Re: A simple single line, triple-quoted comment is giving syntax error. Why?

From	Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups	comp.lang.python
Subject	Re: A simple single line, triple-quoted comment is giving syntax error. Why?
Date	2015-03-28 19:06 +0100
Organization	PointedEars Software (PES)
Message-ID	<2864857.XXlpeINscL@PointedEars.de> (permalink)
References	(6 earlier) <1504323.jUKfeKbQsP@PointedEars.de> <mailman.181.1427346636.10327.python-list@python.org> <2362875.FIK8ImHTJA@PointedEars.de> <mailman.211.1427394586.10327.python-list@python.org> <cnjgcjFriptU1@mid.individual.net>

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Gregory Ewing wrote:

> Ian Kelly wrote:
>> What I mean is that if you construct a parse tree of "foo" "bar" using
>> that grammar, it looks like this:
>> 
>>      expr
>>        |
>>     STRING+
>>      /   \
>> STRING  STRING
> 
> Not quite -- STRING+ is not a symbol in the grammar, it's
> a shorthand for a combination of symbols. The parse tree
> is actually just
> 
>        expr
>        /   \
>   STRING  STRING

Or if there is a single STRING,

  expr
   |
  STRING

AISB.

>> Not like this:
>> 
>>     expr
>>      |
>>    STRING+
>>     /  \
>>  expr  expr
>>   |      |
>> STRING  STRING
> […]
> 
> To get a parse tree like the above, there would need to be
> a production like this in the grammar:
> 
>     expr ::= expr+
> 
> There is no such production, so that parse tree is impossible.
> QED.

Correct.

> What you seem to be doing in your mind

Who is “you” here?  You appear to be confused, probably supported by your 
unconventional quotation style, who made which statement.

> is taking this production:
> 
>     expr ::= STRING
> 
> and using it "backwards" to justify expanding any occurrence
> of STRING into expr.

No.

> But grammars don't work that way. You're committing a logical fallacy:
> just because some exprs are STRINGs doesn't mean that all STRINGs are 
> exprs.

The fallacy – a straw man – is yours here.  Nobody – in particular not me – 
claimed that “STRING” can *only* be produced by “expr” in the first place.

Production rules in a formal grammar of the form

  expr           → … | any_goal | …
  any_goal       → … | any_other_goal | …
  any_other_goal → … | STRING+ | …

where the steps in-between are optional, mean *exactly* that (*all*) 
“STRING”s are “expr”s – but that not all “expr”s are “STRING”s.  Because it 
follows from the rules above that these production chains – *but not _only_ 
these* – are possible:

  a) expr ⇒ … ⇒ STRING
  b) expr ⇒ … ⇒ STRING STRING
  c) expr ⇒ … ⇒ STRING STRING STRING

and so on.  The truth of the statement “(All) STRING( literal)s are 
expr(ession)s” is what follows from the possibility of the first production 
chain.  (That does not preclude the possibility that “STRING”s are also an 
element of another set.)

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

Thread

A simple single line, triple-quoted comment is giving syntax error. Why? Aditya Raj Bhatt <adityarajbhatt@gmail.com> - 2015-03-18 10:46 -0700
  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Laurent Pointal <laurent.pointal@laposte.net> - 2015-03-18 19:04 +0100
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Laurent Pointal <laurent.pointal@laposte.net> - 2015-03-18 19:06 +0100
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-18 20:53 +0100
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-18 20:56 +0100
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Terry Reedy <tjreedy@udel.edu> - 2015-03-18 17:47 -0400
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-19 09:58 +1100
          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-03-19 10:23 +1100
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Aditya Raj Bhatt <adityarajbhatt@gmail.com> - 2015-03-18 11:23 -0700
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Igor Korot <ikorot01@gmail.com> - 2015-03-18 14:45 -0400
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Laurent Pointal <laurent.pointal@laposte.net> - 2015-03-18 20:08 +0100
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Tim Roberts <timr@probo.com> - 2015-03-28 16:34 -0700
  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-18 21:21 +0100
  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Tim Chase <python.list@tim.thechases.com> - 2015-03-18 13:15 -0500
  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ben Finney <ben+python@benfinney.id.au> - 2015-03-19 11:06 +1100
  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Denis McMahon <denismfmcmahon@gmail.com> - 2015-03-19 00:53 +0000
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-22 04:14 +0100
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-03-22 14:36 +1100
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-22 04:49 +0100
          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-03-22 15:11 +1100
            Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 05:35 +0100
              Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-25 23:09 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 17:45 +0100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 12:29 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 19:54 +0100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 17:27 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-28 19:20 +0100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-29 01:56 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-29 12:41 +0200
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-31 01:23 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? sohcahtoa82@gmail.com - 2015-03-31 16:10 -0700
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-04-01 10:36 +1100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-31 17:43 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-04-02 23:31 +0200
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? sohcahtoa82@gmail.com - 2015-04-02 15:26 -0700
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-02 18:21 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-03 16:40 +1100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-04-03 16:57 +1100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Marko Rauhamaa <marko@pacujo.net> - 2015-04-03 10:13 +0300
                r"\"" ??? (was A simple single line, triple-quoted comment) Rustom Mody <rustompmody@gmail.com> - 2015-04-03 05:52 -0700
                Re: r"\"" ??? (was A simple single line, triple-quoted comment) Chris Angelico <rosuav@gmail.com> - 2015-04-04 01:40 +1100
                Re: r"\"" ??? (was A simple single line, triple-quoted comment) Rustom Mody <rustompmody@gmail.com> - 2015-04-03 08:14 -0700
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-03 09:25 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-03 00:40 -0600
                Re: A simple single line, triple-quoted comment is giving syntax   error. Why? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-03-27 11:43 +1300
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-28 19:06 +0100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 12:32 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 19:56 +0100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-27 10:15 +1100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Tim Chase <python.list@tim.thechases.com> - 2015-03-26 18:47 -0500
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 19:38 -0600
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Marko Rauhamaa <marko@pacujo.net> - 2015-03-27 07:34 +0200
              Re: A simple single line, triple-quoted comment is giving syntax error. Why? Dave Angel <davea@davea.name> - 2015-03-26 01:23 -0400
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 18:25 +0100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 18:30 +0100
          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Marko Rauhamaa <marko@pacujo.net> - 2015-03-22 12:00 +0200

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