§2. Surface syntax decision

Generated from docs/LANGUAGE.md §2. This is the normative specification; for a guided tour see the Language and Neurosymbolic sections.

Decision: Python-style indentation syntax (: + indent blocks), def/decorators/ keyword arguments retained — struct + enum + traits (§3.9) replace class; explicitly not a superset; PEG grammar with soft keywords; edition field in the manifest from v0.1.

Rationale (12 §1–2):

• The founder base and the ML ecosystem are Python-first; adoption empirics say familiarity, interop, and existing code dominate intrinsic features (Meyerovich & Rabkin, OOPSLA 2013).
• Code LLMs transfer best to languages syntactically close to a high-resource anchor (MultiPL-T); a Pythonic Sema maximizes zero-shot prior transfer and makes the Python→Sema synthetic-corpus pipeline nearly mechanical.
• Mojo proved the superset promise is a trap and “Pythonic + interop” is the stable landing (Mojo FAQ); Codon’s documented list of dynamic features that break static compilation (monkey-patching, metaclasses, heterogeneous collections, dynamic member addition — Codon) is adopted as Sema’s published “differences from Python” divergence spec.
• New constructs are def-modifiers (simulate def, like async def) or soft keywords under a PEG grammar (CPython PEP 617/ PEP 622 precedent), so post-1.0 keywords never break identifiers; a Rust-style edition mechanism (Edition Guide) handles hard promotions.

Rejected alternatives: braces family (Rust/Go surface) — abandons the audience and the LLM prior for tooling benefits tree-sitter already neutralizes; Python superset — Codon/Mojo demonstrate the compatibility tax and credibility burn; new exotic syntax — zero evidence any intrinsic-syntax bet has ever driven adoption (12 §7).

Keyword disposition versus the brief (evidence-driven changes flagged; details in §5 and the Decision record):

Brief keyword	Disposition
semantics	kept (expression predicate + block guard)
simulate	kept (simulate def modifier)
testable	changed: verification is default-on; assure/@no_verify replace the opt-in keyword
policy	kept (declaration + attachment)
monitor	kept (declaration form), collision risk documented
native	split: native = bind (C ABI/FFI), ported = toolchain-deterministic translation
self-healing	scoped, not global: supervise/heal blocks; repair-only, “extend/grow” scoped out (D14); phase sequencing per ROADMAP

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