Roberto Zunino wrote:
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.
Zun, This seems even worse to me. A is not inhabited so how can 42 be of type A? BTW this doesn't compile in 6.6. I suppose I could read my example as "if there is anything that matches Range BITSTRING _ _ then do the following" and since nothing can ever match it then it's a redundant case. But I would expect the compiler to at least warn me that nothing can ever match this. Dom.