Re: [Haskell-cafe] opengl type confusion

On Sun, Jun 16, 2013 at 4:42 PM, wrote:

On Sun, 16 Jun 2013 16:15:25 -0400 Brandon Allbery wrote:

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On Sun, Jun 16, 2013 at 4:03 PM, wrote:

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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

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.

No magic involved, although some automation is. Take a look at the `default` keyword in the Haskell Report (this is the "defaulting" I mentioned earlier). http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-790004.3... The "default `default`" is `default (Integer, Double)` which means that it will try to resolve a numeric literal as type Integer, and if it gets a type error it will try again with type Double. You should use this same mechanism to make numeric literals work with OpenGL code: neither Integer nor Double will produce a valid type for the expression, but at the same time the compiler cannot infer a type because there are two possibilities (GLfloat and GLdouble). You could therefore add a declaration `default (Integer, Double, GLdouble)` so that it will try GLdouble to resolve numeric literals when neither Integer nor Double will work.

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) ?

The first part I just answered. As to the second, a conversion *is* happening, implicitly as defined by the language; the question being, to what type. A numeric literal has type (Num a => a), implemented by inserting a call to `fromIntegral` for literals without decimal points and `fromRational` for others. But the compiler can't always work out what `a` is in (Num a => a) without some help (the aforementioned `default` declaration). -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net