ANTLR4 : clean grammar and tree with keywords (aliases ?)

Question

I am looking for a solution to a simple problem.

The example :

SELECT date, date(date)
FROM date;

This is a rather stupid example where a table, its column, and a function all have the name "date".

The snippet of my grammar (very simplified) :

simple_select
    : SELECT selected_element (',' selected_element) FROM from_element ';'
    ;

selected_element
    : function
    | REGULAR_WORD
    ;

function
    : REGULAR_WORD '(' function_argument ')'
    ;

function_argument
    : REGULAR_WORD
    ;

from_element
    : REGULAR_WORD
    ;


DATE:     D A T E;
FROM:     F R O M;
SELECT:   S E L E C T;

REGULAR_WORD
    : (SIMPLE_LETTER) (SIMPLE_LETTER | '0'..'9')*
    ;

fragment SIMPLE_LETTER
    : 'a'..'z'
    | 'A'..'Z'
    ;

DATE is a keyword (it is used somewhere else in the grammar). If I want it to be recognised by my grammar as a normal word, here are my solutions :

1) I add it everywhere I used REGULAR_WORD, next to it. Example :

selected_element
    : function
    | REGULAR_WORD
    | DATE
    ;

=> I don't want this solution. I don't have only "DATE" as a keyword, and I have many rules using REGULAR_WORD, so I would need to add a list of many (50+) keywords like DATE to many (20+) parser rules : it would be absolutely ugly.

PROS: make a clean tree

CONS: make a dirty grammar

2) I use a parser rule in between to get all those keywords, and then, I replace every occurrence of REGULAR_WORD by that parser rule. Example :

word
    : REGULAR_WORD
    | DATE
    ;

selected_element
    : function
    | word
    ;

=> I do not want this solution either, as it adds one more parser rule in the tree and polluting the informations (I do not want to know that "date" is a word, I want to know that it's a selected_element, a function, a function_argument or a from_element ...

PROS: make a clean grammar

CONS: make a dirty tree

Either way, I have a dirty tree or a dirty grammar. Isn't there a way to have both clean ?

I looked for aliases, parser fragment equivalent, but it doesn't seem like ANTLR4 has any ?

Thank you, have a nice day !

Answer 1

There are four different grammars for SQL dialects in the Antlr4 grammar repository and all four of them use your second strategy. So it seems like there is a consensus among Antlr4 sql grammar writers. I don't believe there is a better solution given the design of the Antlr4 lexer.

As you say, that leads to a bit of noise in the full parse tree, but the relevant non-terminal (function, selected_element, etc.) is certainly present and it does not seem to me to be very difficult to collapse the unit productions out of the parse tree.

As I understand it, when Antlr4 was being designed, a decision was made to only automatically produce full parse trees, because the design of condensed ("abstract") syntax trees is too idiosyncratic to fit into a grammar DSL. So if you find an AST more convenient, you have the responsibility to generate one yourself. That's generally straight-forward although it involves a lot of boilerplate.

Other parser generators do have mechanisms which can handle "semireserved keywords". In particular, the Lemon parser generator, which is part of the Sqlite project, includes a %fallback declaration which allows you to specify that one or more tokens should be automatically reclassified in a context in which no grammar rule allows them to be used. Unfortunately, Lemon does not generate Java parsers.

Another similar option would be to use a parser generator which supports "scannerless" parsing. Such parsers typically use algorithms like Earley/GLL/GLR, capable of parsing arbitrary CFGs, to get around the need for more lookahead than can conveniently be supported in fixed-lookahead algorithms such as LALR(1).

Answer 2

This is the socalled keywords-as-identifiers problem and has been discussed many times before. For instance I asked a similar question already 6 years ago in the ANTLR mailing list. But also here at Stackoverflow there are questions touching this area, for instance Trying to use keywords as identifiers in ANTLR4; not working.

Terence Parr wrote a wiki article for ANTLR3 in 2008 that shortly describes 2 possible solutions:

This grammar allows "if if call call;" and "call if;".

grammar Pred;

prog: stat+ ;

stat: keyIF expr stat
    | keyCALL ID ';'
    | ';'
    ;

expr: ID
    ;

keyIF : {input.LT(1).getText().equals("if")}? ID ;

keyCALL : {input.LT(1).getText().equals("call")}? ID ;

ID : 'a'..'z'+ ;
WS : (' '|'\n')+ {$channel=HIDDEN;} ;

You can make those semantic predicates more efficient by intern'ing those strings so that you can do integer comparisons instead of string compares.

The other alternative is to do something like this

identifier : KEY1 | KEY2 | ... | ID ;

which is a set comparison and should be faster.

Normally, as @rici already mentioned, people prefer the solution where you keep all keywords in an own rule and add that to your normal identifier rule (where such a keyword is allowed).

The other solution in the wiki can be generalized for any keyword, by using a lookup table/list in an action in the ID lexer rule, which is used to check if a given string is a keyword. This solution is not only slower, but also sacrifies clarity in your parser grammar, since you can no longer use keyword tokens in your parser rules.