我以为我开始理解Haskell中的IO,直到遇到以下问题.
我有以下函数,它返回类型IO Float:
getFundPrice :: Int -> Int -> IO Float
getFundPrice fund date = do
priceList <- getFundPrice' fund date
let h = head priceList
return h
函数getFundPrice'使用takusen数据库库并返回类型IO [Float]的列表.
我可以使用以下方法使用Hunit成功测试getFundPrice函数:
p <- getFundPrice 120 20100303
assertEqual
"get fund price"
10.286
(p)
困扰我的问题是以下函数定义:
lastPos :: Int -> (Shares,Float) -> Period -> Fund -> (Shares,Float)
lastPos endDate begPos [] fund = (fst begPos, trnPrice)
where trnPrice = do
price <- getFundPrice fund endDate
return price
我在尝试编译时遇到的错误是"无法匹配预期类型Float' against inferred typeIO …
我有一个现有的程序,它接受命令行参数(用户名,密码,日期),然后使用该Network.HTTP.Conduit库将xml消息发布到服务器.然后我解析结果,做一些工作并使用blaze-html写出文件.
这一切都像一个魅力; 但是,我以为我会使用haskeline密码不可见.我能够创建一个命令行程序来获取用户提供的值并打印出来,但是如果我调用使用管道的函数,它就永远不会返回.
这是违规代码:
main = runInputT defaultSettings loop
where
loop :: InputT IO ()
loop = do
Just username <- getInputLine "WM username: "
Just password <- getPassword (Just '*') "WM password: "
Just date <- getInputLine "Date (YYYYMMDD): "
outputStrLn "querying WM..."
clients <- lift $ getWMClients username password
outputStrLn "successfully retrieved client list from WM..."
let outHeader = renderHeader date username
reportString <- mapM (\x -> createString x clients) cList
lift $ writeFile …我正在努力通过xml-conduit将响应从http-conduit转换为XML文档.
该doPost函数接受XML Document并将其发布到服务器.服务器使用XML文档进行响应.
doPost queryDoc = do
runResourceT $ do
manager <- liftIO $ newManager def
req <- liftIO $ parseUrl hostname
let req2 = req
{ method = H.methodPost
, requestHeaders = [(CI.mk $ fromString "Content-Type", fromString "text/xml" :: Ascii) :: Header]
, redirectCount = 0
, checkStatus = \_ _ -> Nothing
, requestBody = RequestBodyLBS $ (renderLBS def queryDoc)
}
res <- http req2 manager
return $ res
以下工作并返回'200':
let pingdoc = Document (Prologue [] …Haskell新手在这里.我正在尝试使用http-enumerator通过HTTP上的XML查询服务.我能够连接并发送xml格式的请求,并接收xml格式的响应.
查询成功后,服务器将发回一个响应
我正在努力的是处理FAULT文档中指出的异常的正确方法.我正在尝试使用Either,但没有成功.
我在下面编写的ghci:
import Network.HTTP.Enumerator
import Network.HTTP.Types
import qualified Data.ByteString.Lazy as L
import Data.ByteString.UTF8
import Text.XML.Light
hostname = "https://server..."
doPost username password token = do
let q = QName "SYSTEM" Nothing Nothing
let attribs = [Attr {attrKey = QName "user" Nothing Nothing, attrVal = username},
Attr {attrKey = QName "password" Nothing Nothing, attrVal = password},
Attr {attrKey = QName "token" Nothing Nothing, attrVal = token}]
let doc = Element {elName=q, elAttribs=attribs, elContent= [], elLine=Nothing}
req0 <- parseUrl …我正在关注yesod wiki上的yesod教程,并点击链接到外部css框架(蓝图)的墙.
我使用脚手架工具创建了网站,到目前为止,所有内容都使用'yesod devel'正常工作.
我已将蓝图文件下载到static/css/blueprint中,并将以下内容添加到default-layout-wrapper.hamlet:
!!!
<html>
<head
<title>#{pageTitle pc}
<link rel=stylesheet type=text/css media=screen href=@{StaticR css_blueprint_screen_css}>
<link rel=stylesheet type=text/css media=print href=@{StaticR css_blueprint_print_css}>
^{pageHead pc}
<body
^{pageBody pc}
我得到的错误是:
Foundation.hs:98:27:
Not in scope: `css_blueprint_screen_css'
In the result of the splice:
$(hamletFile "hamlet/default-layout-wrapper.hamlet")
To see what the splice expanded to, use -ddump-splices
In the first argument of `hamletToRepHtml', namely
`$(hamletFile "hamlet/default-layout-wrapper.hamlet")'
In the expression:
hamletToRepHtml
($(hamletFile "hamlet/default-layout-wrapper.hamlet"))
Foundation.hs:98:27:
Not in scope: `css_blueprint_print_css'
In the result of the splice:
$(hamletFile "hamlet/default-layout-wrapper.hamlet")
To see …