Recap: Interpreter Pattern

One class per rule and a tree per sentence; the parsing is someone else's job.

Interpreter represents a small language as classes and evaluates its sentences as trees.

Intent. Represent a grammar as classes, and interpret sentences in it.
Participants. An abstract expression with interpret(context), terminal expressions for the leaves, non-terminal expressions that combine others, and a context to evaluate against.
The tree. Non-terminals hold other expressions, so the structure is Composite's; evaluation runs from the leaves up.
In the wild. Python's ast module builds the same kind of tree for its own code, and then compiles it rather than walking it.
The trap. The pattern does not parse; text becomes a tree through a parser, written or generated.
When not. A large grammar, evaluation that must be fast, or a language that already exists.

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