Haskell is known for its very strong static type checking, which eliminates a lot of runtime errors.

But the following simple program crashes at runtime:

data D = A | B

f A = True

main = print (f B)

I understand this has nothing to do with type checking, but why can’t the compiler give a warning about this? Or is this by design or because it is impossible to check with more complex recursive data types?

Thanks,

Peter