On 01/20/12 13:23, Edward Z. Yang wrote: > In Haskell 1.4 g would not be in MonadZero because (a,b) is unfailable > (it can't fail to match). But the Haskell 1.4 story is unattractive > becuase > a) we have to introduce the (new) concept of unfailable > b) if you add an extra constructor to a single-constructor type > then pattern matches on the original constructor suddenly > become > failable > > (b) is a real killer: suppose that you want to add a new constructor and > fix all of the places where you assumed there was only one constructor. > The compiler needs to emit warnings in this case, and not silently transform > these into failable patterns handled by MonadZero...
Okay, great, that explains two things that had not been clear to me: first, that the notion of "unfailable" was not removed from the language so much as not added in the first place, and second, that if "unfailable" *had* been added to the language then this would have created the serious risk that adding a new constructor to a type could change the meaning of your code by changing formerly irrefutable pattern matches into potential sources of mzeros. Thanks! Greg _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe