Re: [Haskell-beginners] Error compiling OpenGL tutorial part 2

17 Aug 2012

      Hi,

You're right it was order of operations with points leading to the
error. Without the application or parenthesizing '(points 7)' the
evaluation becomes '((mapM_ a points) 7)'.

I still don't fully understand the error though.

Thanks!

On Fri, Aug 17, 2012 at 7:06 AM, Andrey Yankin  wrote:
...
Hi!
Try to group points and 7 with parentheses or put $ before points,
since points became a function at this step in tutorial.
But error message is a real mystery.
How could the actual type of mapM_ be unit?
Can somebody here explain this, please?
Is unit a monad?
2012/8/16 Darren Grant :
...
I am currently following the OpenGL tutorial here:
http://www.haskell.org/haskellwiki/OpenGLTutorial2
But in the current incarnation of Display.hs
(http://pastebin.com/g4NekDVh) I am getting the following error:
$ cabal install
Resolving dependencies...
Configuring mygl-0.1...
Preprocessing executables for mygl-0.1...
Building mygl-0.1...
[3 of 5] Compiling Display          ( Display.hs,
dist\build\mygl\mygl-tmp\Display.o )
Display.hs:13:9:
    Couldn't match expected type `IO a0' with actual type `()'
    In the return type of a call of `mapM_'
    In a stmt of a 'do' expression:
        mapM_
          (\ (x, y, z)
             -> preservingMatrix
              $ do { color
                   $ Color3 ((x + 1.0) / 2.0) ((y + 1.0) / 2.0) ((z +
1.0) / 2.0);
                     translate $ Vector3 x y z;
                   cube (0.1 :: GLfloat) })
          points
          7
    In the expression:
      do { clear [ColorBuffer];
           scale 0.7 0.7 (0.7 :: GLfloat);
           mapM_
             (\ (x, y, z)
                -> preservingMatrix
                 $ do { color
                      $ Color3 ((x + 1.0) / 2.0) ((y + 1.0) / 2.0) ((z
+ 1.0) / 2.0);
                      .... })
             points
             7;
           flush }
cabal.exe: Error: some packages failed to install:
mygl-0.1 failed during the building phase. The exception was:
ExitFailure 1
Not being too familiar with the Haskell syntax I'm not sure how to go
about decomposing the code to find the source of the problem.
Is there any general advice that might help me with this sort of situation?
Thank you!
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