| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
| Extensions |
|
Control.Monad.Result
Description
This monad transformer extends a monad with the ability to fail.
Synopsis
- class Monad m => MonadError e (m :: Type -> Type) | m -> e where
- throwError :: e -> m a
- catchError :: m a -> (e -> m a) -> m a
- liftEither :: MonadError String m => Either String a -> m a
- data ResultT m a where
- runResultT :: Functor m => ResultT m a -> m (Either String a)
- mapResultT :: (Functor m, Functor n) => (m (Either String a) -> n (Either String b)) -> ResultT m a -> ResultT n b
- type Result = ResultT Identity
- pattern Result :: Either String a -> Result a
- runResult :: Result a -> Either String a
- pattern Error :: String -> Result a
- pattern Success :: a -> Result a
- result :: (String -> b) -> (a -> b) -> Result a -> b
- fromEither :: Either String a -> Result a
- toEither :: Result a -> Either String a
- fromSuccess :: a -> Result a -> a
- toMonadFail :: MonadFail m => Result a -> m a
- module Control.Monad
- module Control.Monad.Fix
- module Control.Monad.Trans
The class
class Monad m => MonadError e (m :: Type -> Type) | m -> e where #
The strategy of combining computations that can throw exceptions by bypassing bound functions from the point an exception is thrown to the point that it is handled.
Is parameterized over the type of error information and
the monad type constructor.
It is common to use Either StringMonadError class.
You can also define your own error type and/or use a monad type constructor
other than Either StringEither IOErrorMonadError
class.
(If you are using the deprecated Control.Monad.Error or
Control.Monad.Trans.Error, you may also have to define an Error instance.)
Methods
throwError :: e -> m a #
Is used within a monadic computation to begin exception processing.
catchError :: m a -> (e -> m a) -> m a #
A handler function to handle previous errors and return to normal execution. A common idiom is:
do { action1; action2; action3 } `catchError` handlerwhere the action functions can call throwError.
Note that handler and the do-block must have the same return type.
Instances
liftEither :: MonadError String m => Either String a -> m a Source #
Lift Either into MonadError.
The ResultT monad transformer
data ResultT m a where Source #
A monad transformer that is similar to ExceptT except a MonadFail instance.
MonadFail=ResultT.throwE
Bundled Patterns
| pattern ResultT :: Functor m => m (Either String a) -> ResultT m a | Construct and destruct |
Instances
mapResultT :: (Functor m, Functor n) => (m (Either String a) -> n (Either String b)) -> ResultT m a -> ResultT n b Source #
Map the unwrapped computation using the given function.
The Result monad
result :: (String -> b) -> (a -> b) -> Result a -> b Source #
Case analysis for the Result type.
Examples
>>>let s = Success 0>>>let e = Error "critical">>>result ("Bad: " ++) (("OK: " ++) . show) s"OK: 0">>>result ("Bad: " ++) (("OK: " ++) . show) e"Bad: critical"
fromSuccess :: a -> Result a -> a Source #
Convert Result aa with a default value.
Re-exports
module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans