我喜欢通过制作像计算器这样的小工具来学习一门新语言。
虽然我已经搜索了很多关于特定案例的惯用示例（例如数组和列表的惯用用法），但我不知道如何将它们放在一起以惯用的方式编写这个小计算器。
所以这是我的代码：
(defn pre-process [s]
"Seperate operands with operators and replace ( with l, ) with r"
(re-seq #"\d+|[\+\-\*\/lr]"
(clojure.string/replace s #"\(|\)" {"(" "l" ")" "r"})))
(defn calc-once [stk]
"Take one operator from operator stack and apply it to
top two numbers in operand stack"
(let [opt (:opt stk)
num (:num stk)
tmp-num (pop (pop num))
tmp-opt (pop opt)
last-two-num [(peek (pop num)) (peek num)]
last-opt (peek opt)]
(assoc stk
:num (conj tmp-num (apply (eval last-opt) last-two-num))
:opt tmp-opt)))
(defn clean-stk [stk]
(loop [stk stk]
(if (> (count (:opt stk)) 1)
(recur (calc-once stk))
(peek (:num stk)))))
(defn calc
"A simple calculator"
[s]
(clean-stk
(reduce
(fn [stk item]
(let [item (read-string item)
operators #{'+ '- '* '/}
prio {'+ 0 ; Define operator priority here
'- 0
'* 1
'/ 1
'l -1
'r -1
'dummy -2}
add-to-num #(assoc %1 :num (conj (:num %1) %2))
add-to-opt #(assoc %1 :opt (conj (:opt %1) %2))
item-prio (get prio item)
last-prio #(get prio (peek (:opt %)))]
(cond
(number? item) ; It's number
(add-to-num stk item)
(get operators item) ; It's operator
(loop [stk stk]
(if (<= item-prio (last-prio stk))
(recur (calc-once stk))
(add-to-opt stk item)))
(= 'l item) ; (
(add-to-opt stk item)
(= 'r item) ; )
(loop [stk stk]
(if (not= (peek (:opt stk)) 'l)
(recur (calc-once stk))
(assoc stk :opt (pop (:opt stk)))))
:else
(println "Unexpected syntax: " item))))
(apply (partial list {:num '() :opt '(dummy)}) ;; Basic structure of stack
s))))
调用后：
(calc (pre-process (read-line))))
它可以计算如下：
(1 + 3) * ( 4 + 4)
32
我认为我的代码可以通过
消除那些
cond或者
尝试将其
{:num '() :opt '()}变成更易于访问的数据结构
，但我不知道。
希望有人可以给我一些建议或指出我的代码（或我的问题的语法：P）的问题。
=====================================谢谢:)========== =======================
谢谢你们的帮助。我修改了我的代码，现在看起来好多了。但我还有一些问题：
- 我应该将一些不太通用的函数（例如
add-to-num）放入全局变量中吗？ - 有没有人发现有时在 FP 中命名一个函数非常困难？特别是对于那些非泛型函数。
这是我的新代码：
(def prio
{'+ 0 ; Define operator priority here
'- 0
'* 1
'/ 1
'l -1
'r -1
'dummy -2})
(def operators #{'+ '- '* '/})
(defn pre-process [s]
"Seperate operands with operators and replace ( with l, ) with r"
(re-seq #"\d+|[\+\-\*\/lr]"
(clojure.string/replace s #"\(|\)" {"(" "l" ")" "r"})))
(defn calc-once [stk]
"Take one operator from operator stack and apply it to
top two numbers in operand stack"
(let [opt (:opt stk)
num (:num stk)
tmp-num (pop (pop num))
tmp-opt (pop opt)
last-two-num [(peek (pop num)) (peek num)]
last-opt (peek opt)]
(assoc stk
:num (conj tmp-num (apply (eval last-opt) last-two-num))
:opt tmp-opt)))
(defn process-stk [stk checker fn-ret]
(loop [stk stk]
(if (checker stk)
(recur (calc-once stk))
(fn-ret stk))))
(defn calc
"A simple calculator"
[s]
(process-stk
(reduce
(fn [stk item]
(let [item (read-string item)
add-to-num #(assoc %1 :num (conj (:num %1) %2))
add-to-opt #(assoc %1 :opt (conj (:opt %1) %2))
item-prio (get prio item)
last-prio #(get prio (peek (:opt %)))]
(cond
(number? item) ; It's number
(add-to-num stk item)
(get operators item) ; It's operator
(process-stk stk #(<= item-prio (last-prio %))
#(add-to-opt % item))
(= 'l item) ; (
(add-to-opt stk item)
(= 'r item) ; )
(process-stk stk #(not= (peek (:opt %)) 'l)
#(assoc % :opt (pop (:opt %))))
:else
(println "Unexpected syntax: " item))))
(apply (partial list {:num '() :opt '(dummy)}) ;; Basic structure of stack
s))
#(> (count (:opt %)) 1)
#(peek (:num %))))