Question

我有几个运行的计算会抛出不同类型的异常。我怎样才能在do块中链接它们而不评估每一个，然后将结果模式匹配为Either ex result？

type IntComp ex = ExceptT ex IO Int

local :: IntComp IOException
local = ExceptT $ map length <$> tryIO (readFile "file")

web :: IntComp WebException
web = ExceptT $ map length <$> tryAny (getWebsite "file")

comp :: (Exception ex) => IntComp ? -> IntComp ? -> IntComp ?
comp local web = ExceptT $ do
res1 <- local
res2 <- web
return (res1 + res2)

由于它们都有Exception的实例，我认为必须有某种方式以类似的方式编写comp函数？

Answer 1

如果您有两种例外类型EOne和ETwo，则可以这样做：

twoExceptions :: ExceptT (Either EOne ETwo) m a
twoExceptions = do
    fmapL Left computationThatFailsWithEOne -- :: ExceptT EOne m b
    fmapL Right computationThatFailsWithETwo -- :: ExceptT ETwo m c
    ...

Answer 2

comp :: (Exception ex1, Exception ex2) => IntComp ex1 -> IntComp ex2 -> IntComp SomeException
comp local web = do
    res1 <- withExceptT toException local
    res2 <- withExceptT toException web
    return (res1 + res2)

Chain ExceptT与do block中的Left不同

2 个答案: