Clojure的eval没有 "see"本地符号

Question

我正在 Clojure 中尝试 eval:

(let [code_as_data '(if (< sequ) on_true on_false)
      sequ [1 3 5]
      on_true "sequence is sorted in ascending order"
      on_false "sequence is NOT sorted"]
  (eval code_as_data))

CompilerException java.lang.RuntimeException: Unable to resolve symbol: sequ in this context, compiling:(/tmp/form-init3253735970468294203.clj:1:25)

如何定义符号以便 eval“看到”它们？

Answer 1

向运行时由 eval 生成的代码提供本地数据的最简单方法是生成一个接受参数的表单。

(let [code-as-data '(fn [sequ on-true on-false]                                 
                      (if (apply < sequ)                                        
                        on-true                                                 
                        on-false))                                              
      f (eval code-as-data)]                                                    
  (f [1 3 5]                                                                    
     "sequence is sorted in ascending order"                                    
     "sequence is NOT sorted"))

当然，由于函数是将运行时值插入已知形式的标准方法，因此实际上根本不需要使用 eval。无需 eval 即可更简单地表达相同的功能:

(let [f (fn [sequ on-true on-false]                                             
          (if (apply < sequ)                                                    
            on-true                                                             
            on-false))]                                                         
  (f [1 3 5]                                                                    
     "sequence is sorted in ascending order"                                    
     "sequence is NOT sorted"))

在实际代码中，仅当需要在运行时生成逻辑时(例如，如果用户提供新算法)，才需要 eval 版本。如果期望用户将代码编写为函数很麻烦，您可以采取折衷方案:

(defn code-with-context                                                         
  [body sq t else]                                                              
  (let [f (eval (list 'fn '[sequ on-true on-false] body))]                      
    (f sq t else)))                                                             



(code-with-context (read-string "(if (apply < sequ) on-true on-false)")         
                   [1 3 5]                                                      
                   "sequence is sorted in ascending order"                      
                   "sequence is NOT sorted")