From: Albert Schlef <albertschlef@...>
Date: 2008-12-28T21:08:54+09:00
Subject: Re: Syntax error I don't understand

Dave Bass wrote:
>
> "When in doubt, use parentheses." This avoids having to learn
> complicated rules

Matt Harrison wrote:
>
> It would be my guess ruby is trying to do "a << rand" and then finds
> an extra integer stuck on the end.

No doubt. But could anybody describe this special case more accurately 
so I recognize it in the future? It even happens for "2 + rand 3". I 
don't want to add parentheses in all my function calls, because I 
understand that's contrary to Ruby's coding style.

I had a peek at a simplified BNF of ruby:

http://web.njit.edu/all_topics/Prog_Lang_Docs/html/ruby/yacc.html

and I see:

  ARG      : ...
           | ARG `<<' ARG
           | ...
           | ARG `+' ARG
           | ARG `-' ARG
           | ARG `*' ARG
           | ARG `/' ARG
           | ...
           | PRIMARY

  PRIMARY  : ...
           | FUNCTION
           | ...

  FUNCTION : ...
           | OPERATION [`(' [CALL_ARGS] `)']
           | ...

  COMMAND  : ..
           | OPERATION CALL_ARGS
           | ...

And according to this, "a << rand 3" indeed is a syntax error because 
"rand 3" isn't a FUNCTION but a COMMAND. Since a COMMAND isn't a 
PRIMARY, it can't stand for an ARG.

Nevetheless, I wonder if I should have known this without checking out 
the BNF. Shouldn't it be "common knowledge" that you can't do "2 + rand 
3" in Ruby?
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