[Haskell] Better Exception Handling
jules at jellybean.co.uk
Wed Nov 24 14:44:37 EST 2004
On 24 Nov 2004, at 19:16, Scott Turner wrote:
> Each error type is an instance of Hierarchical, so that its errors may
> considered part of a larger category of errors. In the instance
> 'parent' specifies how the error appears if it is caught by a handler
> expecting then next more general error type.
Dynamic seems to be a sledgehammer for this particular nut.
Can we not have a type class for 'subtyping' (like your hierarchical).
Can multi-parameter typeclasses do this? We want:
class SubType a b where
inject :: a -> b
instance SubType a b,SubType b c => SubType a c where
inject :: a -> c
inject some_a = inject ((inject some_a) :: b)
and then the definition of `except` requires that the SubType relates
the type of the handler and the type of the actual error?
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