It comes from my knowledge of functional programming and the problems that come from trying to do this stuff by passing dictionaries. I'm not a Scala programmer, so maybe I've missed some subtleties, but I doubt it. Here's Edward Kmett:
"Since you can pass any dictionary anywhere to any implicit you can't rely on the canonicity of anything. If you make a Map or Set using an ordering, you can't be sure you'll get the same ordering back when you come to do a lookup later. This means you can't safely do hedge unions/merges in their containers. It also means that much of scalaz is lying to itself and hoping you'll pass back the same dictionary every time."
You get the same issue with Ocaml's implementation of polymorphic tree-based maps, where you lose type-safety. This is why you don't want to pass dictionaries for this stuff, but instead want to use modules and functors, which allow you to emulate existential types, higher-order kinding and higher-rank polymorphism.
"Since you can pass any dictionary anywhere to any implicit you can't rely on the canonicity of anything. If you make a Map or Set using an ordering, you can't be sure you'll get the same ordering back when you come to do a lookup later. This means you can't safely do hedge unions/merges in their containers. It also means that much of scalaz is lying to itself and hoping you'll pass back the same dictionary every time."
You get the same issue with Ocaml's implementation of polymorphic tree-based maps, where you lose type-safety. This is why you don't want to pass dictionaries for this stuff, but instead want to use modules and functors, which allow you to emulate existential types, higher-order kinding and higher-rank polymorphism.