Editing Diagrams/Dev/Constraints (section)

=== Why not simply parameterize over the scalar value and fix our two-dimensional vector type to be V2? ===
* The existing datatype was packed, and I though that should be preserved (and indeed it is, in Diagrams.TwoD.Types.Double). That was what I put in my proposal (or its comments, rather), and so that was what I implemented. It wasn't really any effort, since there was already a type family (V) to use. The diagrams types are already highly polymorphic, so adding more polymorphism shouldn't hurt (much).
* Later, someone told me on IRC that the packed datatype wasn't that useful. Although the type family does let R2 be packed, it doesn't let us pack the other data types, such as SizeSpec2D.  Looking at libraries such as vector and text, it seems that the general approach for packing types is to create a data family of packed types, create marshaling/unmarshalling functions, and then optimise them out (a process known as "fusion"). If we go the fusion route, then the packed R2 types are unnecessary (the data family takes care of them) and so all we need is V2. Terminology note: since e.g. String -> ByteString is called 'pack', "packed" datatypes are those that use unboxed types. This might appear to conflict with current GHC terminology which uses the UNPACK pragma to signify unboxing. However, the opposite of UNPACK is NOUNPACK, not PACK, so they are technically not in conflict. I suggest removing the "UN" part so that the pragmas are PACK and NOPACK respectively.
* Suppose we did change out the parameters of v for parameters of s and change the usages of v to usages of (V2 s). The V type family will remain in use for other reasons, such as abstracting over 2D vs 3D, so now the library has different API's for 2D, 3D, and generic code. But it already did (generic vs. R2Ish vs. R2Ish). The (R2Ish v) constraints change  to (ScalarIsh (V s)) constraints. But writing them over and over again is boring, so we define R2Ish again. Except this time, since V2 / V3 are in the types already, we can actually just define RIsh (no dimension). So all that actually needs to change are the data types and the constraint synonym name.
* So now I'm thinking it's possible and useful to just always use V2. In particular, I can't write the generic Renderable instance unless all types have the same scalar type parameter as their last type argument, which using V2 uniformly makes easy but using V makes hard.