Use the exceptions package about rest

[hesselink]

We need the error type in the type signature, to be able to say things about it (like the serialization formats supported). With exceptions like those in exceptions, you don't get this: the possible exception types are hidden. So I don't think this is possible to do easily.

[dpwiz]

Well... Apparently, exceptions provide an instances for ErrorT m and other transformers so the only monad required to provide Throw/Cath/Mask instances is GenHandler m. So this task is pretty low-priority and needs relevant use cases first. Maybe i'll hit them in my project soon.

[hesselink]

I thought you wanted to remove the ErrorT layer? To be honest, I don't quite see how this would work, or why it would be useful. The GenHandler type now looks like this:

data GenHandler m f where
  GenHandler ::
    { dictionary :: Dict h p i o e
    , handler    :: Env h p i -> ErrorT (Reason e) m (Apply f o)
    , secure     :: Bool
    } -> GenHandler m f

As you can see, the e error type in the handler field is the same as the e in the dictionary field. This is needed to make sure errors have the right serialization formats, can be documented, etc. So if we change it to something like this:

  GenHandler ::
    { dictionary :: Dict h p i o e
    , handler    :: MonadThrow m => Env h p i -> m (Apply f o)
    , secure     :: Bool
    } -> GenHandler m f

Now the e is completely gone from handler, we can throw anything, but we can't document it, serialize it, or even get a hold of it when running the handler.

I'll close the ticket for now, but feel free to reopen it if you have an actual problem/use case related to this.