Personally, I came much less against UndecidableInstances when I was trying to do OOP in Haskell than when I was trying do Prolog-like things at the type level. Things like transitive relations: (If a type B contains an A, and if C contains B, then C contains A, and so on). I kind of abandonned quickly that way of reasoning ^^.
2011/6/7 Yitzchak Gale <g...@sefer.org> > I wrote: > >>> You almost never want to use UndecidableInstances > >>> when writing practical programs in Haskell. > >>> When GHC tells you that you need them, it almost > >>> always means that your types are poorly designed, > >>> usually due to influence from previous experience > >>> with OOP. > > wren ng thornton wrote: > >> That's a bit unfair. There are many kinds of type-level > >> hackery which require UndecidableInstances but are > >> (a) perfectly safe for practical use, and > >> (b) have nothing to do with OOP. > >> One particularly trivial example that comes to mind is: > >> newtype Mu f = Mu (f (Mu f)) > > I agree. I've even used that one (well, something similar > anyway). > > Oleg wrote: > > It seems that UndercidableInstances keep getting a bad rap. > > There are legitimate and decidable applications of > > UndercidableInstances. These applications have nothing to do > > with OOP, or HList for that matter... That extension should not > > be categorically stigmatized. > > I'm sorry if I came across as giving it a bad rap, or > stigmatizing it. That was certainly not my intention. There > are plenty of excellent techniques that use this and other > GHC extensions; accomplished Haskellers should have > them in their toolbox. > > But I stand by my statement, which is coming from my > perspective as a professional Haskell software developer, > not a PL researcher. Perhaps I should clarify it though. > > I am making two claims: > > 1. In everyday practical Haskell programming, it is very > unusual that a technique requiring direct use of > UndecidableInstances is the right tool for the job. Of course, > that observation is colored by my own experience, but I > believe that it is generally true. > > 2. If a person is surprised by the GHC error suggesting > that UndecidableInstances is needed and is struggling with > understanding it, then it is almost certain that person has > inappropriately used OOP thinking in trying to design a > Haskell program. > > Experienced Haskellers familiar with those techniques > do not get the error. Or they get it and say, "Oops, forgot > the UndecidableInstances." > > Whereas most programmers coming to Haskell have had > OOP experience. The first thing that happens when they > try to write a program in Haskell of any significant complexity > is the "UndecidableInstances" error. It happened to me, too. > > Thanks, > Yitz > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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