我在输入 `->' 时遇到解析错误

Posted 2023-02-24

【中文标题】我在输入 `->\' 时遇到解析错误

【英文标题】：I'm getting parse error on input `->'我在输入 `->' 时遇到解析错误

【发布时间】：2014-01-22 13:19:14

【问题描述】：

我正在尝试运行以下代码，它在 -> 处给我解析错误

- Make sure you have the hxt, url and http packages:
        cabal install hxt
        cabal install url
        cabal install http
        cabal install hxt-curl -

import Text.XML.HXT.Core
import Network.HTTP
import Network.URI

weatherDataURL = "http://www.weather.gov/xml/current_obs/KAGC.xml"

retrieveWeatherData = do
  case parseURI weatherDataURL of
    Nothing  -> ioError . userError $ "Invalid URL"
    Just uri -> get uri
 - | Download a URL.  (Left errorMessage) if an error, (Right doc) if success. -
get uri = do
  eresp <- simpleHTTP (Request uri GET [] "")
  case eresp of
    Left _    -> ioError . userError $ "Failed to get " ++ show uri
    Right res -> return $ rspBody res

    -readString :: Attributes -> String -> iostateArrow s b XmlTreeSource read a document
    that is stored in a normal Haskell String the same function as readDocument, but the
    parameter forms the input. All options available for readDocument are applicable for
    readString. -


parseXML doc = readString [ withValidate no --turn off the validation step.
                          , withRemoveWS yes ---- throw away formatting WS
                          ] doc

data Weather = Weather
   location, observationTime,
    summary, windDirection :: String,

    temperature, humidity,
    dewpoint,
    pressure, windSpeed,
    visibility             :: Float 
  deriving (Eq, Show)

atTag tag = deep (isElem >>> hasName tag) -- Selecting all "top level" tag in XML document
text = getChildren >>> getText
textAtTag tag = atTag tag >>> text

getWeather = atTag "current_observation" >>>
        proc x -> do -- proc (arrow abstraction) except that it constructs an arrow instead of a function.
        loc     <- textAtTag "location"          -< x --  reserved symbol used for building commands from an expression of arrow type and an expression to be fed as input to that arrow
        obsTime <- textAtTag "observation_time"  -< x
        summ    <- textAtTag "weather"           -< x
        windDir <- textAtTag "wind_dir"          -< x
        temp    <- textAtTag "temp_c"            -< x
        humi    <- textAtTag "relative_humidity" -< x
        wind    <- textAtTag "wind_mph"          -< x
        pres    <- textAtTag "pressure_mb"       -< x
        dew     <- textAtTag "dewpoint_c"        -< x
        vis     <- textAtTag "visibility_mi"     -< x
    returnA -< Weather
       location        = loc,
        observationTime = obsTime,
        summary         = summ,
        windDirection   = windDir,
        temperature     = read temp,
        humidity        = read humi,
        windSpeed       = read wind * 1.61,
        pressure        = read pres,
        dewpoint        = read dew,
        visibility      = read vis * 1.61 

-- GHCi test:
-- Main> retrieveWeatherData >>= \ doc -> runX (parseXML doc >>> getWeather)

main = do
  doc    <- retrieveWeatherData
  xml    <- return $ parseXML doc
  result <- runX (xml >>> getWeather)
  case result of
    []  -> putStrLn "Unable to parse weather data."
    w:_ -> print w

【问题讨论】：

如果您只显示有语法错误的部分而不是整个文件，对我们来说会容易得多。

【参考方案1】：

我猜¹你的解析错误在这一行：

proc x -> do

如果是这样，这意味着您还没有告诉友好的邻居 Haskell 编译器您希望它支持箭头语法。您可以通过将其放在文件顶部来做到这一点：

-# LANGUAGE Arrows #-

---

¹但是以后请告诉我们，这样我们就不用去猜测了。