Re: [boost] [Output Formatter]: Cascading Style Sheets (CSS) proposal

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-09-17 10:28:19

Vladimir Prus wrote:
>Hi Reece,
>
> >    io::sequence_decorators< char > seq( "; " );
> >    std::cout << '"' << seq.open(      std::cout ) << "\"\n"  // "[ "
> >              << '"' << seq.close(     std::cout ) << "\"\n"  // " ]"
> >              << '"' << seq.separator( std::cout ) << "\"\n"; // "; "
> >
> > where separator( std::cout ) uses the separator value specified by seq 
>and
> > the others use the values set on the std::cout stream.
>
>I don't understand this phrase, could you clarify. What does it mean to 
>"use
>the separator".

"use the separator value": the io::sequence_decorators< char > 
(io::formatter in the review implementation of the library) type holds 3 
io::decoration< char > properties -- open; close; and separator. These are 
used when performing input/output using my library to render a given type.

In the example above, the seq object provides a custom value for the 
separator decoration. The io::decoration< char >::operator()( StreamType & ) 
method will look up the default value of the decoration using the stream 
passed to it unless a custom value is provided. Thus, when accessing the 
separator value using this method, the custom one is chosen instead of the 
default one.

> > To change these
> > defaults, you can use:
> >
> >    std::cout << io::cdecorate( "[{ ", " }]" ); // [{ a, b, c }]
> >
> > The problem with this is that it changes the defaults for all the
> > formatting used, thus you cannot have different defaults for sequences 
>and
> > n-ary types.
>
>I don't undestand this, either. Probably I've missed something in earlier
>messages. What's "n-ary" type?

An n-ary type is a type that has a fixed number of elements. std::pair is a 
2-ary type, boost::math::quaternion is a 4-ary type, and so on.

>Do you provide the ability to use different separators for sequences, sets 
>and maps?

In the review version of the library, there are two sets of default values: 
one for n-ary types formatting as "( a, b )" and one for the other sequence 
types, formatting as "[ a, b, c ]". In the version I am rewriting, I only 
have one set of default formatting: "[ a, b, c ]".

>Then I don't understand why you can
>set different default style for those kinds of object, and cannot 
>selectively
>override the style

Because the way I am storing it on the stream is based on decoration type 
(open; close; separator) and not on the format object type.

> > == Dynamic Styles
> >
> > You can also set CSS properties within script code, e.g.
> >    tag.style.fontFamily = "Times New Roman";
> >
> > This style behaviour is available in the new decorators, for example:
> >    io::wrapper_decorators< char > wrap;
> >    wrap.open = "<< ";
> >    wrap.close = " >>";
>
>What does this decorator "wrap"?

A wrapper_decorator is one that only has open and close decoration values. 
Thus, it wraps the object associated with it (via a format object):

   template< class OutStream, typename T >
   OutStream & write( OutStream & os, const T & elem )
   {
      return( os << open << elem << close );
   }

The sequence_decorators< char > type extends the wrapper_decorator< char > 
type, adding the separator property.

> > == Cascading Style Sheets
> >
> > HTML provides a <style> tag to specify styling across the document 
>without
> > linking to an external file, e.g.:
> >    <style>h1{ color: orange; font-size: 2pc; }</style>
> >
> > The equivalent within my library would be to define the styling on a
> > stream, e.g.:
> >
> >    std::cout
> >       << css::element( "pair" )
> >          [
> >             ( css::attribute( "open"  ) = "<< " )
> >          +  ( css::attribute( "close" ) = " >>" )
> >          ]
> >       << css::element( "container" )
> >          [
> >             css::attribute( "separator" ) = " | "
> >          ]
> >       << io::object( ob ) // output: [ << a, 5 >> | << b, -1 >> ]
> >       << css::element( "pair" )
> >          [
> >             css::attribute( "separator" ) = " = "
> >          ]
> >       << io::object( ob ); // output: [ << a = 5 >> | << b = -1 >> ]
>
>I'm sorry, but I'm worried of the direction you took. For me, the primary
>utility of this library is debugging output. For that, I'd be happy with a
>single style, as long as it's something usable, like YAML
>(http://www.yaml.org/).

I haven't implemented CSS functionality yet. It is my thoughts based on the 
comments by Jonathan.

>I agree that you might want to store and then restore STL container from a
>file, and the library addresses this need to. But I don't understand why
>you would want to customize the formatting style. Basically, you either 
>should
>the data to user, or not. In first case, you need a single usable style. 
>For
>the second case, you might need another "compact" style. I don't see the
>point of customization.

It is currently possible to state what style formatting an object takes 
using the decorate function (format in the review implementation). Thus:

   // review implementation:
   std::cout << io::formatob( vec ).format( "( ", " )" ); // ( a, b, c, d )

Here, the separator is not set and thus takes the default value (the one 
provided by one of the mechanisms outlined above).

You might want to customize formatting for various reasons. One example is 
if you have:
   std::list< std::string > names;
and want to write that out as a HTML ordered list. You can do the following:

   std::cout << io::formatob( names ).format( "<ol><li>", "</li></ol>", 
"</li><li>" );

The examples provided in the library show some advanced formatting that can 
be achieved.

>Finally, is this library going to be used for producing generic text files?
>E.g. code generation. I think no, to generate text you need some text
>template system, and that's way beyond the scope of the library.

It is possible to do this to a limited degree, for example, outputting
   4, 7, 10, -5
   27, 11, 5, 17
   ...

so the data can be read in by a spreadsheet, for example. It is also 
possible to provide a basic form of message localization, reading a 
std::map< std::string, std::string > in from a file. E.g.:

english.loc
   "goodnight" = "Good night. Sweet dreams."
   "..." = "..."
german.loc
   "goodnight" = "Gute nacht. Sussen Schlafen."
   "..." = "..."
spanish.loc
   "goodnight" = "Buenos noches."
   "..." = "..."
portuguese.loc
   "goodnight" = "Boa noite."
   "..." = "..."

int main( ... )
{
   std::ifstream loc( argv[ 1 ]);
   std::map< std::string, std::string > messages;

   loc >> io::formatob( messages,
         io::containerfmt( io::pairfmt
         (
            io::containerfmt().format( "\"", "\"", "" ), // currently 
unsupported
            io::containerfmt().format( "\"", "\"", "" ) // currently 
unsupported
         ).format( " = " ))
      ).format( "", "\n[end]", "\n" );

   std::cout << "goodnight = " << messages[ "goodnight" ] << '\n';
}

>So, I'm not convinced custom style gives you anything. On the other hand, 
>it
>will cost a lot. At the very least, even documenting the above will be
>considerable effort. And given that the library is header-only, this 
>features
>will cost everyone extra compile time. Even is 'css' stuff is in separate
>header, you'd need the hooks for customization.

Custom styling is currently implemented and is relatively cheap. The CSS 
design proposal is - I agree - complex in nature and would give a 
performance penalty. I was discussing a possible implementation to 
Jonathan's proposal.

Regards,
Reece

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