LithosAnanake/docs/working/architecture/SDK-HOWTO-20260819.md

SDK HOWTO — capsules/sdk.4th

Status: WORKING. Part of the v1.9.0 scoping work (FABRIC-2.md section K). Loads the two cookbook capsules (turtle graphics, DoE library) and adds an SDK vocabulary entry point plus FENCE protection on top of them.

Loading it

Kernel-only (it loads other capsules via EXEC, which does not exist in the hosted build), REPL-invoked — not part of init.4th's boot sequence, matching turtle.4th's own choice:

S" sdk.4th" EXEC

This loads turtle.4th and doe.4th, defines an SDK vocabulary with two introspection words (SDK-VERSION, SDK-HELP), then calls FENCE to protect everything loaded so far — the base wordset, both cookbook capsules, and sdk.4th's own words — from FORGET. Prints a one-line confirmation and hint on load; run SDK-HELP for the full word list.

What VOCABULARY actually does here — read before assuming isolation

sdk.4th defines its own words (SDK-VERSION, SDK-HELP) into a named SDK vocabulary via SDK DEFINITIONS. This is organizational, not isolating: this VM's primary word lookup (vm_find_word, used by the ordinary interpreter loop) is a flat, first-character-bucketed scan of the whole dictionary — it does not consult CONTEXT/CURRENT at all. Verified directly before writing this file: a word defined while CURRENT targeted a custom vocabulary remained globally callable immediately after switching back to FORTH DEFINITIONS, exactly as if no vocabulary had been involved. The VOCABULARY/DEFINITIONS/CONTEXT/ORDER/(FIND) machinery is real, standard FORTH-79, and useful for anything that explicitly walks vocabulary chains ((FIND), ORDER) — it just isn't what makes SDK words reachable day to day. Don't build anything on the assumption that loading a second vocabulary hides or scopes its words from the rest of the system.

FENCE — and the bug it surfaced

FENCE ( -- ) is new (this scoping pass) — it raises the dictionary's FORGET boundary to whatever is currently the newest word, so anything defined afterward can be safely FORGETten without reaching back into protected territory. sdk.4th calls it once, at the very end of loading.

Building a direct test for FENCE surfaced a real, severe, pre-existing bug in FORGET itself — completely independent of FENCE, reproducible with the original boot-time fence alone. Forgetting the single newest word wrongly destroyed every other word back to the fence too; forgetting an older word (which correctly cascades to remove newer words, per FORTH-79 semantics) crashed with a SIGSEGV — a use-after-free in the dictionary relink logic. Found and fixed as part of this work; full root-cause writeup in FABRIC-2.md section K. Three POST cases were added to dictionary_manipulation_words_test.c (Module 14) alongside FORGET's own, including the exact regression scenario, so it can't silently return.

Vocabulary

Word	Effect
SDK-VERSION	Prints the release tag (SDK v1.9.0 (scoping)).
SDK-HELP	Prints the full word list for both cookbook capsules, with HOWTO pointers.
FENCE	(Not SDK-specific — a base word, used by this capsule.) Raises the FORGET boundary to the current dictionary top.

Everything from turtle.4th and doe.4th is also available after loading — see their own HOWTOs (TURTLE-GRAPHICS-HOWTO-20260819.md, DOE-LIBRARY-HOWTO-20260819.md) for their vocabularies.

Verification performed

• mkcapsule --lint capsules/ clean (block range 5109–5115, clear of turtle.4th's 5100–5108).
• Hosted-build logic trace: fabric.4th's core blocks + turtle.4th + doe.4th + sdk.4th's own blocks (with its two EXEC lines stripped, since EXEC doesn't exist hosted — the capsules they'd load were concatenated directly instead) piped into the hosted binary. SDK-HELP runs and prints correctly; zero VM errors from any of this content (all errors present in the log are the built-in POST suite's own deliberate error-injection cases, confirmed by cross-checking against the same baseline used for the cookbook capsules' own verification).
• Built cleanly (zero warnings) and baked into the capsule set on all three kernel architectures (amd64/aarch64/riscv64); boot verified clean through POST on all three, 1012 passed / 0 failed / 0 errors identically, dict_hash unchanged from the pre-sdk.4th baseline on all three (expected — sdk.4th isn't autoloaded, so it cannot affect boot-time dictionary content).
• Not yet driven interactively through a live REPL — same practical limitation as turtle.4th's own HOWTO documents (the DoE campaign that auto-runs on every kernel boot blocks the REPL for ~25–30 minutes before serial commands can reach it). The FORTH logic is verified; the interactive load-and-run experience is not.