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

[Mehdi Amini via llvm-dev]

Hi,

I On Oct 11, 2016, at 6:22 PM, Zachary Turner via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> A while back llvm::format() was introduced that made it possible to combine printf-style formatting with llvm streams.  However, this still comes with all the risks and pitfalls of printf.  Everyone is no-doubt familiar with these problems, but here are just a few anyway:
> 
> 1. Not type-safe.  Not all compilers warn when you mess up the format specifier.  And when you're writing your own Printf-like functions, you need to tag them with __attribute__(format, printf) which again not all compilers have.

I’m not very sensitive to the “not all compilers have” argument, however it is worth mentioning that the format may not be a string literal, which defeat the “sanitizer”.

>   If you change a const char * to a StringRef, it can silently succeed while passing your StringRef object to printf.  It should fail to compile!

llvm::format now fails to compile as well :)

However this does not address other issues, like: `format(“%d”, float_var)` 

> 
> 2. Not security safe.  Functions like sprintf() will happily smash your stack for you if you're not careful.  
> 
> 3. Not portable (well kinda).  Quick, how do you print a size_t?  You probably said %z.  Well MSVC didn't even support %z until 2015, which we aren't even officially requiring yet.  So you've gotta write (uint64_t)x and then use PRIx64.  Ugh.
> 
> 4. Redundant.  If you're giving it an integer, why do you need to specify %d?  It's an integer!  We should be able to use the type system to our advantage.
> 
> 5. Not flexible.  How do you print a std::chrono::time_point with llvm::format()?  You can't.  You have to resort to providing an overloaded streaming operator or formatting it some other way.

It seems to me that there is no silver bullet for that: being for llvm::format() or your new proposal, there is some sort of glue/helpers that need to be provided for each and every non-standard type.


> So I've been working on a library that will solve all of these problems and more.

Great! I appreciate the effort, and talking about that with Duncan last week he was mentioning that we should do it :)


> 
> The high level design of my library is borrowed heavily from C#.  But if you're not familiar with C#, I believe boost has something similar in spirit.  The best way to show it off is with some examples:
> 
> 1. os << format_string("Test");   // writes "test"
> 2. os << format_string("{0}", 7);  // writes "7"
> 
> Immediately we can see one big difference between this and llvm::format() / printf.  You don't have to specify the type.  If you pass in an int, it formats it as an int.
> 
> 3. os << format_string("{0} {0}", 7); // writes "7 7"
> 
> #3 is an example of something that cannot be done elegantly with printf.  Sure, you can pass it in twice, but if it's expensive to compute, this means you have to save it into a temporary.

What about: printf(“%0$ %0$”, 7); 

> 
> 4. os << format_string("{0:X}", 255);  // writes "0xFF"
> 5. os << format_string("{0:X7}", 255);  // writes "0x000FF"
> 6. os << format_string("{0}", foo_object); // fails to compile!
> 
> Here is another example of an improvement over traditional formatting mechanisms.  If you pass an object for which it cannot find a formatter, it fails to compile.
> 
> However, you can always define custom formatters for your own types.  If you write:
> 
> namespace llvm {
>   template<>
>   struct format_provider<Foo> {
>     static void format(raw_ostream &S, const Foo &F, int Align, StringRef Options) {
>     }
>   };
> }
> 
> Then #6 will magically compile, and invoke the function above to do the formatting.  There are other ways to customize the formatting behavior, but I'll keep going with some more examples:
> 
> 7. os << format_string("{0:N}", -1234567);  // Writes "-1,234,567".  Note the commas.

Why add commas? Because of the “:N”?
This seems like localization-dependent: how do you handle that?

What happens with the following?

os << format_string("{0:N}", -123.455);


> 8. os << format_string("{0:P}", 0.76);  // Writes "76.00%"
> 
> You can also left justify and right justify.  For example:
> 
> 9. os << format_string("{0,8:P}", 0.76);  // Writes "  76.00%"
> 10. os << format_string("{0,-8,P}", 0.76);  // Writes "76.00%  "
> 
> And you can also format complicated types.  For example:
> 
> 11. os << format_string("{0:DD/MM/YYYY hh:mm:ss}", std::chrono::system_clock::now());  // writes "10/11/2016 18:19:11”

11 looks pretty cool in terms of flexibility :)


> I already have a working proof of concept that supports most of the fundamental data types and formatting options such as percents, exponents, comma grouping, fixed point, hex, etc.
> 
> To summarize, the advantages of this approach are:
> 
> 1) Safe.  If it can't format your type, it won't even compile.  
> 2) Concise.  You can re-use parameters multiple times without re-specifying them.
> 3) Simple.  You don't have to remember whether to use %llu or PRIx64 or %z, because format specifiers don't exist!
> 4) Flexible.  You can format types in a multitude of different ways while still having the nice format-string style syntax.
> 5) Extensible.  If you don't like the behavior of a built-in formatter, you can override it with your own.  If you have your own type which you'd like to be able to format, you can add formatting support for it in multiple different ways.
> 
> I am hoping to have something ready for submitting later this week.  If this interests you, please help me out by reviewing my patch!  And if you think this would not be helpful for LLVM and I should not worry about this, let me know as well!

Feel free to add me as a reviewer!

— 
Mehdi

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