Language change proposal

[Joachim Durchholz]

Chris Pressey wrote:
> It's a good idea, but wouldn't it be easier to write code that works on
> different versions if it was done with macros,  e.g.
> 
>   -ifdef(?ERLANG_5_3).
>   % some code that uses bleeding-edge features here
>   -else.
>   -ifdef(?ERLANG_5_2).
>   % some code that uses 5.2-isms here
>   -else.
>   % some code that is dangerously language-version-unspecific here
>   -endif.
>   -endif.

Most definitely *no*.
I have maintained C code that went like this; it was a nightmare. The 
reason was that it offered too many opportunities for "clever writing" 
like this:

if (normal condition) {
   ...
#ifdef VERSION = xxx
   ...
} else {
   ...
#else
   ...
} else {
   ...
#endif
   ...
}

You can't even properly do indentation...

However, there's one thing that /could/ be done: add an erlang_version 
predicate that's guaranteed to be evaluated at compile time. Also, add 
some rules about compile-time evaluation for expressions "known to be 
constant" (these can be very simplicistic, look at the rules for Turbo 
Pascal, for example).

With that, you can write stuff like (excuse the non-Erlang syntax here, 
I've still not gotten around to making my ankles wet...)

    if erlang_version = "5_3"
      % some code that uses bleeding-edge features here
    else
      if erlang_version = "5_2"
        % some code that uses 5.2-isms here
      else
        some code that is dangerously language-version-unspecific here
      endif
    endif

Regards,
Jo