Consider an HTTP request. A get can't have a body, a post can. So you could define data Request b where GET :: Headers -> Request () POST :: H

Consider an HTTP request. A get can't have a body, a post can. So you could define data Request b where GET :: Headers -> Request () POST :: Headers -> body -> Request body As the article says, building up your intuition is the hard bit. You could go further by typing the headers to recognize the type of body you're dealing with.