原标题:在检查所有异常时如何处理异常类型的多个实例?
我有以下导入(注意我的前奏实际上是ClassyPrelude,它使用UnliftIO.Exception)。请注意,System.Logger来自tinylog,这是一个位于fast-logger之上的瘦库。
import Prelude hiding(log)
import System.Logger hiding(log)
import qualified System.Logger as TL
并具有以下功能:
logExceptions :: MonadUnliftIO m => Logger -> m a -> m a
logExceptions logger program = withException program
(\ex -> do
logIt Warn logger ["Logging exception: ", (show ex)]
flush logger
)
将lambda放入具有类型的本地函数可能会使它更加清晰:
logExceptions :: MonadUnliftIO m => Logger -> m a -> m a
logExceptions logger program = withException program logEx
where
logEx :: (MonadUnliftIO m, Exception e) => e -> m ()
logEx ex = do
logIt Warn logger ["Logging exception: ", (show ex)]
flush logger
这导致以下编译错误:
* Could not deduce (Exception e0)
arising from a use of `withException'
from the context: MonadUnliftIO m
bound by the type signature for:
logExceptions :: forall (m :: * -> *) a.
MonadUnliftIO m =>
Logger -> m a -> m a
at src/FDS/Logging.hs:19:1-56
The type variable `e0' is ambiguous
These potential instances exist:
instance Exception SomeException -- Defined in `GHC.Exception.Type'
instance Exception IOException -- Defined in `GHC.IO.Exception'
instance Exception SomeAsyncException
-- Defined in `GHC.IO.Exception'
...plus four others
...plus 30 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
* In the expression: withException program logEx
In an equation for `logExceptions':
logExceptions logger program
= withException program logEx
where
logEx :: (MonadUnliftIO m, Exception e) => e -> m ()
logEx ex
= do logIt Warn logger ...
....
|
20 | logExceptions logger program = withException program logEx
|
最令人担忧的是plus 30 instances involving out-of-scope types
。我可以隐藏这些进口以略微改善这种情况:
import GHC.Exception.Type hiding(SomeException)
import GHC.IO.Exception hiding(IOException, SomeAsyncException)
但是通过并找到所有30多种异常类型并以这种方式掩盖它们似乎并不合理。我假设我在这里做了一些非常错误的事情,或者我真的需要经历并掩盖一切吗?
注意:
logIt
功能只是围绕着tinylx的log
功能的薄包装 - 随意用任何感觉符合人体工程学的替代品。我现在明白我的问题是Exception
论证需要一个具体的类型,因为它是我的问题中所述的多态函数,并且没有调用站点将其缩小到特定类型。 Catch'em all中描述了正确的答案! here,它是使用混凝土SomeException
类型。结果代码是:
logExceptions :: MonadUnliftIO m => Logger -> m a -> m a
logExceptions logger program = withException program logEx
where
logEx :: MonadUnliftIO m => SomeException -> m ()
logEx ex = do
logIt Warn logger ["Logging exception: ", (show ex)]
flush logger