Question

I wish to call the function

glShaderSource ::
 GLuint
 -> GLsizei
 -> GHC.Ptr.Ptr (GHC.Ptr.Ptr GLchar)
 -> GHC.Ptr.Ptr GLint
 -> IO ()

The third argument is the shader program, which is a Haskell string in my program. How do I convert the Haskell String into a GHC.Ptr.Ptr (GHC.Ptr.Ptr GLchar) so that I can call glShaderSource?

Answer 1

You can use withCString from Foreign.C.String to convert a Haskell C string to a temporary C string. The string is allocated at the beginning of the call and deallocated at the end.

withCString s $ \c_string -> let gl_string = castPtr c_string :: Ptr GLchar
                             in glShaderSource a b (foo gl_string) d

The cast is necessary because string marshaling functions use the CChar type, while GL uses the GLchar type. They are both 8-bit signed integers, and I presume that neither the OpenGL library nor the FFI will change its character type in the future. If you’re concerned about pointer casting, you can write your own marshaling function.

On pointer-to-pointer types:

You now have a Ptr GLchar. The right way to make a Ptr (Ptr GLchar) depends on what the OpenGL library expects. For instance, does it expect an array of pointers? Does it call free on some pointers? Does it write to some of those strings? The solution will probably involve some amount of memory allocation and pointer copying.