Re: [Haskell-cafe] GHCi infers a type but refuses it as type signature

OK, I found two papers by the author, Mauro Jaskelioff, that seem relevant. One paper "Modular Monad Transformers" is all category theoretical. Maybe I should read the other one "Monatron: An Extensible Monad Transformer Library". On Thu, Jun 25, 2009 at 12:17 PM, Wei Hu wrote:

Could you or anyone else briefly explain how mmtl solves the combinatorical explosion problem? Reading the source code is not very productive for newbies like me. Thanks!

On Tue, Jun 23, 2009 at 5:34 AM, Luke Palmer wrote:

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On Tue, Jun 23, 2009 at 2:20 AM, wrote:

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Simple: the definition of MonadState uses those extensions.

Thanks, yes it helps and explains all. :^)

I suppose then that if -XFlexibleContexts is indeed required by the standard libraries, it is a "safe" extension, meaning supported by all compilers? Are many such extensions de-facto standard that anyone can enable by default?

Now to answer your question, with 50% less idealistic ranting! There is definitely a canon of "safe" extensions in the community.  Hierarchical libraries (that's not even an -X flag, it's just on), multiparam typeclasses, fundeps are among them.  I can't say whether FlexibleContexts is. However, your question is rather moot here, as you are using the mtl.  It uses UndecidableInstances, whose blessing into the de facto standard would require as a precondition the batshit-insanity of the de facto community. I personally use transformers as my monad library, as it is (as far as I know) the only Haskell 98 monad library on Hackage (you'll hardly notice the more explicit lifts, I promise!).  However, looking a bit more optimistically into the future, I'd say mmtl is the "most likely to succeed", since it solves the combinatorical explosion problem which haunts all the other monad libraries (and at least avoids the extensions which are doomed to failure). Anyway, it is never too early to free yourself of mtl.  Look at the monad libraries on hackage and weigh their pros and cons; the only thing special about mtl is its mediocrity. Luke _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe