Hi all, I'm trying to create a yesod subsite with "private" urls. The subsite exports widgets which reference the urls, but the urls are not intended to be referenced from the master site handlers directly. In order for this to work, the sub site needs to know how to translate it's urls into master urls. I didn't like passing the subsite route as a parameter to every widget function, so I tried a class: class YesodSubRoute sub master where subRoute :: Route sub -> Route master After confronting the dreaded "type families are not injective" error (i.e. "sub" cannot be inferred from "Route sub", etc), I rewrote it as: class YesodSubRoute sub master where subRoute :: sub -> master -> Route sub -> Route master This works, but seems a bit cludgy to me. Looking closer, I realized that mkYesodGeneral never reuses types for Route instances, which means Route actually is injective (as long as instances are only being defined by mkYesodGeneral), the typechecker just doesn't know it. As an experiment, I changed Route to a data family. It went rather smoothly, and I was able to use my original definition of YesodSubRoute (instances of which could in theory be generated by mkYesodGeneral). The only trouble I've had is exporting Route instance constructors imported from another package (specifically yesod-auth in my case), which means every module referencing auth routes has to import Yesod.Helpers.Auth). Relevant commits are below. Does this seem like a sensible approach? Are there better alternatives? thanks, -matt https://github.com/softmechanics/yesod/commit/335722b6b234841492a6fba2eb46bd... https://github.com/softmechanics/yesod-auth/commit/baf9b44978aad4dba9b86cfaf...