2009/10/5 Michael Mossey
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
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