[Haskell-cafe] ghc overlapping instances
Steffen Mazanek
haskell at steffen-mazanek.de
Wed Dec 5 10:29:50 EST 2007
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 at 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 at unibw.de
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