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

Zachary Turner via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 1 05:39:03 PDT 2016

The big problem i see is that to get compile time checking without the UDL
we're going to have to do something like FORMAT_STRING("{0}") where this is
a macro. It just seems really gross. It is true that it is harder to find
the documentation, but that could be alleviated by putting all of this in
its own namespace like llvm::formatv, then one could search the namespace
On Mon, Oct 31, 2016 at 11:41 PM Sean Silva <chisophugis at gmail.com> wrote:

> On Mon, Oct 31, 2016 at 5:21 PM, Chandler Carruth via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> On Mon, Oct 31, 2016 at 3:46 PM Zachary Turner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> Hi all,
> Tentatively final version is up here: https://reviews.llvm.org/D25587
> It has a verbal LGTM, but I plan to wait a bit longer just in case anyone
> has some additional thoughts.  It's a large patch, but if you're
> interested, one way you can help without doing a full-blown review is to
> look at the large comment blocks in FormatVariadic.h and
> FormatProviders.h.  Here I provide a formal description of the grammar of
> the replacement sequences and format syntax.  So you can look at this
> without looking at the code behind it and see if you have comments just on
> the format language.
> Here's a summary of (most) everything contained in this patch:
> 1) UDL Syntax for outputting to a stream or converting to a string.
>     outs() << "{0}"_fmt.stream(1)
>     std::string S = "{0}"_fmt.string(1);
> I continue to have a strong objection to using UDLs for this (or anything
> else in LLVM).
> I think this feature is poorly known by many programmers. I think it will
> produce error messages that are confusing and hard to debug. I think it
> will have a significant negative impact on compile time. I also think that
> it will exercise substantially less well tested parts of every host
> compiler for LLVM and subject us to an increased rate of mysterious host
> compiler bugs.
> I also think it forces programmers to be aware of a "magical" construct
> that doesn't really fit with the rest of the language.
> It isn't that any of these issues in isolation cannot be overcome, it is
> that I think the value provided by the UDL specifically is substantially
> smaller than the cost.
> I would *very strongly* prefer that this is accomplished with "normal" C++
> syntax, and that compile time checking is done with constexpr when
> available. I think that will give the overwhelming majority of the benefit
> with dramatically lower cost.
> +1, the UDL seems a bit too automagical.
> `format_string("{0}", 1)` is not that much longer than
> `"{0}"_fmt.string(1)`, but significantly less magical.
> Simple example: what should I type into a search engine to find the LLVM
> doxygen for the UDL? I know to search "llvm format_string" for the format
> string, but just from looking at a use of the UDL syntax it might not be
> clear that format_string is even called.
> -- Sean Silva
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