Usda Value Parser

usda-value-parser

Use this skill when implementing or verifying USDA right-hand-side values and attribute type validation.

Spec Sources

• specification/file_formats/README.md sections Values, Common Metadata, Attribute Values, and Attribute Types and Value Validation
• specification/foundational_data_types/README.md

Pinned tag / commit: v1.0.1

The pinned specification is normative for value grammar. This skill may add target-native representation, diagnostics, and performance constraints, but it must not broaden where a value form is legal. Consumers decide whether None, PathRef, AssetRef, listOp, time sample, or spline syntax is permitted in their current production context.

Provides

• Atomic values
• Tuple values
• List values
• Dictionary values
• Path refs
• Asset refs
• None as value block where appropriate
• Type-directed validation and coercion for attribute defaults

Contract

This skill owns contracts/handles/usda-value-parser.handle.json and provides the usda_value_parser capability consumed by the USDA spec parser. It consumes USDA lexical services, foundational value storage, path references, and authored listOp storage from the graph rather than defining local equivalents. It also follows contracts/capabilities/semantic-runtime-types.json.

Production-family obligations are factored into:

• contracts/usda-productions/value-syntax.contract.json

The value parser returns target-native semantic values for the spec parser: typed raw values, dictionaries with typed entries, path handles, authored listOps, time-sample maps, spline maps, and value-block sentinels as appropriate for the target language. It must not use Json, tagged JSON, Python dict, std::map<string, Json>, or canonical dump object trees as the domain value representation. Validation adapters may serialize these values to contracts/value.schema.json, but that serialization is not the parser boundary.

Value parsing supplies production building blocks; it does not decide higher level grammar context. For example, parsing a path reference is valid only when the caller's production allows a PathRef, and parsing None is valid only when the caller maps it to a value block or empty authored listOp.

When parsing an attribute declaration and default assignment, validate the parsed value against the declared type if the type is known in the foundational type table.

Unknown attribute types have undefined validation behavior in the spec. For this prototype, preserve the value and emit no type-validation error unless a golden requires otherwise.

Boundary Guards

Defer text lexing to usda-lexical-format.

Defer scalar, array, dictionary, semantic alias, and value-block storage to usd-foundational-values.

Defer path parsing to usd-paths.

Defer authored listOp storage to usd-listops-authored.

Path spellings, token spellings, and name spellings parsed from USDA text are boundary strings. Convert them to target-native path, token, or semantic name values before returning domain data to usda-spec-parser.

Do not return JSON-shaped values to usda-spec-parser; return target-native typed value objects or apply a clearly typed adapter only at test/dump output.

Do not implement value resolution or spline evaluation.

Time sample maps and spline maps are specialized authored values. They should be returned as target-native domain records: time samples keyed by numeric time ordinates with typed field values/value blocks, and splines with typed extrapolation, loop, knot, tangent, and interpolation members. Do not return Python dict/list payloads, std::map<string, Json>, tagged JSON, or canonical dump object trees as the domain representation for these values.

Test Obligations

• scalar defaults
• tuple defaults
• arrays
• dictionaries
• string dictionaries
• path references and asset references
• None value block for attribute defaults
• time sample maps as stored authored values
• spline maps as stored authored values
• type mismatch diagnostic
• numeric coercion where allowed by attribute type validation