[Haskell-cafe] Type system trickery
andrewcoppin at btinternet.com
Mon Jun 22 15:34:35 EDT 2009
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...
More information about the Haskell-Cafe