Control.Monad.Exception.Throws

Defines the Throws binary relationship between types.

Documentation

class (Private l, Exception e) => Throws e l

Throws is a type level binary relationship used to model a list of exceptions.

Usually there is no need for the user to add further instances to Throws except to encode subtyping. As there is no way to automatically infer the subcases of an exception, they have to be encoded manually mirroring the hierarchy defined in the defined Exception instances.

For example, the following instance encodes that MyFileNotFoundException is a subexception of MyIOException :

 instance Throws MyFileNotFoundException (Caught MyIOException l)

Throws is not a transitive relation and every ancestor relation must be explicitly encoded.

                                                 --   TopException
                                                 --         |
   instance Throws MidException   TopException   --         |
                                                 --   MidException
   instance Throws ChildException MidException   --         |
   instance Throws ChildException TopException   --         |
                                                 --  ChildException

SomeException is automatically an ancestor of every other exception type.

Instances

UncaughtException e => Throws e NoExceptions

Exception e => Throws e (Caught SomeException l)

SomeException is at the top of the exception hierarchy . Capturing SomeException captures every possible exception

Exception e => Throws e (Caught e l)

Throws e l => Throws e (Caught e' l)

data Caught e l

A type level witness of a exception handler.

Instances

Exception e => Throws e (Caught SomeException l)

SomeException is at the top of the exception hierarchy . Capturing SomeException captures every possible exception

Exception e => Throws e (Caught e l)

Throws e l => Throws e (Caught e' l)

(Exception e, Monad m) => MonadCatch e (EMT (Caught e l) m) (EMT l m)

Private (Caught e l)

class Exception e => UncaughtException e

Uncaught Exceptions model unchecked exceptions (a la RuntimeException in Java)

In order to declare an unchecked exception e, all that is needed is to make e an instance of UncaughtException

data NoExceptions

Instances

Private NoExceptions

UncaughtException e => Throws e NoExceptions

data ParanoidMode

Instances

Private ParanoidMode