Note: This message was originally posted by ' Shantanu' on the "*Re: Clojure
for large programs*" thread.
I took a look at Shantanu's macros, and I like the concept a lot. But I
would prefer something baked into the :pre condition itself. The reason is
that it just removes a layer of indirection. If you dig into '*
clj/clojure/core.clj*', you can see that the 'fn' macro is using 'assert' to
test these conditions. Assert allows error messages to be applied, ie:
*user => (assert false) *
*user => (assert false "fubar") *
However, (defmacro fn ...) assumes that just the boolean condition is being
passed in, A). But I'd like to have the option to pass in a message B).
*A) *
**
*(def fubar *
* (fn []*
* {:pre [ (true? false) ] }*
* (println "Hello World")))*
*(fubar)*
*
*
*B) *
*(def thing *
* (fn []*
* {:pre [ [(true? false) "A false message"] ] }*
* (println "Hello World")))*
*(thing)*
I reworked the 'fn' macro, only for the :pre condition, as a demonstration (see
here <http://pastebin.com/fETV1ejJ>). The calling semantics don't change
that much. Is there any interest in putting this into core? I'd
use Shantanu's workaround otherwise, or in the interim.
Thanks
Tim Washington
twash...@gmail.com
416.843.9060
On Sun, Jul 3, 2011 at 11:42 AM, Shantanu Kumar <kumar.shant...@gmail.com>wrote:
>
>
> On Jul 3, 7:39 pm, Timothy Washington <twash...@gmail.com> wrote:
> > I'm using pre / post assertions quite a bit in a project I'm building.
> And I
> > too would love to see better or custom error messages for each assertion.
>
> That should be possible with a macro. For example, I use this:
>
> https://bitbucket.org/kumarshantanu/clj-miscutil/src/acfb97c662d9/src/main/clj/org/bituf/clj_miscutil.clj#cl-1009
>
> Maybe you need something like this(?):
>
> (defmacro verify-my-arg
> "Like assert, except for the following differences:
> 1. does not check for *assert* flag
> 2. throws IllegalArgumentException"
> [err-msg arg]
> `(if ~arg true
> (throw (IllegalArgumentException. ~err-msg))))
>
> Then use it thus:
>
> (defn foo [m]
> {:pre [(verify-my-arg "m must be a map" (map? m))]}
> (println m))
>
> Regards,
> Shantanu
>
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