Chapter 8

Output Schemas and Structural Queries

Parser.OutputSchema describes the possible values of a checked grammar. A structural query searches the values of an actual parse result. Together they let a tool inspect the shape of a grammar and find values without generating code or serializing a result. This chapter defines both.

The Go APIs in this chapter are experimental and MAY change incompatibly. The schema JSON format is versioned; this chapter defines version 2.

#Scope and Terminology

A schema is a description of the output of a grammar: its declared struct types and the inferred value shape of each rule. A pattern (QueryPattern) is a programmatic description of a value and of the values reachable from it. A query is a compiled pattern, evaluated against a parse result. A match is one occurrence of a value in the result that satisfies the pattern, together with its captures.

A parser compiled with source information records the rule that produced each node and the labels of its captures. Rule constraints need it.

#Contract

#Output Schemas

p.OutputSchema(opts...) returns the schema of the parser's grammar.

• The default schema contains the configured start rule, the rules it calls and the types that their output shapes reference. WithAllSchemaRules() also includes unreachable rules and all declared types. WithStart changes the default root.
• A type entry is either a declared struct type, with ID declared/<Name>, or the inferred value of a rule, with ID inferred/<rule>. Aliases are not entries: their shapes appear where they are used.
• sourceInfo records whether the parser was compiled with source information.
• Entries, fields, join alternatives and captures have a stable order, so repeated exports of one checked grammar encode to identical JSON bytes.
• Export needs checked grammar metadata, which only the grammar source has. A parser loaded from a compiled grammar holds none, in any mode: its export fails with an error that matches pego.ErrSchemaUnavailable, and pego schema rejects a compiled grammar. Export the schema from the grammar source.

#Shapes

A shape describes a set of values. Each kind allows only the members listed.

A node shape requires value. Each entry of captures names a capture of the node and gives its shape and its locations: paths lists the index paths of the elements of the node's value that the capture labels, and side is true when the capture is held beside the value rather than as one of its elements. A capture has at least one location. Captures are static: the schema describes them whether or not the parser records labels.

A tuple of no elements is the unit value.

#Schema JSON, Version 2

Decoding requires exact field-name case and rejects unknown fields, duplicate keys at every object level, unsupported formats and versions and incomplete envelopes.

schema.Validate() performs the full version 2 validation of a schema, whether it was decoded from JSON or built in Go. It checks that:

• type IDs, rule names, field names and the capture names of a node are nonempty and unique;
• each type ID is declared/<name> or inferred/<name> and agrees with its origin, and every declared type has a struct shape;
• shape kinds and scalar names are supported, and each shape has only the members of its kind;
• list and optional shapes have their element, and node shapes their value;
• every capture has a location, and no index of a path is negative;
• references resolve, and inline shapes are acyclic (cycles through named references are allowed);
• Start names a rule of Rules, and the Result of each rule is a reference exactly to inferred/<rule-name>, which exists as a type entry.

Omitted, nil and empty member slices are equivalent.

Validation checks structure and canonical relationships. Type names alone do not establish compatibility between grammars.

#Comparing Schemas

pego.DiffOutputSchemas(before, after) validates both complete operands and compares their contents by type ID and rule name. It returns the added, removed and changed entries in sorted order; for equal schemas its Types and Rules slices are empty. The comparison ignores the order of entries, of struct fields, of join alternatives, of captures and of capture paths, and envelope fields such as Start and sourceInfo; tuple element order and named references are significant. A diff is descriptive: it does not classify compatibility.

#Patterns

QueryPattern constrains a value and the values reachable from it.

• Elements sees through nodes as x[i] does: on a node it applies to the elements of the node's value.
• The fields of an error value are message and expected, both strings.
• Constraints that no value can satisfy together, such as Category: "int" with Fields, are rejected when the query is compiled.
• Patterns are copied when compiled. There is no text query language and there are no text predicates.

#Compiling Queries

pego.CompileStructuralQuery(pattern, schema) compiles a query. A supplied schema is validated first, and the compilation rejects patterns that no described value can match: unknown type and rule names, fields that a type does not have and categories or types that a value cannot have. Nested patterns are checked against the shape of the field, element or node value to which they apply.

• A join or optional shape accepts a pattern that matches any one of its alternatives. With Value *(Pair | Box), a Value pattern of type Pair with Pair fields, of type Box or of category nil compiles, while one of category string does not.
• A pattern applied to a value of unknown shape (top or the query root) is checked only against its own type's fields.
• An error value appears only where the shape includes error or missing, so a pattern of type or category error compiles only there. A missing token is an error value: the patterns of errors, with their fields, apply to a missing shape.
• For an optional scalar field such as Value *string, the categories string and nil compile and other known categories do not.
• A pattern with a Rule constraint compiles only against a schema whose sourceInfo is true, and only for a rule the schema has.
• A nil schema defers these checks to evaluation. Evaluation always checks the runtime values, because a caller MAY mutate a result after parsing.

pego.PrepareQuerySchema(schema) validates a schema once and returns a prepared schema whose CompileStructuralQuery(pattern) performs the same pattern checks without validating again. A prepared schema owns a copy of the schema and is safe for concurrent use.

#Evaluation

StructuralQuery.Matches(root, opts...) accepts a parse result and returns all matches or an error.

• Matches visit each value before its outgoing edges: the elements of a tuple, of a list or of a node's value by index, then the captures of a node that are held beside its value, by name, and the fields of a struct by name. A shared value can produce several matches.
• Each QueryCapture holds the captured value, its immediate containing Parent and an owned Path relative to the root, made of element, capture and field edges. Captured values and parents belong to the input graph. The graph MUST be stable while results are used and MUST NOT be mutated concurrently.
• A cycle on the active path matches pego.ErrQueryCycle. Sharing without a cycle is allowed.
• A pattern with a Rule constraint on a result that has nodes but records no rule, because the parser was compiled without source information, matches pego.ErrQueryNoSourceInfo.
• pego.WithQueryContext(ctx) supplies a context; cancellation returns the error of the context. pego.WithQueryWorkLimits(pego.WorkLimits{MaxSteps: n}) bounds traversal, matching, path copies and result construction; exhaustion matches pego.ErrWorkLimit. Zero limits are unlimited. MaxInputWork is unused, because a query inspects no source text. The step count is abstract and MAY change between versions.
• A query sees only the values that the result retains. Actions and discard operators remove punctuation and comments; a query does not recover them, interpret source positions or highlight.

#Validation and Errors

#Compatibility and Support

Schemas and queries are available only through the Go API of the engine. Generated Go and TypeScript parsers do not expose them.

#Conformance

The schema, validation, diff and query tests of the repository establish the behavior. Repeated export of one grammar is tested for byte equality.