Clojure: if-let and when-let | Hacker News Reader

Clojure: if-let and when-let

blog.jayfields.com

PG has written about if-let, when-let, while-let and the likes in On Lisp, but calls them anaphoric macros instead. They are basically the same macros, but without the use of bindings (more brevity). You should take a look at it if you're interested.

http://lib.store.yahoo.net/lib/paulgraham/onlisp.pdf - Chapter 14 - Anaphoric Macros is on page 189.

Also appears to be available online: http://www.bookshelf.jp/texi/onlisp/onlisp_15.html#SEC99

It's a design choice to value explicit naming (and easier nesting) over brevity.

Anaphoric macros are not idiomatic in Clojure (there is only one: proxy), implicit introduction of locals is frowned upon (and you have to fight the hygienic macro system to create them).

Oh, I am by no means against explicit naming. I'm just recommending people to read chapter 14 of On Lisp if they find if-let and when-let good macros they use often.

They are basically the same macros, but without the use of bindings (more brevity).

So if-let and when-let are not really anaphoric.

In Common Lisp, I use b-when and b-if that Kenny Tilton wrote:

  (defmacro b-when (bindvar boundform &body body)
    `(let ((,bindvar ,boundform))
       (when ,bindvar
         ,@body)))

  (defmacro b-if (bindvar boundform yup &optional nope)
    `(let ((,bindvar ,boundform))
       (if ,bindvar
           ,yup
         ,nope)))

Usage:

  (b-when foo (plusp 1)
	(frob foo))

I find your lack of parentheses disturbing. I'd personally prefer:

  (b-when foo (plusp 1)
    (frob foo))

These implementations also don't handle declarations correctly ;P A surprisingly tricky topic.

Hmm, did you mean...

  (bwhen (foo (plusp 1))
      (frob foo))

bwhen is actually the first version of the macro, heh. It does seem a bit more idiomatic with parens, but I've gotten used to the other.

AND-LET* is a nice generalization: http://srfi.schemers.org/srfi-2/srfi-2.html

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