8.2. GHC-only Extension Libraries

If you rely on the implicit import Prelude that GHC normally does for you, and if you don't use any weird flags (notably -fglasgow-exts), and if you don't import the Glasgow extensions interface, GlaExts, then GHC should work exactly as the Haskell report says (modulo a few minor issues, see Section 6.13).

If you turn on -fglasgow-exts, a new world opens up to you and the compiler will recognise and parse unboxed values properly, and provide access to the various interfaces libraries described here (and piles of other goodies.)