[m-users.] Fwd: Understanding string literals and name tokens

[Xiaofeng Yang]

Thanks for your help. It seems that I've found most answers from the source
code. :lol:

     Best regards,
Xiaofeng Yang

2014-12-14 20:09 GMT+08:00 Julien Fischer <jfischer at opturion.com>:
>
>
> Hi,
>
> On Sat, 13 Dec 2014, Xiaofeng Yang wrote:
>
>  I have confused by the Mercury Reference Manual (14.01.1). I found it
>> seems
>> not so up-to-date with the implementation. Now I have some questions about
>> string literals and name tokens need to be helped.
>>
>> Do the differences between a string literal and a quoted name is only
>>     * quoted by single/double quotes
>>     * '' can appears in a quoted name (but not a string literal), whereas
>> "" can appears in a string literal (but not a quoted name)
>> ?
>>
>
> Almost, except that:
>
> '' _can_ appear in a string literal, it just means two single quote
> characters
> unlike in a quoted name where it represents _one_ single quote character.
>
> Likewise, "" in a quoted name represents two double-quote characters and in
> a string literal represents one double-quote character.
>
>  I found some usage not be mentioned in the refman, e.g.
>>
>> I write "\\" but it interpreted as \\, this is not mentioned.
>>
>
> I'm not sure what you mean by "interpreted" there.  The value given by the
> string literal "\\" is a string containing a single backslash character.
> It may be that you are trying to print this string  it out using the
> predicate
> io.write.  If so, note what the documentation for that predicate says:
>
>         Strings and characters are always printed out in quotes, using
> backslash
>         escapes if necessary.
>
> If you just want to print out the value of the string literal instead of
> its
> syntactic representation use io.write_string or io.print instead.
>
>  I write "\" but it errors while compiling, so how the backslash works?
>>
>
​Yes, this is my mistake.​



>
> That literal is missing a terminating double-quote delimiter.  The escaped
> double-quote does not act as one.  It should be:
>
>     "\""
>


refman: "Strings may also contain backslash escapes. ‘\a’ stands for
“alert” (a beep character), ‘\b’ for backspace, ‘\r’ for carriage-return,
‘\f’ for form-feed, ‘\t’ for tab, ‘\n’ for newline, ‘\v’ for vertical-tab.
An escaped backslash, single-quote, or double-quote stands for itself."


I found that \` is also ​ a backslash escape, but not mentioned in the
refman.



>
> (Which is no different to the way C, C++, C#, Java and many other
> programming
> languages handle the same thing.)
>
>  I write "\143\" and it is interpreted as "c", this is what mentioned in
>> refman 2.3. But I write "\143" is also interpreted as "c",
>>
>
> ​​
> It's a (deliberate) bug in the Mercury implementation.  Unterminated octal
> escapes like "\143" should not be allowed, as the reference manual says,
> but
> the implementation currently does allow them because when the lexer was
> originally written compatibility with NU-Prolog was required and NU-Prolog
> apparently had a bug that lead it to accept unterminated octal escapes.
>
> This should have been fixed a long time ago (since compatibility with
> NU-Prolog
> isn't an issue these days), but I guess the issue hasn't arisen until now
> because octal escapes are so rarely used.  I will fix the lexer up so that
> it rejects unterminated octal escapes.


​This sounds a good news.​



>
>
>  whereas "\43" will interpreted as "#", these are what not mentioned in
>> the refman.
>>
>
> Same issue as above.
>
>  I found the max value for the rule "\x....\" is \xfffff\, and the max
>> value
>> for the rule "\....\" is \x777777\, this is what not mentioned in the
>> refman.
>>
>
> ​​
> The intention is that you should get an error for any character code that
> lies
> outside the Unicode range (0x0 - 0x10ffff).  I'll add something to the
> reference
> manual about this.
>
>
I found that ​0x0 is not a​llowed in the form of \x....\, \...\, \u.... and
\U........, but this is not mentioned in the refman.



> Cheers,
> Julien.
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