如何从 Clojure 中的字符串中获取变量的值？

Question

I was doing a small project in clojure and I wonder if there is something like this:

(let [myvar "hello"] (println (read-var "myvar")))

where the "read-var" function finds that there is a variable with the name passed as string and returns it's value.

I found this load-string function but it seems that doesn't work with let bindings.

Thank you!

Answer 1

I would say that in case you're in need of this kind of behaviour, you're probably not doing it right. in fact i can't even imagine why would someone want to do this in practice

but there is a way

clojure macros have special implicit parameter, called &env , allowing you to get local bindings. So you could use this feature for local vars resolution at runtime:

(defmacro get-env []
  (into {} (map (juxt str identity)) (keys &env)))

notice that this macro doesn't require to know your desired var name at compile time, it rather just lifts the bindings from macro scope to runtime scope:

(let [x 10]
  (let [y 20]
    (get-env)))
;;=> {"x" 10, "y" 20}

(let [a 10
      b 20
      c 30
      env (get-env)]
  [a b c env])
;;=> [10 20 30 {"a" 10, "b" 20, "c" 30}]

even this

(let [a 10
      b 20
      c 30
      env (get-env)]
  (get-env))
;;=> {"a" 10, "b" 20, "c" 30, "env" {"a" 10, "b" 20, "c" 30}}

(let [x 10] (println ((get-env) "x")))
;;=> 10
;;   nil

so the behaviour is dynamic, which could be shown with this fun example:

(defn guess-my-bindings [guesses]
  (let [a 10
        b 20
        c 30]
    (mapv #((get-env) % ::bad-luck!) guesses)))

user> (guess-my-bindings ["a" "zee" "c"])
;;=> [10 :user/bad-luck! 30]

but notice that this get-env effect is limited to the bindings effective at it's expand-time. eg:

(let [x 10
      y 20
      f (fn [] (let [z 30]
                 (get-env)))]
  (f))
;;=> {"x" 10, "y" 20, "z" 30} ;; ok

(def f (let [x 10
             y 20]
         (fn [] (let [z 30]
                  (get-env)))))

(f)
;;=> {"x" 10, "y" 20, "z" 30} ;; ok

but

(let [qwe 999]
  (f))
;;=> {"x" 10, "y" 20, "z" 30} ;; oops: no qwe binding 

Answer 2

I am not aware of some approach to accomplish this if read-var has to be a function. If read-var were a macro and its argument is a string literal it would be possible to implement read-var so that the code that you wrote works. Another approach would be to build a macro read-var that uses eval , but that would also not be possible because eval cannot access local bindings as explained in this answer .

The closest I could come up with that (i) implements read-var as a function and (ii) lets you pass runtime values as arguments to read-var is the following:

(def ^:dynamic context {})

(defmacro with-readable-vars [symbols & body]
  `(binding [context (merge context ~(zipmap (map str symbols) symbols))]
     ~@body))

(defn read-var [varname]
  (get context varname))

and you can now use this code like

(let [myvar "hello"]
  (with-readable-vars [myvar]
    (println (read-var "myvar")))) ;; Prints hello

The difference compared to your code is that you have to declare the variables that should be readable using the with-readable-vars macro. Obviously, you can build another macro that combines let and with-readable-vars if you like:

(defmacro readable-let [bindings & body]
  `(let ~bindings
     (with-readable-vars ~(vec (take-nth 2 bindings))
       ~@body)))

(readable-let [myvar "hello"]
  (println (read-var "myvar")))

The above code assumes you are not using advanced features such as destructuring for your bindings.

Answer 3

Pretty simple here:

  (let [myvar "hello"]
    (println myvar))
  ;=> hello

Please see this sample project , esp. the list of documentation.

---

If you really want to pass the name of the variable as a string, you will need the eval function:

(ns tst.demo.core
  (:use tupelo.core tupelo.test)
  (:require
    [schema.core :as s]
  ))

(dotest
  (let [mydata "hello"]
    (is= "hello" mydata) ; works
  ))

(def myVar "hello") ; creates a Clojure 'Var':  tst.demo.coore/myVar

(dotest
   ; will fail if don't use fully-qualified namespace in string
    (let [parsed (clojure.edn/read-string "(println :with-eval tst.demo.core/myVar)")]
      (eval parsed)
      ;=> `:with-eval hello`
  ))