Question

Today, I got a compiler error when trying to use a lazy pattern when matching on an existential GADT constructor:

An existential or GADT data constructor cannot be used inside a lazy (~) pattern

Why is that limitation? What “bad” stuff could happen, if it were allowed?

Answer 1

Consider

data EQ a b where
  Refl :: EQ a a

If we could define

transport :: Eq a b -> a -> b
transport ~Refl a = a

then we could have

transport undefined :: a -> b

and thus acquire

transport undefined True = True :: Int -> Int

and then a crash, rather than the more graceful failure you get when trying to head-normalise the undefined.

GADT data represent evidence about types, so bogus GADT data threaten type safety. It is necessary to be strict with them to validate that evidence: you can’t trust unevaluated computations in the presence of bottom.