37 lines
1.5 KiB
Haskell
37 lines
1.5 KiB
Haskell
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{- | This is just like "Control.Monad.Error.Class" except you can throw/catch the error of any ErrorT in the monad stack instead of just the top one as long as the error types are different. If two or more ErrorTs in the stack have the same error type you get the error of the top one. -}
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{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, OverlappingInstances #-}
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module Control.Monad.Throw where
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import Prelude hiding (catch)
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import Control.Monad.Reader
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import Control.Monad.Error
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-- | Same as 'MonadError' but without functional dependency so the same monad can have multiple errors with different types
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class (Monad m) => Throw e m where
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throw :: e -> m a
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-- ^ Abort action and throw give exception. Analogous to 'throwError'.
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catch :: m a -> (e -> m a) -> m a
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-- ^ If first action aborts with exception then execute second action. Analogous to 'catchError'
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throwLeft :: (Throw e m) => m (Either e a) -> m a
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-- ^ Execute action and throw exception if result is Left, otherwise return the Right result
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throwLeft = (either throw return =<<)
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instance (Error e) => Throw e (Either e) where
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throw = throwError
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catch = catchError
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instance (Error e, Monad m) => Throw e (ErrorT e m) where
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throw = throwError
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catch = catchError
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instance (Error e, Throw e m, Error x) => Throw e (ErrorT x m) where
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throw = lift . throw
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catch a h = ErrorT $ catch (runErrorT a) (runErrorT . h)
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instance (Throw e m) => Throw e (ReaderT x m) where
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throw = lift . throw
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catch a h = ReaderT $ \x -> catch (runReaderT a x) (flip runReaderT x . h)
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