08. Lenses (FACET v2.1.3)

This page lists normative standard Level-0 lenses from Appendix A and key runtime rules from §9.

Lens model

Each registered lens has:

• name, version
• input_type, output_type (FTS)
• trust_level (0|1|2)
• gas function
• determinism class (pure|bounded|volatile)
• effect_class (required for trust levels 1/2)

Unknown lens → F802. Missing/invalid effect_class for Level-1/2 → F456.

Trust levels

• Level 0: deterministic, no I/O
• Level 1: bounded external behavior with cache contract
• Level 2: volatile external/nondeterministic behavior

Mode rules:

• Pure mode: Level 2 forbidden (F801); Level 1 cache-only (F803 on miss)
• Exec mode: Level 1/2 allowed only if host + policy/guard allow

Standard Level-0 text lenses (Appendix A.1)

• trim() -> string
• lowercase() -> string (locale-independent)
• uppercase() -> string (locale-independent)
• split(separator: string) -> list<string>
• replace(pattern: string, replacement: string) -> string (safe regex subset)
• indent(level: int) -> string (2 spaces × level)

Standard Level-0 data lenses (Appendix A.2)

• json(indent: int = 0) -> string
• keys() -> list<string>
• values() -> list<any>
• map(field: string) -> list<any>
• sort_by(field: string, desc: bool = false) -> list<any>
• default(value: any) -> any
• ensure_list() -> list<any>

Pipeline typing

Each pipeline step must accept previous output type. Type mismatch → F451.

Valid:

@vars
  clean: "  FACET  " |> trim() |> lowercase()
  arr: { a: 1, b: 2 } |> keys()

Invalid:

@vars
  bad: 42 |> trim()

Layout strategy constraints

When a lens pipeline is used as a section strategy:

• Pure mode requires Level-0 only (else F801)
• strategy must be deterministic, idempotent, total for valid NFC+LF strings
• strategy must not depend on locale/time/env/filesystem/network

Regex safety

Any regex-capable lens must use linear-time safe regex behavior (RE2-class or proven subset) to qualify as Level-0.