Is where a canonical doc about ~' or I have to read the core.clj?
On 8月21日, 下午10时40分, Nicolas Oury <nicolas.o...@gmail.com> wrote:
> Clojure whitin a `(...) context resolves the name space of symbols.
> This is most often what you want and prevent some name collisions at
> macro expension.
>
> But as binders can not be name-space resolved this do not allow to write:
>
> `(let [and ....])
>
> this would expand to (let [ns/and ...]) which is forbidden.
>
> The usual solutions (choose the first applicable) :
>
> - use automatic name generation
> `(let [and# ....] and#)
>
> - generate a symbol name:
> (let [my-fresh-sym (gensym "foo_")]
> `(let [~my-fresh-sym ...])
>
> It is mostly useful when the same symbols occurs in a complex
> structure of `(...)s.
>
> - If you really need to choose the name a binding symbol: use ~'symbol.
> This says to the evaluation of `() "replace by exactly symbol"
> So
> (`let [~'and ...]) will expand to (let [and ...] ) this is a problem
> because it can conflict with some other names.
> However, in some case it is very useful/necessary. Especially when you
> want to expose some symbols to the macro user.
>
>
>
>
>
>
>
> On Sat, Aug 21, 2010 at 3:20 PM, limux <liumengji...@gmail.com> wrote:
> > This is some code in a blog of William Gropper, the useage of ~'=
> > confused me, waiting some more detailed explain, advanced thanks
>
> > (defmacro filter
> > [pred query]
> > `(let [~'and (fn[& xs#] (apply str (interpose " AND " xs#)))
> > ~'or (fn[& xs#] (apply str (interpose " OR " xs#)))
> > ~'= (fn[x# y#] (sql-exp "=" x# y#))
> > ~'> (fn[x# y#] (sql-exp ">" x# y#))
> > ~'< (fn[x# y#] (sql-exp "<" x# y#))
> > ~'like (fn[x# y#] (sql-exp "like" x# y#))
> > ~'in (fn[x# xs#]
> > (str (sqlize x#) " IN (" (apply str (interpose ", " xs#)) ")"))]
> > (apply str ~query " where " ~pred)))
>
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