lens-4.15.3: Lenses, Folds and Traversals

Copyright | (C) 2014-2016 Edward Kmett
License | BSD-style (see the file LICENSE)
Maintainer | Edward Kmett <ekmett@gmail.com>
Stability | provisional
Portability | Control.Monad.Error
Safe Haskell | None
Language | Haskell98

Control.Monad.Error.Lens

Contents

- Catching
- Handling
- Trying
- Handlers
- Throwing

Description

Synopsis

- catching :: MonadError e m => Getting (First a) e a -> m r -> (a -> m r) -> m r
- catching_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r
- handling :: MonadError e m => Getting (First a) e a -> (a -> m r) -> m r -> m r
- handling_ :: MonadError e m => Getting (First a) e a -> m r -> m r -> m r
- trying :: MonadError e m => Getting (First a) e a -> m r -> m (Either a r)
- catches :: MonadError e m => m a -> [Handler e m a] -> m a
- data Handler e m r = Handler (e -> Maybe a) (a -> m r)
- class Handleable e m h | h -> e m where
- throwing :: MonadError e m => AReview e t -> t -> m x
- throwing_ :: MonadError e m => AReview e () -> m x

# Catching

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

Catch exceptions that match a given `Prism` (or any `Getter`, 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 => Getter e a     -> m r -> (a -> m r) -> m r
    catching :: MonadError e m => Fold 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 `Prism` (or any `Getter`), discarding the information about the match. This is particuarly useful when you have a ``Prism'` e ()` where the result of the `Prism` or `Fold` 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 => Getter e a     -> m r -> m r -> m r
    catching_ :: MonadError e m => Fold 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 => Fold e a       -> (a -> m r) -> m r -> m r
    handling :: MonadError e m => Getter 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 => Getter e a     -> m r -> m r -> m r
    handling_ :: MonadError e m => Fold 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 `Prism` (or any `Getter`) 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 => Getter e a     -> m r -> m (Either a r)
    trying :: MonadError e m => Fold 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

Handleable e m (Handler e m) # |
Methods handler :: Typeable * a => Getting (First a) e a -> (a -> m r) -> Handler e m r # handler_ :: Typeable * a => Getting (First a) e a -> m r -> Handler e m r #
Monad m => Functor (Handler e m) # |
Methods fmap :: (a -> b) -> Handler e m a -> Handler e m b # (<$) :: a -> Handler e m b -> Handler e m a #
Monad m => Plus (Handler e m) # |
Methods zero :: Handler e m a #
Monad m => Alt (Handler e m) # |
Methods (<!>) :: Handler e m a -> Handler e m a -> Handler e m a # some :: Applicative (Handler e m) => Handler e m a -> Handler e m [a] # many :: Applicative (Handler e m) => Handler e m a -> Handler e m [a] #
Monad m => Semigroup (Handler e m a) # |
Methods (<>) :: Handler e m a -> Handler e m a -> Handler e m a # sconcat :: NonEmpty (Handler e m a) -> Handler e m a # stimes :: Integral b => b -> Handler e m a -> Handler e m a #
Monad m => Monoid (Handler e m a) # |
Methods mempty :: Handler e m a # mappend :: Handler e m a -> Handler e m a -> Handler e m a # mconcat :: [Handler e m a] -> Handler e m a #

class Handleable e m h | h -> e m where #

Both `exceptions` and Control.Exception provide a `Handler` type.

This lets us write combinators to build handlers that are agnostic about the choice of which of these they use.

Minimal complete definition

handler

Methods

handler :: Typeable a => Getting (First a) e a -> (a -> m r) -> h r #

This builds a `Handler` for just the targets of a given `Prism` (or any `Getter`, really).

    catches ... [ handler _AssertionFailed (s -> print $ "Assertion Failed\n" ++ s)
                , handler _ErrorCall (s -> print $ "Error\n" ++ s)
                ]

This works ith both the `Handler` type provided by `Control.Exception`:

    handler :: Getter     SomeException a -> (a -> IO r) -> Handler r
    handler :: Fold       SomeException a -> (a -> IO r) -> Handler r
    handler :: Prism'     SomeException a -> (a -> IO r) -> Handler r
    handler :: Lens'      SomeException a -> (a -> IO r) -> Handler r
    handler :: Traversal' SomeException a -> (a -> IO r) -> Handler r

and with the `Handler` type provided by `Control.Monad.Catch`:

    handler :: Getter     SomeException a -> (a -> m r) -> Handler m r
    handler :: Fold       SomeException a -> (a -> m r) -> Handler m r
    handler :: Prism'     SomeException a -> (a -> m r) -> Handler m r
    handler :: Lens'      SomeException a -> (a -> m r) -> Handler m r
    handler :: Traversal' SomeException a -> (a -> m r) -> Handler m r

and with the `Handler` type provided by `Control.Monad.Error.Lens`:

    handler :: Getter     e a -> (a -> m r) -> Handler e m r
    handler :: Fold       e a -> (a -> m r) -> Handler e m r
    handler :: Prism'     e a -> (a -> m r) -> Handler e m r
    handler :: Lens'      e a -> (a -> m r) -> Handler e m r
    handler :: Traversal' e a -> (a -> m r) -> Handler e m r

handler_ :: Typeable a => Getting (First a) e a -> m r -> h r #

This builds a `Handler` for just the targets of a given `Prism` (or any `Getter`, really). that ignores its input and just recovers with the stated monadic action.

    catches ... [ handler_ _NonTermination (return "looped")
                , handler_ _StackOverflow (return "overflow")
                ]

This works with the `Handler` type provided by `Control.Exception`:

    handler_ :: Getter     SomeException a -> IO r -> Handler r
    handler_ :: Fold       SomeException a -> IO r -> Handler r
    handler_ :: Prism'     SomeException a -> IO r -> Handler r
    handler_ :: Lens'      SomeException a -> IO r -> Handler r
    handler_ :: Traversal' SomeException a -> IO r -> Handler r

and with the `Handler` type provided by `Control.Monad.Catch`:

    handler_ :: Getter     SomeException a -> m r -> Handler m r
    handler_ :: Fold       SomeException a -> m r -> Handler m r
    handler_ :: Prism'     SomeException a -> m r -> Handler m r
    handler_ :: Lens'      SomeException a -> m r -> Handler m r
    handler_ :: Traversal' SomeException a -> m r -> Handler m r

and with the `Handler` type provided by `Control.Monad.Error.Lens`:

    handler_ :: Getter     e a -> m r -> Handler e m r
    handler_ :: Fold       e a -> m r -> Handler e m r
    handler_ :: Prism'     e a -> m r -> Handler e m r
    handler_ :: Lens'      e a -> m r -> Handler e m r
    handler_ :: Traversal' e a -> m r -> Handler e m r

Instances

Handleable SomeException IO Handler # |
Methods handler :: Typeable * a => Getting (First a) SomeException a -> (a -> IO r) -> Handler r # handler_ :: Typeable * a => Getting (First a) SomeException a -> IO r -> Handler r #
Typeable (* -> *) m => Handleable SomeException m (Handler m) # |
Methods handler :: Typeable * a => Getting (First a) SomeException a -> (a -> m r) -> Handler m r # handler_ :: Typeable * a => Getting (First a) SomeException a -> m r -> Handler m r #
Handleable e m (Handler e m) # |
Methods handler :: Typeable * a => Getting (First a) e a -> (a -> m r) -> Handler e m r # handler_ :: Typeable * a => Getting (First a) e a -> m r -> Handler e m r #

# Throwing

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

Throw an `Exception` described by a `Prism`.

    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