A back end for GHC is described by a value of type `GHC.Driver.Backend.Backend`. This type is an enumeration type. The semantics of a particular value constructor is determined by code spread throughout the compiler: while it would be tedious to count exactly how many modules include a `case` discrimination on the `Backend` type, the `GHC.Driver.Backend` module is imported by over 20 other modules. This distributed semantics makes it a bit challenging to add a new value of `Backend` type, as is being contemplated for both JavaScript and WebAssembly targets. One possible remedy would be to keep the existing compiler structure as much as possible, but to make the `Backend` type abstract, centralizing all the `Backend`-dependent functionality in one module. Because of the potential for recursive imports, such a refactoring is not quite possible, but an approximation would probably be good enough.