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.

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.

In any case, the very fact that you refer to "automagic" and "type conversion" indicates that you don't really have an understanding of how Haskell's numeric types work; this will lead you into not only this kind of confusion, but worse problems later. In particular, you're going to get into dreadful messes where you expect Haskell to transparently deal with strange combinations of numeric types as if Haskell were (almost-typeless) Perl or something, and you'll have real trouble getting that code to work until you sit down and figure out how strong typing and Haskell's numeric typeclasses interact.