Storable records offer an efficient flat, packed representation in memory. In particular, field access is constant time (i.e. it doesn't depend on where in the record the field is) and as fast as possible, but updating fields may not be as efficient. The requirement is that all fields of a record have Storable instances.

The implementation leaks into the usual vinyl lens API: the requirement of Storable instances necessitates specialization on the functor argument of the record so that GHC can find all required instances at compile time (this is required for constant-time field access). What we do is allow ourselves to write instances of the RecElem and RecSubset classes (that provide the main vinyl lens API) that are restricted to particular choices of the record functor. This is why the SRec2 type that implements records here takes two functor arguments: they will usually be the same; we fix one when writing instances and write instance contexts that reference that type, and then require that the methods (e.g. rget) are called on records whose functor argument is equal to the one we picked. For usability, we provide an SRec type whose lens API is fixed to ElField as the functor. Other specializations are possible, and the work of those instances can always be passed along to the SRec2 functions.

Note that the lens field accessors for SRec do not support changing the types of the fields as they do for Rec and ARec.