Question

当我只写main = do ...块时，它完美无缺。当我只放置stories = do...块时，它也有效。也许有一个缩进问题。

以下是代码：

stories = do
let str0 = "There once was "
str1 <- ["a princess ", "a cat ", "a little boy "]
let str2 = "who lived in "
return (  str0 ++ str1  )

main = do
let len = length stories
putStrLn ("Enter a number from 0 to " ++ show (len - 1))
n <- readLn
putStrLn ""
putStrLn (stories !! n)

它出了什么问题？

Answer 1

虽然这个事实并不像Python那样广告，但Haskell确实有syntactically significant indentation。在您的情况下，do s主体中的代码必须缩进：

stories = do
    let str0 = "There once was "
    str1 <- ["a princess ", "a cat ", "a little boy "]
    let str2 = "who lived in "
    return (  str0 ++ str1  )

main = do
    let len = length stories
    putStrLn ("Enter a number from 0 to " ++ show (len - 1))
    n <- readLn
    putStrLn ""
    putStrLn (stories !! n)

输入“=”haskell错误时解析错误

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