Another option: create a helper function to do the work and have the macro
call that:
(defn foo* [bar body-thunk]
{:pre [(string? bar)]}
(body-thunk)) ;or whatever
(defmacro foo [bar & body]
`(foo* ~bar (fn [] ~@body))
Justin
On Tuesday, March 20, 2012 9:07:45 AM UTC-4, Chas Emerick wrote:
>
> Your second `foo` call fails in 1.2 as well. If there was ever a time
> when it would have succeeded, it would have been a bug. Since `foo` is a
> macro, it receives its arguments unevaluated, so `(str "baz" 34)` will
> always be received as a list of three values.
>
> The syntax-quote precondition simply expands into a list containing the
> symbol 'clojure.core/string? and the value of bar; this is a logically-true
> value, and so does not trigger any error.
>
> You want something like:
>
> (defmacro foo
> [bar & body]
> `(let [bar# ~bar]
> (when-not (string? bar#) (IllegalArgumentException. "msg"))
> ...))
>
> - Chas
>
> On Mar 20, 2012, at 8:34 AM, Shantanu Kumar wrote:
>
> > Hi,
> >
> > The way preconditions are invoked in Clojure 1.3.0 seems to have
> > changed since Clojure 1.2:
> >
> > (defmacro foo
> > [bar & body]
> > {:pre [(string? bar)]}
> > ...)
> >
> > (foo "bar34" ...) ; doesn't complain, which is OK
> > (foo (str "baz" 34) ...) ; Error! (I wanted this to pass)
> >
> > When I write the precondition like this:
> >
> > {:pre [`(string? ~bar)]}
> >
> > It doesn't seem to check the precondition at all in 1.3.0.
> >
> > Can somebody suggest me what am I missing? I want both the examples
> > above to be verified and passed as OK.
> >
> > Shantanu
> >
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