Haskell主要函数

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'

我究竟做错了什么？

一个内的所有函数do块必须与返回的一元值匹配。你可以写

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

Because print返回一个IO价值。同样，如果您使用的是Maybemonad，你必须做类似的事情

-- 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

这有效是因为lookup and listToMaybe两者都返回一个Maybe，这是整体的返回值do由类型签名指定的块firstElementOf.

看看类型qsort，它只返回[a], not IO something，所以不能直接在里面使用main的阻止。您还可以使用以下命令将其返回值分配给名称let:

main = do
    let result = qsort [1, 3, 2]
    print result