解析錯誤輸入「=」haskell錯誤

Question

當我只寫main = do ...塊時，它完美的工作。當我把只有stories = do...塊它也可以。也許有一個縮進問題。

下面是代碼：

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) 

有什麼不對呢？

Answer 1

儘管這個事實並不像Python那樣廣告，但Haskell確實有syntactically significant indentation。在你的情況在do S中的機構必須對代碼進行縮進：

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)