Re: [Haskell-cafe] MonadLib usage

Hi, When using monadLib, I use newtype deriving to get the Functor, Applicative, and Monad instances for my custom newtyped monad. Those work just fine, and there is nothing unsafe about them. For a custom monad, I usually don't derive MonadLib's effect classes directly. Instead, I tend to define new operations specific to the monad. This has the benefit of abstracting away internal implementation details, which makes it easier to change or extend the monad later. For example, to implement a monad which provides a source of unique identifiers, I might use a state transformer: newtype MyMonad a = MyMonad (StateT Int ...) Now, instead of deriving a "StateM" instance, I would define a custom operation for obtaining new names, something like this: newName :: MyMonad Name newName = MyMonad (do x <- get; set (x + 1); return (mkName x)) This is why GHC's limitation of deriving only the last argument of a class has not been too much of a problem for me. On a more general note, I don't think that there is anything particularly difficult about lifting the "deriving only the last argument" restriction, except for picking a reasonable notation and finding a willing contributor to hack it up. (Generalized generalized deriving anyone :-) ?) If you are interested in some of the issues with generalized newtype deriving in general, this thread has some examples: http://osdir.com/ml/haskell-cafe@haskell.org/2010-03/msg00388.html Hope that this helps, -Iavor On Sun, Jul 18, 2010 at 9:59 AM, Emil Melnikov wrote:

On 2010, July 18, 23:27 Ivan Lazar Miljenovic wrote:

...

When discussing a similar issue with Manuel Chakravarty, he convinced me that cunning newtype deriving is actually rather bad in practice and shouldn't be used as there's a lack of proofs or some such (I can't remember the arguments, but I remember being convinced by them :p).

Hmm...  I can't imagine how it is possible, since new and original types are isomorphic.

Can you give me some pointers to this discussion (links or keywords)?

-- Emil. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe