Abstract First-Class Citizens

[giovinazzo-kevin]

All abstract terms can be represented canonically as a base term.

There should be an interface that allows defining new types in C# as long as a conversion to and from their canonical form is provided.
This interface would also expose the parser, which is currently not extensible, for the purposes of parsing the new type.

After this change is implemented, Dicts and possibly Lists should be rewritten to be abstract first class citizens.

[giovinazzo-kevin]

Infrastructure

• Created the IAbstractTerm interface that handles conversion to canonical form, explanation and unification logic
• Created the AbstractTermParser<T> abstract class and the IAbstractTermParser untyped interface
• Added parameter Action<Parser> configureParser where necessary
• Added method Parser.TryAddAbstractParser<T>(AbstractTermParser<T> parser)
• Modified ITerm to have a Maybe<IAbstractTerm> AbstractForm

Changes

• Implemented and standardized lists through the AbstractList IAbstractTerm and the ListParser<T>. The new types are: List, Set (formerly BracyList), CommaList and, for internal use, UntypedList
• Reimplemented Dict as an abstract term through the new interface and made it depend on the Set parser
• Modified unification and explanation logic to account for abstract terms

Bugs

Some bugs pertaining to list unfolding may still be in hiding at the moment.