[Haskell-cafe] Re: MonadCatchIO, finally and the error monad

Paolo Losi paolo.losi at gmail.com
Mon Oct 18 04:21:31 EDT 2010

On 14/10/2010 12:01, Michael Snoyman wrote:

> I have a recommendation of how to fix this: the MonadCatchIO typeclass
> should be extended to include finally, onException and everything
> else. We can provide default definitions which will work for most
> monads, and short-circuiting monads like ErrorT (and I imagine ContT
> as well) will need to override them.

Hi Michael,

As an haskell newbie, I've got some questions on this matter.

Is it using ErrorT instead of extensible exeptions really necessary?
I've read your comment stating that we cannot pass function arguments
to Exceptions because of the Show class constraint.
But is that really limiting? Wouldn't your use case (e.g.
HTTP redirection in Yesod) be implementable with extensible exceptions?

I would guess that the rule of thumb would be not to mix
extensible exception and ErrorT whenever it is possibile.

I've quickly read your post on inverting transformer stacks
and, from my newbie point of view, I feel that the extra complexity
isn't worth the gain.



My own rule of thumb would be not using

More information about the Haskell-Cafe mailing list