Control.Monad.Error.Class (original) (raw)

Description

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](/package/base-4.16.3.0/docs/Data-Either.html#t:Either "Data.Either")` `String` a

The Error monad (also called the Exception monad).

Synopsis

• class Monad m => MonadError e m | m -> e where
  • throwError :: e -> m a
  • catchError :: m a -> (e -> m a) -> m a
• liftEither :: MonadError e m => Either e a -> m a
• tryError :: MonadError e m => m a -> m (Either e a)
• withError :: MonadError e m => (e -> e) -> m a -> m a
• handleError :: MonadError e m => (e -> m a) -> m a -> m a
• mapError :: (MonadError e m, MonadError e' n) => (m (Either e a) -> n (Either e' b)) -> m a -> n b
• modifyError :: MonadError e' m => (e -> e') -> ExceptT e m a -> m a

Documentation

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](/package/base-4.16.3.0/docs/Data-Either.html#t:Either "Data.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](Control-Monad-Error-Class.html#t:MonadError "Control.Monad.Error.Class") class. You can also define your own error type and/or use a monad type constructor other than `[Either](/package/base-4.16.3.0/docs/Data-Either.html#t:Either "Data.Either")` `String` or `[Either](/package/base-4.16.3.0/docs/Data-Either.html#t:Either "Data.Either")` `IOError`. In these cases you will have to explicitly define instances of the [MonadError](Control-Monad-Error-Class.html#t:MonadError "Control.Monad.Error.Class")class. (If you are using the deprecated Control.Monad.Error orControl.Monad.Trans.Error, you may also have to define an Error instance.)

Methods

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](Control-Monad-Error-Class.html#v:throwError "Control.Monad.Error.Class"). Note that handler and the do-block must have the same return type.

withError :: MonadError e m => (e -> e) -> m a -> m a Source #

[MonadError](Control-Monad-Error-Class.html#t:MonadError "Control.Monad.Error.Class") analogue to the withExceptT function. Modify the value (but not the type) of an error. The type is fixed because of the functional dependency m -> e. If you need to change the type of e use [mapError](Control-Monad-Error-Class.html#v:mapError "Control.Monad.Error.Class") or [modifyError](Control-Monad-Error-Class.html#v:modifyError "Control.Monad.Error.Class").

modifyError :: MonadError e' m => (e -> e') -> ExceptT e m a -> m a Source #

A different [MonadError](Control-Monad-Error-Class.html#t:MonadError "Control.Monad.Error.Class") analogue to the withExceptT function. Modify the value (and possibly the type) of an error in an ExceptT-transformed monad, while stripping the ExceptT layer.

This is useful for adapting the [MonadError](Control-Monad-Error-Class.html#t:MonadError "Control.Monad.Error.Class") constraint of a computation.

For example:

data DatabaseError = ...

performDatabaseQuery :: (MonadError DatabaseError m, ...) => m PersistedValue

data AppError = MkDatabaseError DatabaseError | ...

app :: (MonadError AppError m, ...) => m ()

Given these types, performDatabaseQuery cannot be used directly insideapp, because the error types don't match. Using [modifyError](Control-Monad-Error-Class.html#v:modifyError "Control.Monad.Error.Class"), an equivalent function with a different error type can be constructed:

performDatabaseQuery' :: (MonadError AppError m, ...) => m PersistedValue performDatabaseQuery' = modifyError MkDatabaseError performDatabaseQuery

Since the error types do match, performDatabaseQuery' _can_ be used in app, assuming all other constraints carry over.

This works by instantiating the m in the type of performDatabaseQuery toExceptT DatabaseError m', which satisfies the MonadError DatabaseErrorconstraint. Immediately, the ExceptT DatabaseError layer is unwrapped, producing [Either](/package/base-4.16.3.0/docs/Data-Either.html#t:Either "Data.Either") a DatabaseError or a PersistedValue. If it's the former, the error is wrapped in MkDatabaseError and re-thrown in the inner monad, otherwise the result value is returned.

Since: 2.3.1