[llvm-dev] RFC: General purpose type-safe formatting library

Mehdi Amini via llvm-dev
Thu, 13 Oct 2016 04:16:06 -0000

> On Oct 12, 2016, at 8:33 PM, Zachary Turner <zturner at google.com>
wrote:
>
> AFAICT this appears to be the first time you've clarified that
you're talking about a situation where the compile-time checking happens
using something other than format strings.

I though I was clear in the thread (in the history below) when I wrote
"Maybe the problem is using a string to format this in the first place”
followed by this example:  format(“{0}”, rPad(col_width, my_object));  where the
padding is *not* in the format string.

Another example earlier in the thread was the range (let say printing elts from
10 to 20 in a vector<int>). And instead of a syntax like:

/* Format string is {eltid, separator, <range>} */
print(“{0:,<10-20>}”,  /* std::vector<int> */. v);

And having to actually generate the format string in the first place

std::string format = format_string(“\{0:,<{0}-{1}>\}”, /* begin */ 10, /*
end */ 20);

I rather have something like

print(“{0}”, Range(“, ", v.begin()+10, v.begin()+20));


>   In Pavel's original email, he suggested compile time checking and you
mentioned that I didn't object to it.  But if you go back and read my
response, I said we can do the compile time checking *of the format strings*
using C++14.  So no I didn't object to it in principle, but I never strayed
from the desire to use format strings.
>
> To respond to your other point, no it doesn't make it more flexible
than a non-string based solution.  But does anyone want a non string-based
solution?  We already have one, it's called raw_ostream.  And STL has
another one in iostreams.  sprintf and llvm::format are not more flexible than
streaming operators either, and yet people still flock to them because it yields
the nicest looking code.  James Knight pointed out earlier that "any time
someone invents a new formatting library, everyone always ends up using printf
anyway".  There's a reason for that, and it's because printf is
string-based.  That's what people want.
>
> So if we're talking about string-based versus non string-based, then
yes, I'm married to the idea of a string based solution.
>
> That doesn't mean we can't *also* expose the underlying format
functionality via an additional set of non format based functions.  But
string-based formatting is necessary if there is to be any adoption at all.

I am not convinced that just because previous attempts didn’t success, we’re
stuck forever with printf.

At this point I’m not it is worth continuing the discussion, we can just agree
to disagree on the principle.

—
Mehdi

>
> On Wed, Oct 12, 2016 at 8:19 PM Mehdi Amini <mehdi.amini at apple.com
<mailto:mehdi.amini at apple.com>> wrote:
>> On Oct 12, 2016, at 8:07 PM, Zachary Turner <zturner at google.com
<mailto:zturner at google.com>> wrote:
>>
>> On Wed, Oct 12, 2016 at 12:40 PM Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>> On Oct 12, 2016, at 12:35 PM, Zachary Turner <zturner at
google.com <mailto:zturner at google.com>> wrote:
>>>
>>> You get compile time checking automatically when we can use c++14
though. If you use it with a string literal, you'll get compile time
checking, otherwise you won’t.
>>
>> I understand that, but that doesn’t really address my concerns.
>>
>>>
>>> Here's a different example though. Suppose you're writing a
tool which prints formatted output, and the field width is specified by the
user.
>>
>>
>>> Now you NEED to build the format string at runtime, there's no
other way
>>
>> Maybe the problem is using a string to format this in the first place.
>>
>> For example, you could wrap the object you want to print with an
adaptor in charge of padding to the right till you reach the column width.
>>
>> format(“{0}”, rPad(col_width, my_object));
>>
>> FWIW I do think that literal format strings will handle 90% or more of
uses.  I just don't see the benefit of needlessly banning the other cases.
Because all that's going to happen is someone is going to resort to using
snprintf etc, which is exactly the problem I'm trying to solve.
>
> Sorry but you’re totally missing the point. If there is a need for
dynamism, this should be supported, that’s not the question. My point is that
generating a string that will be parsed by a format function can’t be the only
solution.
>
>>   It's literally no extra effort to support runtime format strings,
and it makes the library more flexible as a result.
>
> No: it does *not* make it more flexible than a non-string based solution
that have the same functionality.
>
> —
> Mehdi
>
>
>
>> I'm willing to start with UDLs only because I think it will get us
quite far, but as soon as I need to pass a format string through an intermediate
function or something like that, I will probably check in the 3 extra lines of
code to add a const char* overload format function.
>>
>> FWIW, there's no easy way to add compile time checking of format
strings until C++14, regardless of whether use UDLs or not.  So that doesn't
change either way.

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