This works for me: {-# LANGUAGE EmptyDataDecls, GADTs #-} module Main where data NoZoo data Zoo newtype X = X Int deriving (Show) newtype Y = Y Char deriving (Show) data Foobar a where Foo :: X -> Y -> Foobar NoZoo Bar :: X -> Y -> Foobar NoZoo Zoo :: Foobar NoZoo -> Foobar Zoo foobar :: Foobar a -> X foobar f = case f of Foo x _ -> x Zoo g -> foobar g main :: IO () main = putStrLn . show $ foobar (Zoo $ Foo (X 1) (Y 'a')) Could you post a test case? On Jun 22, 2009, at 3:34 PM, Andrew Coppin wrote:
Niklas Broberg wrote:
Not nearly as annoying as this:
data Foobar a where Foo :: X -> Y -> Foobar NoZoo Bar :: X -> Y -> Foobar NoZoo Zoo :: Foobar NoZoo -> Foobar Zoo
For some reason, if I do this I get endless type check errors. I have to change the top two back to Foobar a before it will work. *sigh*
Well, that means something very different obviously. It means Zoo constructors can never take Zoo arguments.
...which would be precisely what I want, yes. :-)
Why would that give you type check errors? If it does, you're doing something else wrong.
I think (I'm not sure) it's because of stuff like this:
foobar :: Foobar a -> X foobar f = case f of Foo x y -> ... Zoo g -> foobar g
The first case implies that f :: Foobar NoZoo, while the second implies that f :: Foobar Zoo. Apparently this seemingly reasonable construct does not type-check...
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