[Haskell-cafe] opengl type confusion

[briand at aracnet.com]

On Sun, 16 Jun 2013 16:15:25 -0400
Brandon Allbery <allbery.b at gmail.com> wrote:

> On Sun, Jun 16, 2013 at 4:03 PM, <briand at aracnet.com> wrote:
> 
> > Changing the declaration to GLdouble -> GLdouble -> GLdouble -> IO() and
> > using
> > (0.0::GLdouble) fixes it, and I'm not clear on why it's not automagic.
> >  There are many times I see the
> 
> 
> Haskell never "automagic"s types in that context; if it expects GLdouble,
> it expects GLdouble. Pretending it's Double will not work. It "would" in
> the specific case that GLdouble were actually a type synonym for Double;
> however, for performance reasons it is not. Haskell Double is not directly
> usable from the C-based API used by OpenGL, so GLdouble is a type synonym
> for CDouble which is.
> 
> compiler doing type conversion an numerican arguments although sometimes
> > the occasional fracSomethingIntegralorOther is required.
> >
> 
> I presume the reason the type specification for numeric literals is because
> there is no defaulting (and probably can't be without introducing other
> strange type issues) for GLdouble.
> 

What I was thinking about, using a very poor choice of words, was this :


*Main> let a = 1
*Main> :t a
a :: Integer
*Main> let a = 1::Double
*Main> a
1.0
*Main> :t a
a :: Double
*Main> 

so normally 1 would be interpreted as an int, but if I declare 'a' a Double then it gets "promoted" to a Double without me having to call a conversion routine explicitly.

That seems automagic to me.

(0.0::GLdouble) works to make the compiler happy.  So it appears to be taking care of the conversion automagically.

So maybe a better question, I hope, is:

How can I simply declare 0.0 to be (0.0::GLdouble) and have the functional call work.  Doesn't a conversion have to be happening, i.e. shouldn't I really have to do (realToFrac 0.0) ?

Brian