Question

我正在尝试使用WAI编写一个简单的Fibonacci Web服务器，但我无法弄清楚这些类型。这段代码是我想要做的事情的本质，但它已经破碎了。 getQueryArg函数返回Maybe ByteString，我想在我的fibHandler函数中使用它。

如何正确处理Maybe中的fibHandler？
如何在Maybe上调用putStrLn？我正在尝试fmap，但我似乎无法做到正确。

{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.HTTP.Types
import Network.Wai.Handler.Warp (run)
import Data.ByteString.Lazy.Char8 ()  -- Just for an orphan instance
import Control.Monad.IO.Class (liftIO)
import Data.Conduit
import Data.String.Utils
import Data.ByteString as BS (ByteString, putStrLn)
import Data.ByteString.Char8 as B (unpack)
import Data.Text as T (intercalate, pack, unpack)

app :: Application
app req
    | rawPathInfo req == "/fib" = fibHandler req
    | otherwise = notFoundHandler

fibHandler :: Request -> ResourceT IO Response
fibHandler req = do
    let nStr = getQueryArg (queryString req) "n"
    fmap (liftIO . BS.putStrLn) n
    let n = read nStr
    return $ responseLBS
        status200
        [("Content-Type", "text/plain")]
        (show $ fib n)

fib :: Int -> Int
fib n = foldl (*) 1 [1..n]

getQueryArg :: Query -> BS.ByteString -> Maybe BS.ByteString
getQueryArg [] key = Nothing
getQueryArg ((k,v):qs) key
    | k == key = Just v
    | otherwise = getQueryArg qs key

notFoundHandler :: ResourceT IO Response
notFoundHandler = return $ responseLBS
    status404
    [("Content-Type", "text/plain")]
    "Not found"

main :: IO ()
main = do
    BS.putStrLn $ "http://localhost:8080/"
    run 8080 $ app

[更新：此代码的工作副本位于：https://gist.github.com/3145317]

Answer 1

使用Maybe值最简单的方法是使用case。

case getQueryArg foo bar of
    Nothing -> {- something went wrong, write some code to report an error -}
    Just x  -> {- everything went okay, and x is the result of the successful computation -}

一旦你完成了几十次，你就可以毕业于速记版本：

maybe ({- went wrong -}) (\x -> {- successful x -}) (getQueryArg foo bar)
fromMaybe {- default value -} (getQueryArg foo bar)
traverse_ B.putStrLn (getQueryArg foo bar) -- this trick is a personal favorite

具有WAI的斐波纳契网络服务

1 个答案: