[Haskell-cafe] Speed of Error handling with Continuations vs. Eithers

[Derek Elkins]

On Fri, May 14, 2010 at 4:53 PM, Antoine Latter <aslatter at gmail.com> wrote:
> On Fri, May 14, 2010 at 4:25 PM, Derek Elkins <derek.a.elkins at gmail.com> wrote:
>> You did it wrong.  All you did was Church encode the Either type.
>> Your bind is still doing a case-analysis.  All you have to do is use
>> ContT r (Either e).  The bind implementation for ContT is completely
>> independent of the underlying monad.  It doesn't even require the m in
>> ContT r m to be a functor, let alone a monad.  Therefore the ContT
>> bind doesn't do any case-analysis because it doesn't know anything
>> about the underlying monad.  One way to look at what is happening is
>> to compare it to Andrzej Filiniski's work in "Representing Monads" and
>> "Representing Layered Monads".
>>
>
> Would you then use callCC to get the required short-circuit-on-error behavior?
>
> A church encoding of Either coded as a monad transformer still
> wouldn't hit the inner monad on bind, even if it is weaving the left
> and right continuations together.

callCC wouldn't work well here.  What would work better is another
control operator commonly called 'control' which does not resume if
the passed in continuation isn't invoked.  However, it's usually even
clearer (or at least more concise) in these situations to work with
the continuation passing style directly.

-- fail directly using CPS
fail :: String -> ContT r (Either String) a
fail s = ContT $ \k -> Left s