如何在 Haskell 中编写具有初始条件的递归函数？

Question

在 F# 中我可以这样定义：

let rec sum = function
| [] -> 0
| x::xs -> x + sum xs

看起来很方便。 Haskell中有它的对应吗？

Answer 1

假设您喜欢的功能是 "I don't have to repeat the name sum"，LambdaCase 扩展可以启用此功能：

{-# LANGUAGE LambdaCase #-}

module CaseExample where
import Prelude hiding (sum)

sum = \case
    [] -> 0
    x:xs -> x + sum xs

否则，没有扩展名的语法是

sum [] = 0
sum (x:xs) = x + sum xs

Answer 2

sum :: [Int] -> Int

sum [] = 0
sum (x:xs) = x + sum xs

另一种方式

 Prelude>:{
 Prelude|let sumE  xs' = case xs' of
 Prelude|                     [] -> 0
 Prelude|                     x:xs' -> x + sumE xs'
 Prelude| 
 Prelude|:}
 Prelude> sumE [1,2,3]
 6
 Prelude> sumE []
 0

Answer 3

这将允许您编写多行代码：

  Prelude> :set +m

然后试试这个代码。提示符从 prelude> 变为 prelude| :

Prelude> let sumE  xs' = case xs' of
Prelude|                     [] -> 0
Prelude|                     x:xs' -> x + sumE xs'

如何在 Haskell 中编写具有初始条件的递归函数？

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