Re: [Xsb-development] perlmatch quoting problem - a bug report and patch

[Michael K. <ki...@cs...>]

>    There was an unclear spot in the patch (the chop($whatever); in the middle
>    of that string), which I asked Mitchell to clarify.
> 
> This perl code
>    $whatever = <<'hee'; chop($whatever);
>    bar
>    hee
>    ...other statements...
> is equivalent to
>    $whatever = 'bar'."\n"; chop($whatever); ...other statements...

I see - thanks a lot! I wasn't aware of the
$whatever = <<END; command;
   ...
   END

idiom. I thought that 'command' becomes part of the assigned string and was
wondering what was going on ...


	--michael  


> The chop($whatever) chops off that excess trailing newline.
> Chop is called _after_ the string assignment, and mutates the string.
> The end marker (here "hee") has to occur on a line by itself, so we
> add a trailing newline to the input string (here "bar") to make sure
> this happens, and then strip it away again after the string is assigned
> and before it is used.
> 
> So the patch changes something like
>    "$__text__='%s'"
> into something like
>    "$__text__= <<'ENDj9yq6QC43b'; chop($__text__);\n%s\nENDj9yq6QC43b\n"
> 
> I called the variable "$whatever" in the patch summary because the
> actual variable name varies among perlmatch functions ($__text__,
> $__string__, and $string).
> 
> From a language design standpoint, though here-documents look odd at
> first, they permit use like
>    foo( arg1 , <<'END_ARG2', <<'END_ARG3' , arg4 );
>    arg2...
>    END_ARG2
>    arg3...
>    END_ARG3
> which can make code with large text blocks, or quoting issues, much
> clearer.
> 
> Sorry for the confusion.
> 
> Mitchell Charity
> 
> 
> And btw, just in case anyone out there has been using perlmatch's old
> broken quoting to make it serve as a nice general perl eval(), note
> one can still do so with a (slightly more complex) idiom like:
>    // Flora code
>    #perl_eval(Code,StrVal) :-
>      #join(['ENDj9yq6QC43b
>    $__ret__ = "".do{',Code,'};
>    $__ignore__ = <<ENDj9yq6QC43b;
>    '],Src),
>      #perl_substitute(Src,'s/^/$__ret__/s',StrVal).
> Fyi.  It can be quite useful.
> 
> 
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