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我一直在研究计算机程序的结构和解释,并完成Haskell的练习.前两章很好(github上的代码),但第3章让我更难思考.

首先是谈论管理国家,以银行账户为例.他们定义一个函数make-withdraw通过

(define (make-withdraw balance)
    (lambda (amount)
        (if (>= balance amount)
            (begin (set! balance (- balance amount))
                balance)
            "Insufficient funds")))

这样您就可以执行以下代码:

(define w1 (make-withdraw 100))
(define w2 (make-withdraw 100))

(w1 50)
50

(w2 70)
30

(w2 40)
"Insufficient funds"

(w1 40)
10

我不确定如何在Haskell中模仿这个.我首先想到了一个使用State monad的简单函数:

import Control.Monad.State

type Cash    = Float
type Account = State Cash

withdraw :: Cash -> Account (Either String Cash)
withdraw amount = state makewithdrawal where
    makewithdrawal balance = if balance …