{- Recursive instance heads as in ... instance C0 (x,Bool) => C0 x ... are Ok if we allow for typechecking scheme as described in "SYB with class". The main idea is to assume C0 x in proving the preconditions of the body of the clause. This is also works for mutual recursion among type classes and instances to the extent exercised in ditto paper. What about the below example though? Here recursion detours through an extra class in a way that leads to nonterminating typechecking with GHC 6.10.1. Does anyone agree that a constraint resolution scheme like the one mentioned could be reasonably expected to cover this case? Regards, Ralf -} {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-overlapping-instances #-} {-# OPTIONS -fallow-undecidable-instances #-} -- Direct recursion terminates (typechecking-wise) class C0 x where m0 :: x -> () m0 = const undefined instance (C0 x, C0 y) => C0 (x,y) instance C0 Bool instance C0 (x,Bool) => C0 x foo :: () foo = m0 (1::Int) -- Indirect recursion does not terminate (typechecking-wise) class C1 x where m1 :: x -> () m1 = const undefined instance (C1 x, C1 y) => C1 (x,y) instance C1 Bool instance (C2 x y, C1 (y,Bool)) => C1 x class C2 x y | x -> y instance C2 Int Int -- It is this declaration that causes nontermination of typechecking. bar :: () bar = m1 (1::Int)