I have the following program which loops under GHC 6.10.4: http://www.hpaste.org/fastcgi/hpaste.fcgi/view?id=13561#a13561 {-# LANGUAGE DeriveDataTypeable, FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-} module Main where import qualified Data.Data as Data import Data.Typeable (Typeable) import Happstack.Data.Default import Data.Generics.SYB.WithClass.Basics import Data.Generics.SYB.WithClass.Instances () data Proposition = Proposition Expression deriving (Show, Data.Data, Typeable) data Expression = Conjunction (Maybe Expression) deriving (Show, Data.Data, Typeable) -- instance (Sat (ctx [Expression]), Sat (ctx Expression), Sat (ctx Proposition)) => Data ctx Proposition where instance Data DefaultD Proposition where gunfold _ k z c = case constrIndex c of 1 -> k (z Proposition) instance Default Proposition constrExpr :: Constr constrExpr = mkConstr dataTypeExpr "Conjuction" [] Prefix dataTypeExpr :: DataType dataTypeExpr = mkDataType "Expression" [constrExpr] instance ( Data ctx [Expression] , Sat (ctx Expression) , Sat (ctx (Maybe Expression))) => Data ctx Expression where {- instance Data DefaultD Expression where -} gunfold _ k z c = case constrIndex c of 1 -> k (z Conjunction) dataTypeOf _ _ = dataTypeExpr instance Default Expression e :: Expression e = defaultValueD dict main = print e I wish to explain the *many* ways in which it is mysterious. If you load the program into GHCi and evaluate 'e' it will hang. If you compile the program and run it, it will output <<loop>>. This behavior seems annoying, but not very weird. But, here is where it gets fun: 1. if you load the program into GHCi and eval 'e' it will hang. But, if you load the program and type, '(defaultValueD dict) :: Expression' at the prompt, it works fine! 2. if you remove the (Data DefaultD Proposition) instance, it works fine. (Even though Expression does not refer to Proposition in any way) 3. if you simply change the definition of 'gunfold' in the 'Data ctx Proposition' instance to, error "foo". The application works fine. That's right, if you change the body of a function that isn't even being called, evaluating 'e' starts working. (Even though Expression does not refer to Proposition in any way. And even though that gunfold instance is never actually called). 4. if you change the constraint on, Data ctx Expression, from (Data ctx [Expression]) to (Data ctx Expression) it works fine. (Or remove it all together). 5. if you change 'instance (Data DefaultD Proposition) where' to the line above it which is commented out, it works fine. 6. if you change the type of Proposition to, data Proposition = Proposition (Expression, Expression), then it works fine. So far I have only tested this in GHC 6.10.4. Any idea what is going on here? I can't imagine how changing the body of functions that aren't being called would fix things... - jeremy