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