prometeu-studio/discussion/lessons/DSC-0007-pbs-learn-to-discussion-lessons-migration/LSN-0020-pbs-runtime-values-identity-memory-boundaries-legacy.md

id	ticket	title	created	tags
LSN-0020	pbs-learn-to-discussion-lessons-migration	PBS Runtime Values, Identity, and Memory Boundaries Legacy Import	2026-03-27	compiler pbs legacy-import runtime values identity memory

Context

Legacy import from docs/compiler/pbs/learn/03. Runtime Values, Identity, and Memory Boundaries.md.

This lesson preserves the runtime-facing mental model that keeps PBS lowering, diagnostics, and future optimization work aligned on aliasing, retention, and ownership.

Key Decisions

Value categories are explicit

What: PBS distinguishes copied payload values, identity-bearing values, and carrier-only values.

Why: Later lowering and diagnostics need a stable answer to whether a construct preserves aliasing, creates retention, or just transports an existing payload.

Trade-offs: The model is intentionally qualitative rather than layout-specific, which keeps it stable but less byte-accounting-friendly.

Host boundary semantics remain explicit

What: Host interaction is stack-only across the boundary, but host-backed resources still count as identity-bearing on the PBS side.

Why: PBS needs a clean boundary without pretending that host ownership disappears once a value crosses into language semantics.

Trade-offs: This is semantically clean, but it requires implementers to avoid naive “stack-only means no ownership complexity” shortcuts.

Patterns and Algorithms

• Treat scalars as copied payload without user-visible identity.
• Treat structs, services, and host-backed resources as identity-bearing.
• Treat tuples, optional, and result as carriers that do not create identity of their own.
• Interpret cost visibility semantically:
  • allocation-bearing,
  • retention-bearing,
  • copy versus aliasing,
  • host-boundary crossing,
  • trap possibility.
• Keep future lifetime-control and concurrency surfaces out of core-v1 unless explicitly claimed.

Pitfalls

• Assuming carrier types create fresh identity.
• Confusing qualitative runtime guarantees with exact allocator or collector promises.
• Assuming stack-only host crossing eliminates host ownership concerns.
• Silently treating future-profile surfaces as current supported behavior.

Takeaways

• PBS runtime contracts are qualitative on purpose.
• Identity, aliasing, and retention are the stable facts that matter most for maintenance.
• Host authority and PBS semantic identity can coexist without contradiction.