However, if your are using ExtendedDefaultRules then you are likely to know you are leaving the clean sound world of type inference.
First of all, ExtendedDefaultRules is enabled by default in GHCi. Second, my example will work without ExtendedDefaultRules, in pure Haskell98. It is even shorter: instance Num Char main = do x <- return [] let y = x print . fst $ (x, abs $ head x) -- let dead = if False then y == "" else True return () The printed result is either [] or "". Mainly, if the point is to demonstrate the non-compositionality of type inference and the effect of the dead code, one can give many many examples, in Haskell98 or even in SML. Here is a short one (which does not relies on defaulting. It uses ExistentialQuantification, which I think is in the new standard or is about to be.). {-# LANGUAGE ExistentialQuantification #-} data Foo = forall a. Show a => Foo [a] main = do x <- return [] let z = Foo x let dead = if False then x == "" else True return () The code type checks. If you _remove_ the dead code, it won't. As you can see, the dead can have profound, and beneficial influence on alive, constraining them. (I guess this example is well-timed for Obon). For another example, take type classes. Haskell98 prohibits overlapping of instances. Checking for overlapping requires the global analysis of the whole program and is clearly non-compositional. Whether you may define instance Num (Int,Int) depends on whether somebody else, in a library you use indirectly, has already introduced that instance. Perhaps that library is imported for a function that has nothing to do with treating a pair of Ints as a Num -- that is, the instance is a dead code for your program. Nevertheless, instances are always imported, implicitly. The non-compositionality of type inference occurs even in SML (or other language with value restriction). For example, let x = ref [];; (* let z = if false then x := [1] else ();; *) x := [true];; This code type checks. If we uncomment the dead definition, it won't. So, the type of x cannot be fully determined from its definition; we need to know the context of its use -- which is precisely what seems to upset you about Haskell.
To stirr action, mails on haskell-cafe seem useless.
What made you think that? Your questions weren't well answered? What other venue would you propose?