Rewrite repr things (ie the Algebras) with ADTs

[bmyerz]

We currently have very strict rules about Algebra classes, such as repr that can be used to construct the object, and yet we write the same code over and over for each new operator. MacroPy has algebraic data types to avoid much of this.

https://github.com/lihaoyi/macropy#case-classes

While writing an overly complicated rule match and hating it, I came across macropy. The second benefit of these macros is that you can do case pattern matches, which would simplify and better-document the checks of rules. Find any compiler/interpreter written in a functional-friendly language like OCaml or Scala and you'll see pattern matching on algebraic data types.

[bmyerz]

Limitations in case classes: https://github.com/lihaoyi/macropy#limitations

in particular,

• since __slots__ is used, we cannot rely on adding attributes dynamically. This is generally a good thing! But I know there is some logic that will have to be rewritten
• no inheritance. How do we work around this? As we have to inherit from LogicalAlgebra for certain rules to work.
  • we may be able to use just pattern matching and forgo case classes
  • or we may be able to use case class for Algebra elements then inherit from it (not sure how this works with __slots__ for additional attributes)
  • inheritance of case classes in scala are frowned upon anyway and there are ways around it; we just have to refactor lots of code to get this to work
• (possible deal breaker) pattern matching is still in alpha https://github.com/lihaoyi/macropy#pattern-matching and appears to have been so for awhile
• other possibilities http://www.grantjenks.com/docs/pypatt