because "ssdf" is a string but B("asd") is user-defined sum type. So in the original example, you can't unify "ssdf" and the int 33, but in the re-written vers

because "ssdf" is a string but B("asd") is user-defined sum type. So in the original example, you can't unify "ssdf" and the int 33, but in the re-written version B("sfd") and A(33) have the same type and can be unified.