[antlr-interest] Function Expressions

[Jeff Hair]

This does indeed work as expected. Wouldn't have been able to come up with
that on my own. I started down the path of putting function calls in between
term and booleanExpression, and that was *sort of* working. But I also
uncovered some other issues with properties (what you are calling
projection). I sort of understand what's going on with this code via
referencing the last example under "Rewrite rules" at
http://www.antlr.org/wiki/display/ANTLR3/Tree+construction. But I don't
understand it fully. Perhaps I just need some time to digest it. I only
discovered ANTLR like 3 weeks ago, so there's still a lot I have to learn.
I'm surprised I've gotten as far as I have without help, actually...

On Wed, May 4, 2011 at 6:19 PM, John B. Brodie <jbb at acm.org> wrote:

> Greetings!
>
> On Wed, 2011-05-04 at 12:27 -0400, Jeff Hair wrote:
> > Hello all,
> >
> > I have a simple C/JavaScript-style grammar for my interpreter project.
> Right
> > now, functions can be called via identifier(), or identifier(param1,
> > param2). This works fine for simple cases, but in my language functions
> are
> > first-class objects. I'm in the process of redoing my identifier logic
> for
> > properties, arrays, and function calls. I've gotten the first two
> working.
> >
> > I'm trying to allow expressions to be callable as functions, so I can do
> > stuff like createFunction()(), where createFunction would be a function
> that
> > returns a function. Another example would be (1 + 1)(). Obviously that
> > should throw an error, but it should be a permissible language construct.
> > JavaScript allows this.
> >
> > The func.g file in my gist is a pared down version of the language
> grammar,
> > with only the relevant rules in it. Understandably, it fails with the
> > following errors:
> >
> > [java] error(210):  The following sets of rules are mutually
> left-recursive
> > [boolNegation, unary, add, mult, relation, term, expression]
> > [java] error(206): /home/user/955488/func.g:66:2: Alternative 1: after
> > matching input such as IDENT '(' decision cannot predict what comes next
> due
> > to recursion overflow to relation from expression
> > [java] error(201): /home/user/955488/func.g:66:2: The following
> alternatives
> > can never be matched: 2
> >
> > https://gist.github.com/955488 demonstrates the issue. I've stripped out
> > everything except the expression rules. The gist can be cloned as a git
> repo
> > and then built via Ant + Ivy.
> >
> > I understand why it's failing. There's a conflict between the IDENT
> > expression and IDENT '(' ')' for function calls. What I'm trying to
> figure
> > out is how to allow both identifiers and function calls. If I figure that
> > out, it should give me the rest of what I need. Any help would be
> > appreciated.
> >
>
> I think that basically you want Application (I come from the lambda
> calculus and function invocation is called Application therein) to be a
> post-fix operator.
>
> If you look at grammars for Java or C or other C-like languages you will
> see (I believe) that indexing into an array and/or projecting a field
> out of a tuple (record) and probably others-like-them are post-fix
> operators.
>
> So I would recommend the following (tested, but not using your
> incomplete gist framework) --- replacing your term rule with:
>
> //Expressions
> primary
>   : IDENT
>   | '('! expression ')'!
>   | INTEGER
>   | DOUBLE
>   | BOOLEAN
>   ;
>
> term
>   : (primary -> primary) ( suffix[$term.tree] -> suffix )*
>   ;
>
> suffix [CommonTree term] :
>   ( x='(' modifiers? ')' -> ^(APPLICATION[$x,"A"] {$term} modifiers?) )
>  | ( x='[' modifiers  ']' -> ^(INDEXING[$x,"I"] {$term} modifiers) )
>  | ( x='.' (p=IDENT|p=INTEGER) -> ^(PROJECTION[$x,"P"] {$term} $p) )
>  ;
>
> modifiers : expression (','! expression)* ;
>
>
>
> with an appropriate tokens{} section defining  the APPLICATION;
> INDEXING; and PROJECTION imaginary tokens.
>
> the above is a little complicated in order to get the imaginary token
> representing the suffix operator to be a tree root.
>
> note that the stuff in the []'s after the imaginary token name is for
> error reporting and/or tree pretty printing. keep the $x but change the
> string to suite your need.
>
> hope this helps
>    -jbb
>
>
>
>