Hi everybody, I am torturing the ghc type inference extensions, and I think i managed to break something, either in ghc or (more likely) in my brain: | {-# language FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-} | | module Main | where | | class Table table cell where | toLists :: table -> [[cell]] | fromLists :: [[cell]] -> table | | instance Table [[cell]] cell where | toLists = id | fromLists = id | | -- why i am using a class (not relevant for my question) - | -- data FastTable a = FastTable ... | -- instance Table (FastTable a) a where ... | | instance (Table a c, Show c) => Show a where | showsPrec p t = showParen (p > 10) $ showString "fromLists " . shows (toLists t) ghc 7.0.3 sais: | Overlapping instances for Show [[cell0]] | arising from a use of `shows' | Matching instances: | instance Show a => Show [a] -- Defined in GHC.Show | instance (Table a c, Show c) => Show a | -- Defined at /home/mf/tmp/z.hs:18:10-38 | In the second argument of `(.)', namely `shows (toLists t)' I would have thought that there is on overlap: the instance in my code above defines how to show a table if the cell is showable; the (allegedly) overlapping instance defines how to show lists of alpha if alpha is showable. So I was expecting the reduction to go something like: . table ~> [[cell]] (via whichever instance of class Table applies) . [[cell]] ~> [cell] (via instance "Show a => Show [a]") . [cell] ~> cell (via instance "Show a => Show [a]") But it gets worse. I decided to ignore the problem for now and be happy with at least showing the first cell in the upper left corner of the table: | instance (Table a c, Show c) => Show a where | showsPrec p t = showParen (p > 10) $ showString "fromLists " . shows (head . head $ toLists t) to that, ghc 7.0.3 sais: | /home/mf/tmp/z.hs:19:66: | Context reduction stack overflow; size = 21 | Use -fcontext-stack=N to increase stack size to N | $dShow :: Show c19 | $dShow :: Show c18 | [...] shouldn't type inference be able to determine that | ((head . head $ toLists t) :: c and use the instance guaranteed by the instance declaration's class constraint (Show c)? I suspect that there should be a type annotation that would make this approach possible, but even then I am curious what this behavior means. Should I upgrade to a more grown-up release of ghc? Any hints or pointers appreciated. (: Thanks, Cheers, Matthias