haskell sum type multiple declaration error

Question

data A=A
data B=B
data AB=A|B

Which makes a sum type AB from A and B.

but the last line induces a compile error \"multiple declarations of

Answer 1

Because the type of the value created using data constructor A or B will be ambiguous. When I have a = B for instance, what is the type of a? It is A or AB?

You should consider using different data constructor as follows:

data A = MkA
data B = MkB
data AB = A A | B B