Hi,
Thanks for creating this proposal and doing a thorough analysis of the
issues, including the package breakage analysis.
On Thu, Sep 16, 2010 at 10:23 AM, Ross Paterson
* simple monads are now aliases for monad trasformers applied to Identity, e.g.
newtype Reader r a = Reader { runReader :: r -> a }
is replaced by
type Reader r = ReaderT r Identity
reader :: (r -> a) -> Reader r a reader f = ReaderT (Identity . f)
runReader :: Reader r a -> r -> a runReader m = runIdentity . runReaderT m
Rationale: This avoids repetition in the interfaces of both transformers and the proposed mtl-2. It makes transformers more useful on its own, and also saves clients of mtl from defining instances for both Reader r and ReaderT r and ensuring that they are consistent.
I would like to know if this additional abstraction has a noticeable performance cost. Does anyone know of a benchmark that could be used to test this? Perhaps one involving State. Paying careful attention to inlining should help remove any overhead. == Transition issues ==
* 41 failed with the new mtl: * 11 because they defined their own Applicative instances (which can now be deleted) * 11 because of the changed constraint on Functor instances * 15 that used the constructors of base monads (which can be trivially replaced) * 3 that defined instances for base monads * 1 that defined an overlapping Error instance
Should we try to get these 41 package to add an upper bound on their mtl dependency before mtl-2 is released so they don't break? The authors could then migrate their code over to mtl-2 as soon as it has been published. If this proposal is approved, we should create a small wiki with migration tips. Cheers, Johan