Add -fmono-pat-binds, and make it the default
In Haskell 98, pattern bindings are generalised. Thus in (f,g) = (\x->x, \y->y) both f and g will get polymorphic types. I have become convinced that generalisation for pattern-bound variables is just a bridge toof far. It is (I claim) almost never needed, and it adds significant complication. (All the more so if we add bang patterns.) So the flag -fmono-pat-binds switches off generalisation for pattern bindings. (A single variable is treated as a degnerate funtction binding.) Furthremore, as an experiment, I'm making it the default. I want to see how many progarms fail with monomorphic pattern bindings. You can recover the standard behaviour with -fno-mono-pa-binds.