如何在Haskell中捕获读取异常？

Question

在以下Haskell代码中:

data Cmd =  
    CmdExit |  
    CmdOther  
    deriving (Read, Show)

guiString2Cmd s =  
    (return (read s :: Cmd)) `catch` \(e :: SomeException) -> return CmdExit

如果我做:

guiString2Cmd "CmdOther"

一切正常.但是,如果我这样做:

guiString2Cmd "some wrong string"

代码崩溃而不是评估为CmdExit.

如何让代码处理异常而不是崩溃？

Answer 1

使用reads函数,即total,并将失败案例包装为Maybe,如下所示:

maybeRead :: Read a => String -> Maybe a
maybeRead s = case reads s of
    [(x, "")] -> Just x
    _         -> Nothing

maybeRead 是一种非常通用的方法来进行安全解析.