Format specs idea

[Georg Wrede]

Andrei Alexandrescu wrote:
> Nick Sabalausky wrote:
>> "BCS" <ao at pathlink.com> wrote
>>> Reply to Nick,
>>>
>>>> "Andrei Alexandrescu" <SeeWebsiteForEmail at erdani.org> wrote in message
>>>> news:gsiqdr$1csj$2 at digitalmars.com...
>>>>
>>>>> BCS wrote:
>>>>>> One option would be to not throw an error if the format string uses
>>>>>> indexing formats (e.i. out of order formatting)
>>>>>>
>>>>> Yah, that's an option I considered. Maybe it's the best way to go.
>>>>>
>>>> That would be far too clumbsy, unless you made it into two separate
>>>> functions.
>>>>
>>>> For instance (psuedocode):
>>>> auto userInput = getUserInput()
>>>> // userInput now contains "{Name} at {Address}", zip deliberately
>>>> ignored
>>>> writefln(userInput, name, address, zip); // They're used in-order, but
>>>> there
>>>> shouldn't be an error
>>> They are in order but are listed by name so the error doesn't throw. 
>>> The case where the error would be thrown is where the only format 
>>> strings used are the "get the next arg" kind.
>>>
>> I was just using names for illustrative purposes. Also, I was under 
>> the impression that printf-style "get the next arg" formatting codes 
>> were the only ones writef supported. Is this not so?
>>
> Since a few versions ago writef supports positional arguments with Posix 
> syntax.
> 
> Andrei

Positional format specifications really make a huge difference, 
especially where one needs to have the user interface in several 
languages. Thanks for them!!

Now, for the case without positional format specifications, we could 
(and I guess, should) have a solution to "the wrong number of arguments" 
problem. If we have:

  - a non-printing format spec
  - a munch-the-remaining-ones format spec
  - an ignore-too-few-arguments format spec

we could cover all the needed cases.

The first one lets you use one parameter without printing it. (Maybe you 
don't want to print the middle part of somebody's name.) The second is 
used when you don't want to print possibly many remaining arguments, say 
the address of someone, or the country.

The last one, is for the case where you use a single format string in 
several contexts, some of which you suspect may not contain enough 
information to be formally correct, but where you are satisfied to print 
an empty string '' (that is, not print anything) for them.

This situation might arise when the format string originates from 
outside the program.

** When none of the latter two are used, and there is the wrong number 
of arguments, I think a runtime exception should be thrown.

The only time where I think a compile time error is warranted, is when 
the format string is a string literal, with the wrong number of arguments.


The programmer may put the ignore-too-few-arguments format spec anywhere 
in the format string. Its effect starts from that point on.