As I am the culprit of having introduced it with a naive example, I'd
better admit it may not be very useful in practical scenarios across a
wide variety of use cases. For example, when there is an assertion
error with message "`m` should be a map" 14 levels down the stack, I'd
really wish it said "`m` -- Expected: map, Found: vector [:foo :bar]"
so that I can debug it quickly.
Pre-conditions and Post-conditions are a valuable debugging aid, and
to enable that we need very precise information. Unfortunately passing
a string error message cannot encapsulate enough error context. A more
complex example can be where the correctness of input must be
determined collectively (in association with other args) -- in those
cases one can only fall back on comparing input values and raise
IllegalArgumentException accordingly.
Regards,
Shantanu
On Jul 11, 10:40 pm, Timothy Washington <twash...@gmail.com> wrote:
> 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...
>
> > 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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