Design idea: string interpolation

[jonmeow]

Per discussion on #1811, there's interest in string interpolation but it's not an area that's been closely examined. This is an area where a proposal would be helpful.

https://en.wikipedia.org/wiki/String_interpolation gives some information and provides some cross-language examples which may be helpful for proposal background and thinking about options. It's important to consider the syntax that multiple languages are looking at.

In Rust, I think https://www.rustnote.com/blog/format_strings.html and rust-lang/rfcs#2795 may provide some interesting ideas and alternatives to think about. Note though that while Print is using meta-programming in Rust, the leads have expressed interest in exploring non-meta-programming-based solutions first.

As a possible data source, it may be worth poking at a few sourcegraph searches (such as this one) to see what developers are doing today.

[OlaFosheimGrostad]

I would like to give this a go.

[Niikelion]

Javascript have pretty basic, but easy to understand and use string interpolation(template strings): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals . C# syntax, while being more complex(it incorporates option to specify data formats) is also worth considering in my opinion: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated .
If you want to discuss syntax and/or possible ways to approach implementation I will be more than happy to help.

[OlaFosheimGrostad]

C# syntax, while being more complex(it incorporates option to specify data formats) is also worth considering in my opinion: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated . If you want to discuss syntax and/or possible ways to approach implementation I will be more than happy to help.

Yes, I am thinking that maybe $"…" is a good solution. I am trying to map out various use scenarios and various string-buffer-targets in order to get good performance. High level languages create many temporary strings, and it would be nice if we could avoid that for some use cases.

I don't know which discord channel we can use for interpolated strings yet, but feel free to share ideas in this issue, I read it regularly.

[OlaFosheimGrostad]

We might end up needing to support C++20 std::formatter, so it is possible that our hands are tied as far as number formatting goes. On the upside, the C++20 formatting spec is pretty non-controversial, a lot like Python for instance.

[Niikelion]

I don't think that we are constrained by std::formatter syntax in designing string interpolation, but I agree that it is a good idea to get some inspiration from it. Format in interpolation could be specified like this: $"{pi: 10f}" and would roughly translate to std::format("{:10f}", pi);. I think that we can skip all cases that require passing additional arguments to format, like std::format(":{}.{}f",pi, 10, 5); and hope that value : value is not a valid value expression in Carbon.

[OlaFosheimGrostad]

I don't think that we are constrained by std::formatter syntax in designing string interpolation

We might be if we want to support formatting of C++ types, see chrono for instance: https://en.cppreference.com/w/cpp/chrono/local_t/formatter

(Constrained in the sense that we can do a rewrite to what C++ formatter expects.)

[Tomttth]

I was going through wikipedia and found this groovy feature interesting:

final n = 52
def sentence = "n is ${n%2 == 0 ? "even" : "odd"}"
println sentence

It would be good if we can compute the value in the string literal itself.

we can fine tune in this way for carbon:

$"n%2==0?'even':'odd'"

[OlaFosheimGrostad]

$"n%2==0?'even':'odd'"

Carbon appears to be using if cond then value else value for conditional expressions, so one option might be to write this as:

let sentence:auto = $"n is { if n%2 == 0 then "even" else "odd" }";

[OlaFosheimGrostad]

We now have a channel on Discord, feel free to share your thoughts here:

https://discord.com/channels/655572317891461132/1009125434929201172

[geoffromer]

That Groovy example talks about developers in sexist and ageist terms, and we don't do that here. I know it's copied verbatim from Wikipedia, but that kind of language can still be hurtful when it's inside quotation marks. I've replaced it with a more innocuous example in the comments above (and also fixed the formatting, because the $s were confusing GitHub).

[holmbuar]

This is what I see when clicking the link to Discord from this thread. Am I doing it wrong?