[Haskell-cafe] CPS Error Monad

[Antoine Latter]

On Tue, Nov 2, 2010 at 2:11 PM, Brandon Moore <brandon_m_moore at yahoo.com> wrote:
>> From: Max Cantor <mxcantor at gmail.com>
>> Sent: Tue, November 2, 2010 7:58:49 AM
>>
>> FYI,
>>
>> I  implemented an error monad using this church-encoded either  instead of the
>>conventional either.  my thought was that since you skip the  conditional at
>>each bind call you'd get better performance.  I was quite  mistaken.
>
> That's surprising, I think LogicT gains significant performance from that sort
> of CPS conversion.
>
> Was your code like this?
>
> newtype ErrorC e a = ErrorC (runErrorC :: forall r . (e -> r) -> (a -> r) -> r)
>
> instance Monad (ErrorC e) where
>  return x = ErrorC (\fail succeed -> succeed x)
>  ErrorC part1 >>= f =
>     ErrorC (\fail succeed -> part1 fail (\a -> runErrorC (f a) fail succeed))
>
> instance MonadError e (ErrorC e) where
>  throwError e = ErrorC (\fail succeed -> fail e)
>  catchError m handler = ErrorC (\fail succeed ->
>     runErrorC m (\err -> runErrorC (handler err) fail succeed) succeed)
>

In a monad transformer, performing a CPS translation has the benefit
of being able to implement bind, return and other effects without
having to pass through operations on the wrapped monad. This could be
part of the performance improvement you're seeing.

Antoine