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

From: Vladimir Prus (ghost_at_[hidden])
Date: 2004-09-20 08:58:57

Hi Reece,

>> > 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.

Ok.

>>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 ]".

That's bad, I'd rather prefer different formatting styles for sets and
vectors.

>>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.

Why? Is there any specific difficulty with using format object type (or
"kind") as key?

>> > == 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 );
>    }

Is this a method of 'wrapper_decorators'?
>>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).

Ok, aside from naming, the above is fine.

> 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>" );

Is this a practical use case? Well, you can print 'names' this way, but if
you want to print 'Person', you'll need much more powerfull tools. Anyway,
if the overhead is small, no problem.

>>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. 

Agreed, that's reasonable.

> 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';
> }

That's more like an artificial example, I think. For practical uses, the
message catalog would have to contain comments, at least, and that again
requires much more complex parser.

>>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.

I'll try to write a complete review soon,
so stay tuned ;-)

- Volodya