Control.Monad.Error.Lens

Catching

catching :: MonadError e m => Getting (First a) e a -> m r -> (a -> m r) -> m r

Catch exceptions that match a given ReifiedPrism (or any ReifiedGetter, really).

catching :: MonadError e m => Prism' e a     -> m r -> (a -> m r) -> m r
catching :: MonadError e m => Lens' e a      -> m r -> (a -> m r) -> m r
catching :: MonadError e m => Traversal' e a -> m r -> (a -> m r) -> m r
catching :: MonadError e m => Iso' e a       -> m r -> (a -> m r) -> m r
catching :: MonadError e m => ReifiedGetter e a     -> m r -> (a -> m r) -> m r
catching :: MonadError e m => ReifiedFold e a       -> m r -> (a -> m r) -> m r

catching_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r

Catch exceptions that match a given ReifiedPrism (or any ReifiedGetter), discarding the information about the match. This is particularly useful when you have a Prism' e () where the result of the ReifiedPrism or ReifiedFold isn't particularly valuable, just the fact that it matches.

catching_ :: MonadError e m => Prism' e a     -> m r -> m r -> m r
catching_ :: MonadError e m => Lens' e a      -> m r -> m r -> m r
catching_ :: MonadError e m => Traversal' e a -> m r -> m r -> m r
catching_ :: MonadError e m => Iso' e a       -> m r -> m r -> m r
catching_ :: MonadError e m => ReifiedGetter e a     -> m r -> m r -> m r
catching_ :: MonadError e m => ReifiedFold e a       -> m r -> m r -> m r

Handling

handling :: MonadError e m => Getting (First a) e a -> (a -> m r) -> m r -> m r

A version of catching with the arguments swapped around; useful in situations where the code for the handler is shorter.

handling :: MonadError e m => Prism' e a     -> (a -> m r) -> m r -> m r
handling :: MonadError e m => Lens' e a      -> (a -> m r) -> m r -> m r
handling :: MonadError e m => Traversal' e a -> (a -> m r) -> m r -> m r
handling :: MonadError e m => Iso' e a       -> (a -> m r) -> m r -> m r
handling :: MonadError e m => ReifiedFold e a       -> (a -> m r) -> m r -> m r
handling :: MonadError e m => ReifiedGetter e a     -> (a -> m r) -> m r -> m r

handling_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r

A version of catching_ with the arguments swapped around; useful in situations where the code for the handler is shorter.

handling_ :: MonadError e m => Prism' e a     -> m r -> m r -> m r
handling_ :: MonadError e m => Lens' e a      -> m r -> m r -> m r
handling_ :: MonadError e m => Traversal' e a -> m r -> m r -> m r
handling_ :: MonadError e m => Iso' e a       -> m r -> m r -> m r
handling_ :: MonadError e m => ReifiedGetter e a     -> m r -> m r -> m r
handling_ :: MonadError e m => ReifiedFold e a       -> m r -> m r -> m r

Trying

trying :: MonadError e m => Getting (First a) e a -> m r -> m (Either a r)

trying takes a ReifiedPrism (or any ReifiedGetter) to select which exceptions are caught If the Exception does not match the predicate, it is re-thrown.

trying :: MonadError e m => Prism' e a     -> m r -> m (Either a r)
trying :: MonadError e m => Lens' e a      -> m r -> m (Either a r)
trying :: MonadError e m => Traversal' e a -> m r -> m (Either a r)
trying :: MonadError e m => Iso' e a       -> m r -> m (Either a r)
trying :: MonadError e m => ReifiedGetter e a     -> m r -> m (Either a r)
trying :: MonadError e m => ReifiedFold e a       -> m r -> m (Either a r)

Handlers

catches :: MonadError e m => m a -> [Handler e m a] -> m a

This function exists to remedy a gap between the functionality of Control.Exception and Control.Monad.Error. Control.Exception supplies catches and a notion of Handler, which we duplicate here in a form suitable for working with any MonadError instance.

Sometimes you want to catch two different sorts of error. You could do something like

f = handling _Foo handleFoo (handling _Bar handleBar expr)

However, there are a couple of problems with this approach. The first is that having two exception handlers is inefficient. However, the more serious issue is that the second exception handler will catch exceptions in the first, e.g. in the example above, if handleFoo uses throwError then the second exception handler will catch it.

Instead, we provide a function catches, which would be used thus:

f = catches expr [ handler _Foo handleFoo
                 , handler _Bar handleBar
                 ]

data Handler e m r

You need this when using catches.

Constructors

Handler (e -> Maybe a) (a -> m r)

Instances

Monad m => Functor (Handler e m)

Defined in Control.Monad.Error.Lens

Methods

fmap :: (a -> b) -> Handler e m a -> Handler e m b

(<$) :: a -> Handler e m b -> Handler e m a

Throwing

throwing :: MonadError e m => AReview e t -> t -> m x

Throw an Exception described by a ReifiedPrism.

throwing l ≡ reviews l throwError

throwing :: MonadError e m => Prism' e t -> t -> a
throwing :: MonadError e m => Iso' e t   -> t -> a

throwing_ :: MonadError e m => AReview e () -> m x

Similar to throwing but specialised for the common case of error constructors with no arguments.

data MyError = Foo | Bar
makePrisms ''MyError
throwing_ _Foo :: MonadError MyError m => m a