Question

module Main where

qsort :: Ord a => [a] -> [a]
qsort [] = []
qsort (x : xs) = qsort smaller ++ [x] ++ qsort larger
                 where
                   smaller = [a | a <- xs , a <= x]
                   larger  = [a | a <- xs , a >  x]

main = do qsort [1,3,2]

我收到以下错误

Couldn't match expected type `IO t0' with actual type `[a0]'         
In the expression: main
When checking the type of the function `main'

我做错了什么？

Answer 1

do块中的所有函数必须与返回的monadic值匹配。你可以改为写

main = do
    print (qsort [1, 3, 2])

因为print返回IO值。同样，如果您使用Maybe monad，则必须执行类似

的操作

-- lookup :: Eq k => k -> [(k, v)] -> Maybe v
-- listToMaybe :: [a] -> Maybe a

firstElementOf :: Eq q => k -> [(k, [v])] -> Maybe v
firstElementOf key assocMap = do
    v <- lookup key assocMap
    first <- listToMaybe v
    return first

这是有效的，因为lookup和listToMaybe都返回Maybe，这是do块的返回值，由{{1}的类型签名指定}}

查看firstElementOf的类型，它只会返回qsort，而不是[a]，因此无法直接在IO something的do块中使用。您还可以使用main：

将其返回值分配给名称

let

Haskell主要功能

1 个答案: