>His point about `Either a b` was perhaps further from the mark. It is not a data type that represent logical disjunction. That's what the logical connective, disjunction, is for. Haskell doesn't have union types to my knowledge. Either is not a connective. It's a BiFunctor. His point that it's not "associative" or "communtative" or what-have-you simply doesn't make sense. In fact he calls Either "malarky" or, charitably, a "misnomer."

I don't agree with Hickey, but isn't there a connection between Either as a basic sum type and logical disjunction via the curry-howard correspondence?

And wouldn't "forall a b. Either a b" be the bifunctor, since it has a product type/product category as it's domain, while the result "Either X Y" (where X and Y are concrete types, not type variables) has the semantics of logical disjunction ie. it represents a type that is of type X or type Y?