Type classes are open, so nothing prevents someone from adding an
instance 'C0 T1' and calling 't' with constructor 'B', causing a crash
due to the missing pattern.
Erik
On Thu, Jan 12, 2012 at 21:40, Tom Hawkins
Let's say I have:
data T0 data T1
data T a where A :: T T0 B :: T T1
Then I can write the following without getting any non-exhaustive pattern match warnings:
t :: T T0 -> String t a = case a of A -> "A"
However, if I use type classes to constrain the constructors, instead of using the phantom types directly:
class C0 a instance C0 T0
class C1 a instance C1 T1
data T a where A :: C0 a => T a B :: C1 a => T a
Then I get a non-exhaustive pattern match warning on 't'. How come? (I'm using GHC 7.0.4)
It appears that the intelligent pattern analysis of the first example is a relatively recent addition [1].
-Tom
[1] http://hackage.haskell.org/trac/ghc/ticket/3476
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