I have been working on improving the way we describe the control flow of Haskell programs. As the post is rather long, it is split in several parts:

1. State of the art: briefly survey the state of the art (unchecked exceptions, Either, EitherT, etc.) and the issues I am trying to solve.
2. Variant: describe a new Variant data type (a parameterized open sum type). Basically a value of type Variant '[X,Y,Z] can contain any value whose type is X, Y or Z.
3. Flow with Variant: describe how we can use the Variant data type to solve control flow issues.