Re: [HOpenGL] Set polygon mode?

On Sun, 22 Jul 2007, Sebastian Sylvan wrote:

On 21/07/07, Hugh Perkins wrote:

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Hi, question, how do I do "Gl.glPolygonMode(Gl.GL_BACK, Gl.GL_LINE);" in HOpenGl?

Maybe something like "polygonMode $= (Line,Line)" ?

(By the way, what does "$=" do?)

...

From the docs ( http://www.haskell.org/ghc/docs/latest/html/libraries/index.html ):

polygonMode :: StateVar (PolygonMode, PolygonMode)

So apparently it's a value of type "StateVar ...". Right, so look up StateVar and we see that it's a data type wich is an instance of HasGetter and HasSetter. So it appears that it models some sort of mutable state variable. In this case we want to set the variable, so HasSetter seems like the right place to look:

class HasSetter s where ($=) :: s a -> a -> IO ()

There's no textual description in the doc, but from the type and name of the class it looks like this takes a statevar, a value, and sets the state variable to the value. So indeed the following should do what you want:

polygonMode $= (Line,Line)

And now you should also know what that operator does. It's just a convenient way of setting OpenGL state variables (and you can also find e.g. ($~) which modifies a state variable by the function you pass it).

I quite like the $= operator. Makes it clear you're playing around with an underlying state variable. In the C binding it's often not clear you're doing this which can lead to unexpected behaviours. Charles,