Copyright | (c) Michael Weber <[email protected]> 2001 (c) Jeff Newbern 2003-2006 (c) Andriy Palamarchuk 2006 (c) Edward Kmett 2012 |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | [email protected] |

Stability | experimental |

Portability | non-portable (multi-parameter type classes) |

Safe Haskell | Safe |

Language | Haskell2010 |

- Computation type:
- Computations which may fail or throw exceptions.
- Binding strategy:
- Failure records information about the cause/location of the failure. Failure values bypass the bound function, other values are used as inputs to the bound function.
- Useful for:
- Building computations from sequences of functions that may fail or using exception handling to structure error handling.
- Zero and plus:
- Zero is represented by an empty error and the plus operation executes its second argument if the first fails.
- Example type:
`Either String a`

The Error monad (also called the Exception monad).

An exception to be thrown.

Minimal complete definition: `noMsg`

or `strMsg`

.

Nothing

Creates an exception without a message. The default implementation is `strMsg ""`

.

Creates an exception with a message. The default implementation of `strMsg s`

is `noMsg`

.

Error IOException | |

Defined in Control.Monad.Trans.Error | |

ErrorList a => Error [a] | A string can be thrown as an error. |

class Monad m => MonadError e m | m -> e where Source

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 String`

as the monad type constructor for an error monad in which error descriptions take the form of strings. In that case and many other common cases the resulting monad is already defined as an instance of the `MonadError`

class. You can also define your own error type and/or use a monad type constructor other than `Either String`

or `Either IOError`

. In these cases you will have to explicitly define instances of the `MonadError`

class. (If you are using the deprecated Control.Monad.Error or Control.Monad.Trans.Error, you may also have to define an `Error`

instance.)

throwError :: e -> m a Source

Is used within a monadic computation to begin exception processing.

catchError :: m a -> (e -> m a) -> m a Source

A handler function to handle previous errors and return to normal execution. A common idiom is:

do { action1; action2; action3 } `catchError` handler

where the `action`

functions can call `throwError`

. Note that `handler`

and the do-block must have the same return type.

liftEither :: MonadError e m => Either e a -> m a Source

Lifts an `Either e`

into any `MonadError e`

.

do { val <- liftEither =<< action1; action2 }

where `action1`

returns an `Either`

to represent errors.

Since: mtl-2.2.2

© The University of Glasgow and others

Licensed under a BSD-style license (see top of the page).

https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/mtl-2.2.2/Control-Monad-Error-Class.html