Hi, Stefan and Isaac, thx for providing quick advice. @Stefan: Unfortunately I have to use a list. @Isaac: I do not get it. Could you please provide a short example of your approach? The question still remains. Which arguments do I have ghc to start with to get the same behavior than hugs with -98 +o (here it works). I provide my example for testing purposes: module Test where import Test.QuickCheck import Monad(liftM,liftM2) type Program = [Stmt] data Stmt = Text | IfElse Program Program | While Program deriving (Eq, Show) instance Arbitrary [Stmt] where arbitrary = sized genProg instance Arbitrary Stmt where arbitrary = sized genStmt genStmt::Int->Gen Stmt genStmt 0 = return Text genStmt 1 = return Text genStmt 2 = oneof [return Text, return (While [Text])] genStmt n | n>2 = oneof ([return Text, liftM While (genProg (n-1))]++ [liftM2 IfElse (genProg k) (genProg (n-k-1))|k<-[1..n-2]]) genProg::Int->Gen Program genProg 0 = return [] genProg 1 = return [Text] genProg n | n>1 = oneof ((liftM (\x->[x]) (genStmt n)):[liftM2 (:) (genStmt k) (genProg (n-k))|k<-[1..n-1]]) prop_ConstructParse progr = True where types = progr::Program main = mapM_ (\(s,a) -> putStrLn s >> a) [("flowchart construct and parse", test prop_ConstructParse)] 2007/12/4, Stefan O'Rear <stefanor@cox.net>:
On Tue, Dec 04, 2007 at 03:36:20PM +0100, Steffen Mazanek wrote:
Hello,
I want to quickcheck a property on a datatype representing programs (=[Stmt]) and need to define a specific instance
instance Arbitrary [Stmt]
(mainly to restrict the size of the list).
In quickcheck an instance Arbitrary of lists is already defined. Which parameters do I have to give ghc such that it accepts such an instance? In hugs -98 +o is enough. I have tried -XOverlappingInstances, -XFlexibleInstances and also -XIncoherentInstances, however I still got an overlapping instances error for this declaration.
You shouldn't use lists if you need to have special instance behavior - lists are for perfectly ordinary sequences of things. If a program is just a bunch of unrelated statements, then use [], otherwise use a custom (new)type.
Stefan
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-- Dipl.-Inform. Steffen Mazanek Institut für Softwaretechnologie Fakultät Informatik Universität der Bundeswehr München 85577 Neubiberg Tel: +49 (0)89 6004-2505 Fax: +49 (0)89 6004-4447 E-Mail: steffen.mazanek@unibw.de