docs(dd): DD-017 RS274NGC parametric programs — Proposed (#189 phase 7) - #263
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Drafts the design for the largest remaining #189 gap: LinuxCNC/CAM parametric programs — numbered/named parameters (#100 = 5), expressions (X[#1 + sin[30]]), and O-word subroutines/loops/conditionals — which the forward-only line processor drops today. Key decisions for acceptance (D1–D8): - D1: on-demand program-buffered interpreter engaged ONLY when #/[ /O-words appear; FDM + simple CNC keep the untouched forward-only fast path (byte-identical). One motion/classification core; the interpreter only feeds it. - D5: two new DD-003 limits (maxProgramIterations, maxCallDepth) so a while[1] or infinite recursion can't hang/OOM the worker — bounded, no eval, no I/O. - Spec-derivable from RS274NGC (NIST IR 6556) / LinuxCNC — hardware only re-confirms; parity targets only, no spec text copied (RR-004 §6). Proposed only — architecture-sensitive, so blocked until Accepted per governance. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Phase 7, scoped as a DD for your acceptance
The largest remaining #189 gap: real LinuxCNC / CAM output uses the RS274NGC programming layer — parameters (
#100 = 5), expressions (X[#1 + 2*sin[30]]), and O-word subroutines/loops/conditionals. The parser lexes these away today (Number('[#1+5]')→NaN, dropped), so a parametric program renders partial or empty geometry.Unlike the laser semantics, this is fully spec-derivable (RS274NGC / NIST IR 6556, as-implemented by LinuxCNC) — build it right from the spec; hardware only re-confirms. But it turns the forward-only line processor into a small interpreter (symbol table + expression evaluator + control flow that re-runs/skips lines), which is architecture-sensitive → DD before code, per governance.
The decisions I'd want your call on
#,[, or leading O-word is detected. Everything else (all FDM, most simple GRBL/laser CNC) keeps the untouched forward-only fast path → byte-identical. One motion/classification core; the interpreter only feeds resolved lines into it.maxProgramIterations(~1M) andmaxCallDepth(~50) — soo while [1]or infinite recursion can't hang/OOM the worker. Pure arithmetic evaluator, noeval, no I/O.sub/call,if,while/do/repeat).parametricProgramcapability + per-construct disclosure; FDM byte-identical gate + new synthetic parametric goldens.Non-goals: external subroutine files / Fanuc
M98(no I/O), persistent.varparams, machine-config mutation, full conformance.Four open questions for acceptance are listed in §7 (capability name, limit defaults, system-param allow-list, loop-form scope). Nothing is built — this is Proposed; on acceptance the phased plan (§5) is: (1) expressions+params, (2) O-word flow+limits, (3) subroutines, (4) capability/docs.
Docs-only; links green; added to the DD index.
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