[Haskell-cafe] Do we need Monad fail (or MonadFail)?
Monads seem to use fail in one of three ways: -Regular monads leave it at the default definition of error -MonadPlus sometimes redefines it to mzero -IO redefines it to failIO Are there any other definitions of fail? If not, does the special case of IO really need a class-level definition, or could there be another way of dealing with failed pattern matches? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Do we need Monad fail (or MonadFail)?
I'd love for the compiler to give an error (or maybe just a warning) in the case that I have a pattern match in a monad that just blows up (throws an exception) on a pattern match failure. Currently there's no way to know the behavior of failed pattern match failures without looking at the instance for the current monad. And what if you're writing monad agnostic (higher-order polymorphism?) code? If there were a MonadFail class, you could explicitly codify that you expect a monad to NOT crash your program in the case of a pattern match. For example: someFunction :: (MonadState m, MonadFail m) = a - m a someFunction arg = do [x,y,z] - action arg return z Of course one of the laws for MonadFail would be that fail never throws an exception. The compiler couldn't exactly enforce it, but we're used to expecting sane instances that obey laws. I think IO would be the exception (no pun intended) to this and be an instance of MonadFail, but just throw an exception on fail. Since you can only catch exceptions in the IO monad, this sounds reasonable to me. --Jonathan On Tue, Dec 21, 2010 at 2:49 AM, John Smith volderm...@hotmail.com wrote: Monads seem to use fail in one of three ways: -Regular monads leave it at the default definition of error -MonadPlus sometimes redefines it to mzero -IO redefines it to failIO Are there any other definitions of fail? If not, does the special case of IO really need a class-level definition, or could there be another way of dealing with failed pattern matches? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Do we need Monad fail (or MonadFail)?
On Tue, Dec 21, 2010 at 08:31:08AM -0700, Jonathan Geddes wrote: I'd love for the compiler to give an error (or maybe just a warning) in the case that I have a pattern match in a monad that just blows up (throws an exception) on a pattern match failure. You will be interested to know that everything you ask for already was in Haskell ages ago: http://www.cs.auckland.ac.nz/references/haskell/haskell-report-1.4-html/exps.html#do-expressions They decided to get rid of it in Haskell 98, for reasons that someone else can probably explain. Lauri ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Do we need Monad fail (or MonadFail)?
everything you ask for already was in Haskell ages ago: those were the days ... where the method in Functor method was called map, and zero was a method of, guess what, MonadZero... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Do we need Monad fail (or MonadFail)?
I'd be really interested in learning the rationale behind those changes. I'm sure it wasn't done for capricious or arbitrary reasons, but I can't help but see it as a step back. --Jonathan Geddes (sent from android mobile) On Dec 21, 2010 8:47 AM, Lauri Alanko l...@iki.fi wrote: On Tue, Dec 21, 2010 at 08:31:08AM -0700, Jonathan Geddes wrote: I'd love for the compiler to give... You will be interested to know that everything you ask for already was in Haskell ages ago: http://www.cs.auckland.ac.nz/references/haskell/haskell-report-1.4-html/exps.html#do-expressions They decided to get rid of it in Haskell 98, for reasons that someone else can probably explain. Lauri ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org ... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
