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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