While what you say is true, GHC is the de facto Haskell standard. At least the last time I looked, most widely used and well respected Haskell code would only work in GHC using various extensions. That's the actual state of Haskell: GHC with non-standard extensions is the effective standard.
An extension that GHC provides, and therefore part of the effective standard, is Safe Haskell (http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/safe-...). A Safe (compiled with -XSafe, containing the LANGUAGE Safe pragma, or inferred) module may not define any manual instances of Typeable.
