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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