[Haskell-cafe] ghc overlapping instances

[Isaac Dupree]

Steffen Mazanek wrote:
> 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)]

is prop_ConstructParse the only thing that breaks when you remove the 
instance Arbitrary [Stmt] where arbitrary = sized genProg, or have I 
missed something?  If that's all, try this (untested) :

prop_ConstructParse = forAll (sized genProg) (\progr -> True)

and similarly for other properties.

Or, you _can_ use a newtype for quickcheck-only, something like this:

newtype P = P { unP :: Program }
instance Show P where show = show . unP
instance Arbitrary P where arbitrary = sized genProg . unP
prop_ConstructParse (P progr) = True


Isaac