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

Wed Oct 12 12:23:50 PDT 2016

> On Oct 12, 2016, at 12:08 PM, Zachary Turner <zturner at google.com> wrote:
> 
> I thought I did.  :)  Passing format strings between functions is very useful.  For example, imagine wanting to write a function like printRange(const char *Fmt, std::vector<int> Items);

I’m not sure I understand your example? 
Do you mean you want the range to be in the format? If so Why? I would rather write something like:

printRange(“{per_elts_fmt}”, /* separator */ “, ", begin, end);

> This isn't possible if your format string MUST be a string literal

I haven’t seen a convincing example yet to support this. I may miss the obvious, but you haven’t shown it either.
One could find a way to *compose* format in a compile-time-safe more efficiently.

> Equally importantly, I don't see a good reason to disallow runtime format strings.

No compile time checking, bug hiding, not robust.
(i.e. you may not “crash”, but you may still don’t print what you want / expect in every case).

— 
Mehdi

> 
> On Wed, Oct 12, 2016 at 11:59 AM Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> On Oct 12, 2016, at 11:38 AM, Zachary Turner <zturner at google.com <mailto:zturner at google.com>> wrote:
>> 
>> I don't object to compile time checking *as long as it doesn't severely detract from brevity*. 
> 
> 
>> At the same time, I do object to *preventing* runtime format strings.
> 
> You haven’t answered: why?
> 
> — 
> Mehdi
> 
> 
>> 
>> When we have C++14, we can make every member of StringRef constexpr, and at that point we will get compile time checking mostly "for free" without preventing runtime format strings.  For example, given a constexpr-aware implementation of StringRef, if you were to write: os.format("literal format", a, b, c) you would get all the compile time checking, such as ensuring that the number of arguments matches the highest index in the format string, and ensuring there are enough arguments for every placeholder.  But if you wrote os.format(s, a, b, c) you would still get runtime checking of the format strings.
>> 
>> As long as the runtime implementation doesn't exhibit UB when things don't match up, and it kindly asserts to warn you of the problem in the test suite, support runtime format strings can be very helpful.  For example, it could allow you to wrap a call to format in some other function, like:
>> 
>> template<typename... Ts>
>> void wrap_format(const char *Format, Ts &&... Args) {
>>    dbgs().format(Format, ConvertArg(Args)...);
>> }
>> 
>> On Wed, Oct 12, 2016 at 11:24 AM Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>> On Oct 12, 2016, at 7:12 AM, Zachary Turner via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> 
>>> Ahh, UDLs also wouldn't permit non literal format strings, which is a deal breaker imo
>> 
>> Why?
>> Somehow the goal pursued by Pavel (which you didn’t object per-se) is to provide *compile* time checking.
>> This imply that you cannot decouple the construction of the format and the argument list.
>> 
>> — 
>> Mehdi
>> 
>>> On Wed, Oct 12, 2016 at 7:03 AM Zachary Turner <zturner at google.com <mailto:zturner at google.com>> wrote:
>>> I'm not sure that would work well. The implementation relies on being able to index into the parameter pack. How would you do that if each parameter is streamed in?
>>> 
>>> "{0} {1}"_fs(1, 2)
>>> 
>>> Could perhaps work, but it looks a little strange to me.
>>> 
>>> Fwiw i agree format_string is long. Ideally it would be called format, but that's taken.
>>> 
>>> Another option is os.format("{0}", 7), and have format_string("{0}", 7) return a std::string.
>>> On Wed, Oct 12, 2016 at 6:43 AM Aaron Ballman <aaron at aaronballman.com <mailto:aaron at aaronballman.com>> wrote:
>>> >> 1. os << format_string("Test");   // writes "test"
>>> >> 2. os << format_string("{0}", 7);  // writes "7"
>>> >
>>> >
>>> > The "<< format_string(..." is ... really verbose for me. It also makes me
>>> > strongly feel like this produces a string rather than a streamable entity.
>>> 
>>> I wonder if we could use UDLs instead?
>>> 
>>> os << "Test" << "{0}"_fs << 7;
>>> 
>>> ~Aaron
>>> 
>>> >
>>> > I'm not a huge fan of streaming, but if we want to go this route, I'd very
>>> > much like to keep the syntax short and sweet. "format" is pretty great for
>>> > that. If this is going to fully subsume its use cases, can we eventually get
>>> > that to be the name?
>>> >
>>> > (While I don't like streaming, I'm not trying to fight that battle here...)
>>> >
>>> > Also, you should probably look at what is quickly becoming a popular C++
>>> > library in this space: https://github.com/fmtlib/fmt <https://github.com/fmtlib/fmt>
>>> >
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