Throws is a type level binary relationship
used to model a list of exceptions.
There are two cases in which the user may want
to add further instances to Throws.
1. To encode subtyping.
2. To declare an exception as unexplicit (a programming error).
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
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.
instance Throws MidException (Caught TopException l) -- |
instance Throws ChildException (Caught MidException l) -- |
instance Throws ChildException (Caught TopException l) -- |
SomeException is automatically
an ancestor of every other exception type.
- Programming Errors
In order to declare an exception E as a programming error, which should
not be explicit nor checked, use a Throws instance as follows:
instance Throws e l