Hi,
I'm sending this to g-h-bugs because there is certainly a ghc bug,
however any answer may also be interesting to readers of g-h-user so
consider cc-ing them if appropriate.
I am running GHC-4.01 (compiled by myself with GHC 4.01) on a linux
box.
I am trying to create a polymorphic list, each item in the list can
be of different types but the type must be an instance of
PictureObject so I know I can invoke PictureObject's methods on the
elements of the list. Assuming I am on the right track, is there any
way of getting the same result with the latest release of hugs?
I thought I could do this with existential types, however my test
program below has problems. Firstly, if compiled with -DDUMPCORE
then the executable dumps core (Is there any way I can get useful
information from this core?)
Secondly, if compiled without -DDUMPCORE I get the following error
messages:
exist.hs:73:
My brain just exploded.
I can't handle pattern bindings for existentially-quantified constructors.
In the binding group
MkGenPic obj = item
In a pattern binding:
main
= do putStr $ (getPictureName obj)
where
item = head obj_list
MkGenPic obj = item
exist.hs:67:
Ambiguous type variable(s) `pot'
in the constraint `PictureObject pot'
arising from use of `getPictureName' at exist.hs:67
Compilation had errors
(btw. whose brain? :-)
Thanks for any help.
Kevin
module Main where
data Coordinate3D = Coord3D {cx, cy, cz::Double}
deriving (Eq, Show)
-- We Represent a line by two coordinates which it passes through.
data Line = MkLine Coordinate3D Coordinate3D
class PictureObject pot where
-- Returns ordered (rel to 0 0 0) of points where the object
-- intersects the given line.
intersectLineObject :: pot -> Line -> [Coordinate3D]
getPictureName :: pot -> String
data Sphere =
Sphere Coordinate3D -- Centre
Double -- Radius
Double -- ambient coeff
Double -- diffuse coeff
Double -- specular coeff
Double -- phong specular exponent
deriving (Eq, Show)
intersectLineSphere :: Sphere -> Line -> [Coordinate3D]
intersectLineSphere sp line = []
instance PictureObject Sphere where
intersectLineObject = intersectLineSphere
getPictureName _ = "Sphere"
data Cube =
Cube Coordinate3D -- Origin corner
Coordinate3D -- Opposite corner
Double -- ambient coeff
Double -- diffuse coeff
Double -- specular coeff
Double -- phong specular exponent
deriving (Eq, Show)
intersectLineCube :: Cube -> Line -> [Coordinate3D]
intersectLineCube cube line = []
instance PictureObject Cube where
intersectLineObject = intersectLineCube
getPictureName _ = "Cube"
sphere :: Sphere
sphere = Sphere (Coord3D 1 1 1) 1 1 1 1 1
cube :: Cube
cube = Cube (Coord3D 1 1 1) (Coord3D 2 2 2) 1 1 1 1
data GenPic = forall pot. (PictureObject pot) => MkGenPic pot
obj_list:: [GenPic]
obj_list = [MkGenPic sphere, MkGenPic cube]
putName :: PictureObject pot => pot -> IO ()
putName x = putStr $ getPictureName x
main :: IO ()
main = do
#ifdef DUMPCORE
sequence $ map (\(MkGenPic el) -> putStr $ getPictureName el) obj_list
#else
putStr $ getPictureName obj
where MkGenPic obj = item
item = head obj_list
#endif
