On 05/01/06, John Meacham <[EMAIL PROTECTED]> wrote: > independent of anything else, giving up error messages on pattern match > failures in do notation is not acceptable. so, if the split were to > happen, having two methods in MonadZero, one which takes a string > argument, would be needed. > > John
Why does that have to be built into the class specification? Already, GHC can give line/column numbers for failed pattern matches in lambdas, it should be able to do the same for failed pattern matches in do-syntax in a similar way. I've never run into a case where I wanted the pattern match failure error message in the form of a String, wrapped in my monad datatype directly. If you were going to go that route, wouldn't a proper data type be preferred anyway, rather than a String? I can't actually do anything with that string other than to display it, since the report doesn't standardise its structure. Further, these cases seem sufficiently rare that if you're interested in catching information about where the pattern match failed, and using it in your program, you'd probably be better off actually handling failure directly, rather than relying on some system built into do-notation. Otherwise, what's wrong with just letting the thing throw an exception with a meaningful error message? The whole reason things switched to this from the way that they were in Haskell 98 is that apparently people found it confusing that refutable pattern matches in their do-blocks were causing these extra MonadZero typeclass constraints. That may be true, though I'm not sure it's really a language problem so much as a teaching problem. Further, pattern matches can switch from being irrefutable to refutable simply by modifing/extending a datatype, and some found it confusing that they now had a bunch of MonadZero errors cropping up after extending their type. Now, I think that this is a bit of a poor example. In reality, you probably want those error messages -- they serve as a major warning that your code is now probably incorrect, as it fails to deal with a potential case in your data type. Improve the error messages in the compiler if it's confusing. (People should be more aware that algebraic data types are not meant to be easily extensible. This is another good reason to want proper records, as they should take care of situations in which one expects data to be extended after the fact.) Does anyone have a real example of a nontrivial use of fail where the string argument was really important and which wouldn't be better served by MonadError, or by just throwing an exception? Usually when I define my monads, I try to ignore 'fail' as much as possible. In most cases, I just leave it completely undefined, as there's no sensible way to embed strings into my type, or even to fail properly at all. Personally, I'd be most happy with just going back to the Haskell 1.4 way of handling do-notation, and I see the reasons for adopting 'fail' in the first place as a bit specious. - Cale _______________________________________________ Haskell mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell
