Haskell主要函数

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 block 中的所有函数都必须与返回的一元值匹配。你可以改为写

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

因为print返回一个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

这是可行的，因为 lookup 和 listToMaybe 都返回一个 Maybe，它是整个 do 的返回值> 由 firstElementOf 的类型签名指定的 block 。

看qsort的类型，它只返回[a]，而不是IO some，所以不能直接使用在 main 的 do block 内。您还可以使用 let 将其返回值分配给名称:

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