如何在Haskell中打印列表的各个元素？

我是Haskell的初学者，我想知道是否可以打印给定列表中的单个元素。我试图解决这个问题，但失败了。代码如下：

main :: IO()
main = do
    let list = [1,2,3]
    let size = length list
    let temp = print_elem list size
    print temp


print_elem :: [Int] -> Int -> Int
print_elem xs x = do
    let size = length xs
    let idx = size - x
    let element = xs !! idx
    putStr (show element)
    putStr(" ")
    let dummy = print_elem (xs (x-1))
    return " "

我想印一些像这样的东西

1 2 3

如果我简单地使用

putStr（showlist）

它将显示

[1,2,3]

，我不希望这样

但当我运行此代码时，会出现多个错误

printelem.hs:14:5: error:
    * Couldn't match expected type `Int' with actual type `IO b0'
    * In a stmt of a 'do' block: putStr (show element)
      In the expression:
        do let size = length xs
           let idx = size - x
           let element = xs !! idx
           putStr (show element)
           ....
      In an equation for `print_elem':
          print_elem xs x
            = do let size = ...
                 let idx = ...
                 let element = ...
                 ....
   |
14 |     putStr (show element)
   |     ^^^^^^^^^^^^^^^^^^^^^

printelem.hs:16:29: error:
    * Couldn't match expected type `Int -> [Int]'
                  with actual type `[Int]'
    * The function `xs' is applied to one argument,
      but its type `[Int]' has none
      In the first argument of `print_elem', namely `(xs (x - 1))'
      In the expression: print_elem (xs (x - 1))
   |
16 |     let dummy = print_elem (xs (x-1))
   |                             ^^^^^^^^

---

您如何解决此问题？

您的想法过于迫切。首先，您需要一个字符串列表，而不是整数列表。那是地图：

> map show [1,2,3]
["1","2","3"]

接下来，您希望将它们合并到一个单独的、空格分隔的字符串中。这是

Data.List.interlate

：

> import Data.List
> intercalate " " (map show [1,2,3])
"1 2 3"

然后可以将其传递到

putStrLn

（

print

将为您提供现有字符串的字符串表示形式）：

一种方法是

> mapM_ putStr (intersperse " " (map show [1,2,3])) >> putStrLn ""
1 2 3
it :: ()

这将逐个打印元素，并通过

show

转换为字符串，其间穿插空格

> intersperse " " (map show [1,2,3])
["1"," ","2"," ","3"]
it :: [[Char]]

mapM

在每个元素上映射一个

IO

动作构造函数

putStr:：String->IO（）

，并将它们作为一个动作组合序列执行

---

最后，此代码打印换行符。

在列表上使用

map

和

show

的原因是什么？

插入

需要一个字符串列表（或者更准确地说，

插入x

需要一个与

x

类型相同的值列表）<代码>插入“[1，2，3]”不进行类型检查。

> intersperse " " (map show [1,2,3])
["1"," ","2"," ","3"]
it :: [[Char]]