Comments on: Clojure: when-let macro

By: Mark Needham

Mark Needham — Sat, 16 Jul 2011 10:11:00 +0000

@mdeboard:disqus thanks for the feedback!

I was trying to use the language that I see used elsewhere to describe what I understand of the macro but as you point out that doesn’t help!
In normal English the when-let macro will execute the second argument passed to it assuming that the value assigned to the value inside the [] is not nil.

By: Anonymous

Anonymous — Sat, 16 Jul 2011 04:33:00 +0000

“‘when-let’ is used when we want to bind an expression to a symbol and only execute the body provided as the second argument to the macro if that symbol evaluates to true.”
I really appreciate you taking the time to post this info but cloudy writing like this is endemic to posts about Clojure. Clojure experts (or at least people who can reliably program in Clojure, e.g. not me) seem to take great pleasure in obfuscating what they mean with bizarre sentence structure.

In other words: Huh?

By: Patrick Logan

Patrick Logan — Wed, 09 Dec 2009 03:19:20 +0000

“why don’t you use destructuring for your argument list?”

There’s some code in the core lib that could be written more simply if the order of some functions and macros in that file were rearranged. I ran into that adding a…

(cond …
( => fn)
…)

clause to the cond macro. I started puling things up toward the top of the file to make coding that more simple.

http://patrickdlogan.posterous.com/updated-clojure-cond-macro-with-test-value-ap

I’ve since not used that kind of clause enough to propose it and all the rearranging of the core.

By: Mark Needham

Mark Needham — Tue, 08 Dec 2009 18:16:15 +0000

Ah, I dunno! I just copied that ‘when-let’ macro out of the core library

By: Ola Bini

Ola Bini — Tue, 08 Dec 2009 17:59:54 +0000

Just out of curiosity, why don’t you use destructuring for your argument list? Then you can remove your assertions AND your extra bindings:

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(defmacro when-let
“bindings => binding-form test

When test is true, evaluates body with binding-form bound to the value of test”
[[form tst] & body]
`(let [temp# ~tst]
(when temp#
(let [~form temp#]
~@body))))

By: Patrick Logan

Patrick Logan — Tue, 08 Dec 2009 16:52:19 +0000

Line 12 is establishing the binding to the symbol used in the when’s body, i.e. the symbol ‘a in your example.

The symbol ‘a is only bound to 2 *after* the test for 2 being a truthy value.