, > > > > AFAIK), I tend to avoid it because I think it makes the code fairly > > > > unreadable (yes, more unreadable than what I posted). I normally use a > > > > helper function and pass the arguments (similar to a foo and foo-1 > > > > function when doing recursion). > > > > > --------------------- > > > > * backquote == syntaxquote > > > > > ** I think I used nested backquotes one time in common lisp for > > > > something, but I can't remember *why* it was necessary, in particular. > > > > I think it is the case where you want to pass a symbol from one macro- > > > > environment to the next, say generating a bunch of accessors... --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. 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