Re: [Boost][Proposal] library for enhanced switch syntax.

From: рустам абдумаликов (vigosslive2_at_[hidden])
Date: 2019-11-11 17:39:28

Phil Endecott wrote:
> Specifically thinking about strings, there are numerous ways
> that it could be done. Starting with
> (1): the obvious sequence of if-else-if:
> ...
> (2): binary search using nested switches:
> ...
> (3) character-by-character comparison using nested switches:
> ...
> (4) converting to a 64-bit integer, for strings of up to 8 characters:
> ...

Firstly my implementation was never particularly about string in "switch"
it is just one of the applications.
Secondly, example 3-4 assume additional implementation( can take quite a
time to do it right ), it isn't like you have it out of the box,
this is exactly what my statement was about( example 1-2 don't count, they
are non-scalable ).
Lets rather consider something that is more complex:

-- Example_1 "Parse file by extension":

switch( hash_it( get_file_extention( file_name ) ) )
{
case "cpp"HASH:
case "cc"HASH:
case "c++"HASH:
case "cxx"HASH:
case "C"HASH:
{ ... }
break;
case "hpp"HASH:
case "hh"HASH:
case "h++"HASH:
case "hxx"HASH:
case "H"HASH:
{ ... }
break;
}

// OR

// I think this way it is much readable
eswitch( get_file_extention( file_name ) ) >>
case_( any_from( "cpp", "cc", "c++", "cxx", "C" ) ) >> [&]{...} >>
case_( any_from( "h", "hpp", "hh", "h++", "hxx", "H" ) ) >> [&]{...};

// OR
// even this could be possible( but not yet implemented )
eswitch( get_file_extention( file_name ) ) >>
case_( "cpp", "cc", "c++", "cxx", "C" ) >> [&]{...} >>
case_( "h", "hpp", "hh", "h++", "hxx", "H" ) >> [&]{...};

-- Example_2 "Parse HTTP header with Regex":

// O_o too much boilerplate
    if( std::regex_match( line, "^.+ 200 .+$"_r ) ){...}
else if( std::regex_match( line, "^.+ 404 .+$"_r ) ){...}
else if( std::regex_match( line, "^.+: .+$"_r ) ){...}
else { terminate(); }

// OR

// I think this way your intention is clearer
eswitch( line ) >>
case_( "^.+ 200 .+$"_r ) >> []{...} // match for "HTTP/1.1 200 OK"
case_( "^.+ 404 .+$"_r ) >> []{...} // match for "HTTP/1.1 404 Not Found"
case_( "^.+: .+$"_r )    >> [&]{...} >> // match for "key: value"
default_ >> []{ terminate(); };

-- Example_2 "match in range of values":

// Error prone approach
if( value >=  1 && value <= 10 ){ Print("Value in a range[1,10]"); }
if( value >= 11 && value <= 20 ){ Print("Value in a range[11,20]"); }
else { Print('?'); };

/// OR

eswitch( value ) >>
        case_( _1.in( 1, 10 ) )  >> []{ Print("Value in a range[1,10]"); }
 >>
        case_( _1.in( 11, 20 ) ) >> []{ Print("Value in a range[11,20]"); }
>>
        default_                 >> []{ Print('?'); };
 // _1 it is lazy index mapping for the params in eswitch

I my opinion "eswitch" looks more readable, more intuitive, less error prone
and furthermore it is extensible.

P.S.
Here
https://github.com/rabdumalikov/proposals/blob/master/eswitch_boost_proposal.pdf
I had described why I think it is important( just in case you've missed it
).