> The interaction between solving class constraints and equalities with type > families is currently rather ad hoc. >
FYI *September 2008* *Unified Type Checking for Type Classes and Type Families*, T. Schrijvers, M. Sulzmann. Presented at the ICFP 2008 poster session (pdf available on Tom's homepage). We propose to encode type class solving as type function solving. Earlier, I suggested the opposite direction. Either way works and leads to a system which can properly deal with any sort of type class and type function interaction plus can be straightforwardly extended to support co-inductive type classes/functions. -Martin We are currently re-designing that interaction and may then make the > fixed-depth restriction more broadly applicable. However, as Tom already > mentioned, the cycle does not involve type families in your example anyway. > > Manuel > > José Pedro Magalhães: > > Hello Lennart, > > Yes, but according to the manual ( > http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#undecidable-instances), > "Termination is ensured by having a fixed-depth recursion stack". So I would > expect at least termination, which I'm not getting (but I guess that can be > due to the type families). > > > Thanks, > Pedro > > On Thu, Dec 4, 2008 at 15:10, Lennart Augustsson <[EMAIL PROTECTED]>wrote: > >> Turning on UndecidableInstances is the same as saying: OK typechcker, >> you can loop if I make a mistake. >> I've not looked closely at your code, but if you turn on that flag, >> looping is probably not a bug. >> >> -- Lennart >> >> 2008/12/4 José Pedro Magalhães <[EMAIL PROTECTED]>: >> > Hello all, >> > >> > Please consider the following code: >> > >> >> {-# OPTIONS -Wall #-} >> >> {-# LANGUAGE FlexibleContexts #-} >> >> {-# LANGUAGE FlexibleInstances #-} >> >> {-# LANGUAGE TypeOperators #-} >> >> {-# LANGUAGE TypeFamilies #-} >> >> {-# LANGUAGE OverlappingInstances #-} >> >> {-# LANGUAGE UndecidableInstances #-} >> >> >> >> module Test where >> >> >> >> -- Some view >> >> class Viewable a where >> >> type View a >> >> to :: a -> View a >> >> >> >> -- Structural representations >> >> data Unit = Unit >> >> data a :+: b = L a | R b >> >> >> >> instance Viewable Unit where >> >> type View Unit = Unit >> >> to = id >> >> >> >> instance (Viewable a, Viewable b) => Viewable (a :+: b) where >> >> type View (a :+: b) = a :+: b >> >> to = id >> >> >> >> -- Worker class >> >> class F' a where >> >> f' :: a -> () >> >> >> >> instance F' Unit where >> >> f' Unit = () >> >> >> >> instance (F a, F b) => F' (a :+: b) where >> >> f' (L x) = f x >> >> f' (R x) = f x >> >> >> >> >> >> -- Dispatcher class >> >> class (Viewable a, F' (View a)) => F a where >> >> f :: a -> () >> >> f = f' . to >> >> >> >> instance F Unit where >> >> f = f' >> >> >> >> instance (F a, F b) => F (a :+: b) where >> >> f = f' >> >> >> >> -- All generic instances >> >> instance (Viewable a, F' (View a)) => F a >> >> >> >> >> >> -- A recursive datatype >> >> data Nat = Zero | Succ Nat >> >> >> >> -- Instance of Viewable >> >> instance Viewable Nat where >> >> type View Nat = Unit :+: Nat >> >> to = undefined >> >> >> >> -- Uncommenting the line below causes the typechecker to loop (GHC >> 6.10.1, >> >> Windows) >> >> --test = f Zero >> > >> > >> > Is this expected behavior or a bug? >> > >> > >> > Thanks, >> > Pedro >> > >> > _______________________________________________ >> > Glasgow-haskell-users mailing list >> > Glasgow-haskell-users@haskell.org >> > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >> > >> > >> > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >
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