Demo — Lisp / Scheme interpreter

• Type: feature
• Status: done — green on pinned v36
• Owner: —
• Opened: 2026-06-19
• Relation: demo-class survivor from idea-demo-app-candidates (was catalog #3). Interpreter family (demo, not library). Flagship-tier coverage, larger than the others. Sibling to feature-demo-vm.

Goal

A small Lisp/Scheme: S-expression reader, evaluator with environments and closures, core special forms (quote, if, lambda, define, let), and a builtin set (arithmetic, cons/car/cdr, comparisons, list). Tail behavior best-effort.

Surface / shape

• value = tagged union / small class hierarchy: number / symbol / cons / closure / nil (exercises variant or class/VMT lane)
• reader (managed strings → tree), Eval(expr, env), environment as a collection / linked frames
• builtins registered via procedural types

Coverage

deep recursion (eval) · collections / hashing (symbol table, environments) · managed strings (reader/printer) · variant or class hierarchy · procedural types (builtins) · dynamic arrays. GC/arena pressure is a real-world stressor.

Acceptance / oracle

• An eval suite (fixed programs → expected printed results), byte-identical across all targets — e.g. recursive factorial, map, closures capturing state.
• Demo: examples/lisp/ runs the suite; optional REPL over stdin/serial.

Constraints

Platonic source; no compiler changes; integer core for the oracle (rationals / floats optional, kept out of the deterministic path). Larger — may land in slices (reader → eval → closures → builtins). No self-host / cross regression.

Log

• 2026-06-19 — Opened in the demo-ticket organization pass.
• 2026-06-22 — DONE (Track B): commit 1c23671 — examples/lisp/lispdemo.pas — S-expression reader, environment evaluator with closures, special forms quote/if/define/ lambda/let/begin, and integer builtins (+,-,*,/,mod,=,<,>,cons,car,cdr,list, null?,not,eq?). Values live in a flat cons-cell ARENA (parallel global Integer arrays, handle = value) — no records-with-pointer-fields, robust like zlib/sat. Deterministic oracle: recursive factorial(6)=720, fib(10)=55, closures (adder), recursive length, higher-order map => (1 4 9 16). Ends ALL OK, FPC-verified identical; wired into make lib-test + make demos. Globals (incl. recursive defines) resolve via top-env fallback; locals shadow lexically. REPL over stdin left optional / not done (not needed for the oracle).