Re: [Haskell-cafe] Bug in GADT Implementation?

Dominic Steinitz wrote:

I would expect

foo r@(Range BITSTRING _ _) x = []

to give an error but it doesn't. Writing

t = Range BITSTRING

gives one so why not the pattern match?

AFAICS, this is because when you construct a value, as in t, you have to provide the required context (Ord in this case) or face an error. OTOH, when you destruct a value, you simply get the context back and you are not required to provide anything, so no type error is generated. This works in HEAD: data A data Foo where F :: Num A => Foo bar :: Foo -> A bar F = 42 Regards, Zun.