This paper presents a way of encoding some kinds of dynamic reconfiguration behaviour in the interface portion of circuit descriptions. This has many advantages. The user of a reconfigurable circuit has some knowledge about the reconfigurable interface of the circuit. Static analysis tools can make better decisions about how to schedule virtual hardware. And most importantly the compiler can automatically synthesize the required interface between reconfigurable portions of the system and the regular portions of the design. Several existing models of dynamic reconfiguration from the literature are captured using our type system extension based on sum types. This is especially important in System-on-Chip (SoC) contexts where a reconfigurable IP block may have to communicate over a non-trivial IP bus like CoreConnectTM .