Introduction

This module implements a parser for the Contextual Query Language (CQL) which is a part of the Search/Retrieval via URL (SRU) and Search/Retrieval via Web Service (SRW) family of protocols. See the Library of Congress spec for details.

Usage

Running make or go build -o . ./... will compile the library and create the command-line tool cql-cli.

The primary purpose of the library is to parse a CQL string and produce cql.Query which is a parse-tree really.

import (
   "fmt"
   "github.com/indexdata/cql-go/cql"
)

func main() {
   var parser cql.Parser
   query, err := parser.Parse("title=hello")
   if err != nil {
      fmt.Fprintln(os.Stderr, "ERROR", err)
   } else {
      fmt.Println(&query)
   }
}

See the cql-cli source for a more complete example.

Conformance

The CQL specification requires that a query consist of a single search term with an optional index and relation:

(index relation) term

If multiple terms are provided in a query, they must be quoted:

(index relation) "term1 term2"

To remain compatible with existing implementations, like yaz or CQL-java, this parser relaxes this requirement and allows for providing multiple unquoted terms in a query:

(index relation) term1 term2

This behavior is enabled by default and can be disabled by setting the Parser{Strict: true} flag.

To allow for multiple terms in a query, the parser disambiguates between terms and relations by assuming that a relation is either:

1. a symbolic relation: =, ==, <>, < , <=, >, >=
2. a built-in named relation from the default context set:
   adj, any, all, exact, encloses, within
3. a custom prefixed relation for which a prefix is declared, e.g:
   > dc = "http://deepcustard.org/" index dc.relation term
   Note that, unlike yaz or CQL-Java, terms that simply contain a . are not be considered relations unless the query includes a prefix declaration.
4. a custom relation, if a custom default context set is declared, e.g:
   > "http://deepcustard.org/" index relation term
   Note that, effectively, any term on the second position is considered a relation when a custom default context is declared.

These rules are applied in both the non-strict (default) and strict parsing modes.