2009/10/5 Michael Mossey <m...@alumni.caltech.edu>: > > > Eugene Kirpichov wrote: >> >> [x,y,t,b,l,r] <- mapM (getStdRandom . randomR) [(-10,10), (-70,70), ...] >> return (BoxBounds ...) > > Thanks. > > I'm curious about the idea of "pattern matching in do-statements that can > fail." This particular pattern cannot fail. I read that the "fail" function > was introduced to Monad in order to handle pattern matches that fail, and > that most members of haskell-cafe seem to think that was a mistake---that > MonadZero should have been used instead. I.e., any do-block with a pattern > that can fail should explicitly have a MonadZero class constraint. > > This leads to my question about detecting pattern matches that could fail. > We can easily prove the above pattern will never fail. I'm wondering if the > compiler infers this.
Try proving it by hand and judge the result with respect to whether it could have been done by the compiler. > And if a future version of Haskell dumps "fail" and > used MonadZero to replace it, would that future Haskell compiler need to > infer, in all cases, whether a pattern can fail? Is it simple enough to make > that correct inference? > > Thanks, > Mike > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Eugene Kirpichov Web IR developer, market.yandex.ru _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
