A DCG for generating Lucene searches

Right. First off, forget everything you know about Lucene's search syntax. Especially the boolean operators, which do not work in any logical way. This library is based on a data type which, as far as I can determine, represents the internal structure of a Lucene query. Basically, a query is a triple of a modifier (Lucene +, -, or <none>), a numerical boost (Lucene ^ operator), and a, for want of a better name, a 'part'. I could have called it a query 'component', but 'part' is a shorter word that means the same thing. A part is either a primitive term coupled with a field name( (:)/2 constructor), or a composite part consisting of a list of sub-queries (comp/1 constructor). So, we have:

:- type query ---> q(modifier,boost,part).
:- type part  ---> comp(list(query))
                 ; field:prim.

:- type modifier ---> plus, minus, none.
:- type boost == nonneg.

Note that the 'field' argument of the (:)/2 part constructor is inherently defaulty: if no field is specified, the search agent fills it in with an application specific default.

The primitives cover all those obtainable using the Lucene syntax and are as follows:

:- type prim  ---> word(word)           % bare, unquoted literal word
                 ; glob(pattern)        % word with wildcards * and ?
                 ; re(pattern)          % regular expression /.../
                 ; fuzzy(word,integer)  % fuzzy word match <...>~N
                 ; range_inc(word,word) % inclusive range [A TO B]
                 ; range_exc(word,word) % exclusive range {A TO B}
                 ; phrase(list(word),integer) % quoted multi word with slop
                 .

Building queries out of these constructors is a bit of a chore, so next we have an term language and associated evaluator which takes an expression and produces a valid query term. This can be thought of as a set of functions which return queries. Every function in the language produces a value of type query. Some of them leave the field and modifier arguments unbound. If they are unbound at the end of the process, they take on default values. The functions and literals are as follows:

<any atomic literal> :: query    % primitive word with unbound modifier and field
(@)  :: atomic -> query          % wildcard pattern with unbound modifier and field
(\)  :: atomic -> query          % regular expression with unbound modifier and field
(/)  :: atomic, number -> query  % fuzzy match with unbound modifier and field
(//) :: list(atomic), number -> query  % quoted phrase with unbound modifier and field
(+)  :: atomic, atomic -> query  % inclusive range with unbound modifier and field
(-)  :: atomic, atomic -> query  % exclusive range with unbound modifier and field
(+)  :: query -> query           % unifies query modifier with plus
(-)  :: query -> query           % unifies query modifier with minus
(:)  :: atom, query              % unifies all field arguments recursively
(^)  :: query, number            % multiplies boost factor
list(query) :: query             % a list of queries evaluates to a composite query
                                 % with unbound modifier.

A few notes are in order.

Unlike Lucene's ~ postfix operator, the (/)/2 operator must have a number for the edit distance parameter. Lucene's default is 2.
Unlike Lucene's bare quoted term, the (//)/2 must have a number to use as a 'slop' parameter. Supplying zero replicates Lucene's treatment of a bare quoted phrase.
(+)/1 and (-)/1 cannot be composed: this is a contradiction that results in error. (+)/2 and (-)/2 are both idempotent.
Prolog's (-)/1 and (+)/2 operates bind tighter than (:)/2 operators, which is the wrong way round for Lucene queries: Prolog reads -F:E as (-F):E, but my expression language needs to see -(F:E). Hence, there is little kludge in the evaluator to catch such terms and re-group the operators.
Two different field names cannot both be applied to the primitive. Considering that the (:)/2 operator is applied recursively into sub-queries, this means that each node in the syntax tree can have at most one field name on the path from it to the root. This is different from Lucene's parser, which allows one field name to override another.
I've taken the liberty of making multiple boosts on the same query combine multiplicatively, much like ordinary mathematical exponentiation. This is different from Lucene, where a later boost overrides an earlier boost. I think this way makes more sense. Thus, the type of such expression can be defined as:

qexpr ---> @atomic; \atomic
         ; atomic/number
         ; list(atomic)//number
         ; atomic + atomic
         ; atomic - atomic
         ; +qexpr ; -qexpr
         ; atom:qexpr
         ; qexpr^number
         ; list(qexpr)
         .
atomic :< qexpr.  % any atomic is a qexpr
query  :< qexpr.  % any query  is a qexpr

Quoting and escaping policy

Ugh. Let us take the 4 different cases in turn:

Plain search terms. The Prolog interface takes any sequence of characters and automatically escapes them for Lucene.
Wildcard terms (@W). These can contain any characters but * and ? are interpreted as wildcards. The can be escaped with \ to be interpreted literally. \ must be escaped with \. A single \ is illegal.
Regexp terms (\RE). Any / characters are escaped automatically. Otherwise, Many characters have special meaning in regular expressions and must be escaped with \ to be inserted literally. Infact, ANY character can be escaped with \ and inserted literally. A single \ is illegal.
Field names. These do not support escape sequences and cannot contain any special characters (ie " /+-&|!(){}[]^\"~?:\").

So that's the basics of it. There might still be some problems in the DCG when it comes to handling character escapes. Somewhat suprisingly, the DCG seems to parse much of the Lucene query syntax more or less correctly, except for the boolean operators, which Lucene does not handle in any sensible way and are best avoided. Also, it does not parse field names applied to componound queries or the postfix '~' operator.

See (though I warn you it will not be especially enlightening) https://lucene.apache.org/core/4_3_0/queryparser/org/apache/lucene/queryparser/classic/package-summary.html#package_description */