On Sun, 16 Jun 2013 22:19:22 +0100 Tom Ellis <tom-lists-haskell-cafe-2...@jaguarpaw.co.uk> wrote:
> On Sun, Jun 16, 2013 at 01:03:48PM -0700, bri...@aracnet.com wrote: > > wireframe :: Double -> Double -> Double -> IO () > > wireframe wx wy wz = do > > -- yz plane > > renderPrimitive LineLoop $ do > > vertex $ Vertex3 0.0 0.0 0.0 > > vertex $ Vertex3 0.0 wy 0.0 > > vertex $ Vertex3 0.0 wy wz > > vertex $ Vertex3 0.0 0.0 wz > [...] > > > > No instance for (VertexComponent Double) > > arising from a use of `vertex' > [...] > > > > Changing the declaration to GLdouble -> GLdouble -> GLdouble -> IO() and > > using > > (0.0::GLdouble) fixes it > > Vertex3 takes three arguments, all of which must be of the same instance of > VertexComponent. Specifying GLdoubles in the signature of wireframe > specifies the types in the last three calls to Vertex3, but (0.0 :: > GLdouble) is still requried on the first to fix the type there. How else > could the compiler know that you mean 0.0 to be a GLdouble and not a > GLfloat? > > Tom > it's curious that (0.0::GLdouble) 0.0 0.0 is good enough and that (0.0::GLdouble) (0.0::GLdouble) (0.0::GLdouble) is not required. I suspect that's because, as you point out, they all have to be the same argument and ghc is being smart and saying if the first arg _must_ be GLdouble (because I'm explicitly forcing the type), then the rest must be too. Meanwhile 4.3.4 about the default is quite interesting. Didn't know about that :-) Thanks very much for the responses ! Brian _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
