如何计算函数调用次数(FP方法) [英] How to count the number of times a function was called, the FP way

问题描述

我目前正在与Haskell合作通过 SICP .练习1.15询问一个函数被调用了多少次.这个想法可能是您应该使用替换方法，但是我想知道如何在代码中使用它.

I am currently working through SICP with Haskell. Exercise 1.15 asks how many times a function is called. The idea is probably that you should use the substitution method, but I would like to know how to do so in code.

在命令式语言中，可以保留一个全局变量，并在每次调用该函数时对其进行递增.但是，您将如何在Haskell中进行操作(或使用纯功能性方法)?

In an imperative language one can keep a global variable and increment it every time the function is called. But how would you go about it in Haskell (or the pure functional way)?

推荐答案

您可以使用Writer monad来完成此操作，前提是可以将对相关功能的所有调用归为一个do块. :

You can use the Writer monad to accomplish this, provided that all of the calls to the function in question can be grouped together into a do block:

import Control.Monad.Writer

myFunc :: Int -> Int -> Writer (Sum Int) Int
myFunc a b = tell (Sum 1) >> return (a + b)

callMyFunc :: ((Int, Int, Int), Sum Int)
callMyFunc = runWriter $ do a <- myFunc 2 3
                            b <- myFunc 8 7
                            c <- myFunc 3 5
                            return (a, b, c)

main = putStrLn $
    "myFunc was called "
        ++ show (getSum $ snd callMyFunc)
        ++ " times and produced "
        ++ show (fst callMyFunc)

哪个输出:

myFunc was called 3 times and produced (5,15,8)

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