Re: blame: mtl MonadReader instance for ContT

Thanks for the pointer. I did come across that. They only call this particular lifting "not natural," which doesn't seem much of a justification - unless they mean natural in a more formal sense that isn't immediately obvious to me. I revisited that paper upon finding the reference in your Delimited Dynamic Binding, the examples of which finally lent some clarity to the differences in the computational behaviors I was dealing with. On Sat, Dec 27, 2008 at 5:14 PM, Chung-chieh Shan wrote:

Nicolas Frisby wrote in article <5ce89fb50812221019p49bdf464rc78c1cdade5099ed@mail.gmail.com> in gmane.comp.lang.haskell.libraries:

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instance (MonadReader r' m) => MonadReader r' (ContT r m) where local f m = ContT $ \c -> do r <- ask local f (runContT m (local (const r) . c))

1) I am hoping this list can recall the reason for putting this design decision into the mtl. Perhaps there's a reason to prefer this particular side-effect interaction.

For what it's worth, this interaction is justified in Section 8.4 of: Sheng Liang, Paul Hudak, and Mark Jones. 1995. Monad transformers and modular interpreters. In POPL '95: Conference record of the annual ACM symposium on principles of programming languages, 333-343. New York: ACM Press. http://web.cecs.pdx.edu/~mpj/pubs/modinterp.html

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