Generating a symbol from a string and applying it as a function

问题

I'm just learning clojure, and I'm hitting a wall.

I'm trying to read an arithmetic expression as an infix string and process it in Clojure.

e.g. "1 + 2" -> (+ 1 2)

I read in the "+" and turn it into a symbol like this:

(def plus (symbol "clojure.core" "+"))

Which appears to work properly, but when I call it, I don't get what I'm expecting:

user=> plus
+

user=> (plus 1 1)
1
user=> (plus 1 2)
2
user=> (plus 1 2 3)
ArityException Wrong number of args (3) passed to: Symbol  clojure.lang.AFn.throwArity (AFn.java:437)

What am I missing here?

回答1:

Symbols have a function attached to them by default. The function that is attached to them by default is look this key up in a map. That is why your plus behaves oddly. It is attempting to look up elements in a map.

(plus 1 1) This is really look the symbol + up in the map 1 and if not found return a default value of 1.

(plus 1 2) Same as above except default value is 2.

clojure docs for symbols

回答2:

What's your reason to write such code? If you want to have function called plus which gonna be + synonym just write (def plus +).

Clojure + is normal function. You can use it like (+ 1 2 3 4 5). There's no reason to turn it into symbol.

Clojure have no operators at all. Only functions and macros.

Still, if you insist on using symbol you need to eval it like so

(def plus (eval (symbol "clojure.core/+"))).

Have a look on class of (symbol "clojure.core/+") and + itself.

(class (symbol "clojure.core/+")) ;;clojure.lang.Symbol

(class +) ;;clojure.core$_PLUS_

Symbols themselves are not callable as functions which are "under this symbols". If you want to "turn symbol into callable function" you have to eval it.

来源：https://stackoverflow.com/questions/19795906/generating-a-symbol-from-a-string-and-applying-it-as-a-function