> > > data HasShow = forall a. Show a => HasShow a > > weird ~(HasShow x) = HasShow x > > Now, what Show context is referred to by the result of "weird undefined"? >
I'd expect bottom, just as with matching ~(x,y) against bottom. The lambda is not the same as the "where" clause. Let's desugar a few > of the code snippets you gave me: [...] > Thanks. I get it now, and I see that the only version that makes it through the type-checker is the one that defeats laziness and therefore generally breaks these nice morphism properties. That algebraic failure seems reason enough to try fixing ghc. - Conal On Sun, Nov 16, 2008 at 2:18 AM, Ryan Ingram <[EMAIL PROTECTED]> wrote: > > My brain just exploded. > > Best compiler error message ever. > > I think that existential quantification with lazy matches can do some > crazy things to the type system. > > > lazyPair ~(a,b) = (a,b). > > So, lazyPair undefined = (undefined, undefined); it makes any pair > "more defined". But what does this do when you have an existential > context? > > > data HasShow = forall a. Show a => HasShow a > > weird ~(HasShow x) = HasShow x > > Now, what Show context is referred to by the result of "weird undefined"? > > > useShow (HasShow x) = show x > > broken = useShow (weird undefined) > > It's likely that there is a way to solve this; you need to force the > pattern match to take place anywhere that the existential type gets > applied (in the System F sense). But I think that doing so is a lot > more difficult for the compiler author, and it's an edge case in the > language. > > The lambda is not the same as the "where" clause. Let's desugar a few > of the code snippets you gave me: > > > fmap g ~(WC f k) = WC f (fmap g k) > => fmap = \g wc -> let (WC f k) = wc in WC f (fmap g k) > > > fmap g wc = WC f (fmap g k) > > where WC f k = wc > => fmap = \g wc -> let (WC f k) = wc in WC f (fmap g k) > (same as above) > > > fmap g = \ (WC f k) -> WC f (fmap g k) > => fmap = \g wc -> case wc of (WC f k) -> WC f (fmap g k) > (i.e. the same as not using a lazy pattern at all) > > I'll think about this and see if I can come up with a workaround. I'm > not sure it's possible in GHC. > > -- ryan > > > On Sat, Nov 15, 2008 at 10:33 PM, Conal Elliott <[EMAIL PROTECTED]> wrote: > > What is the reasoning behind the ghc restriction that "A lazy (~) pattern > > cannot bind existential type variables"? > > > > This error came up for me in the following code: > > > > -- | Add a continuation. > > data WithCont h b c = forall a. WC (h b a) (a -> c) > > > > instance Functor (WithCont h b) where > > fmap g ~(WC f k) = WC f (fmap g k) > > > > The error message: > > > > A lazy (~) pattern cannot bind existential type variables > > `a' is a rigid type variable bound by > > the constructor `WC' at Data/Zip/FoldL.hs:66:11 > > In the pattern: ~(WC f k) > > > > I also tried this variation: > > > > instance Functor (WithCont h b) where > > fmap g wc = WC f (fmap g k) > > where WC f k = wc > > > > and got this message: > > > > My brain just exploded. > > I can't handle pattern bindings for existentially-quantified > > constructors. > > Instead, use a case-expression, or do-notation, to unpack the > > constructor. > > In the binding group for > > WC f k > > In a pattern binding: WC f k = wc > > > > I can work around these limitations by using a lambda: > > > > instance Functor (WithCont h b) where > > fmap g = \ (WC f k) -> WC f (fmap g k) > > > > which I believe is equivalent. Please correct me if I'm wrong here. > > > > For infix definitions like (<*>), however, this work-around is less > > pleasant. > > For instance, > > > > (<*>) = \ (WC hf hk) (WC xf xk) -> > > WC (hf `zip` xf) (\ (a,a') -> (hk a) (xk a')) > > > > instead of the prettier but forbidden > > > > ~(WC hf hk) <*> ~(WC xf xk) = > > WC (hf `zip` xf) (\ (a,a') -> (hk a) (xk a')) > > > > > > If you're curious what these definitions are about, see > > http://conal.net/blog/posts/enhancing-a-zip/ . > > > > Thanks, - Conal > > > > >
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