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

[Thomas 'PointedEars' Lahn]

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.