[Haskell-cafe] opengl type confusion

[briand at aracnet.com]

On Sun, 16 Jun 2013 22:19:22 +0100
Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> On Sun, Jun 16, 2013 at 01:03:48PM -0700, briand at 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