RFC 0021: Percent String Array Literal with Interpolation

straight-shoota · March 10, 2026, 10:16am

This RFC is about ready to be accepted: RFC 0021: Percent String Array Literal with Interpolation by straight-shoota · Pull Request #21 · crystal-lang/rfcs · GitHub

Here is the full proposal. It’s time for a final glance that everything is in order:

github.com/crystal-lang/rfcs

percent-array-literal-interpolation/text/0021-percent-array-literal-interpolation.md?rgh-link-date=2026-02-25T16%3A10%3A10Z

rfc

---
Feature Name: "percent-array-literal-interpolation"
Start Date: 2026-02-25
RFC PR: "https://github.com/crystal-lang/rfcs/pull/21"
Issue:
---

## Summary

Add a variant of the existing percent array literal (`%w`) with interpolation support, indicated by an upper-case `%W`.

## Motivation

Creating string arrays with the `%w` literal syntax is convenient, but limited because string values must be static.
Introducing dynamic values requires mutating the array afterwards which adds more complexity.
A literal syntax with support for interpolation offers more convenience.

A particular use case is for process command arrays with `Process.run` and similar methods (see [#14773 (comment)]).

[#14773 (comment)]: https://github.com/crystal-lang/crystal/issues/14773#issuecomment-3890019306

This file has been truncated. show original

Carlos · March 11, 2026, 2:35pm

This is a good opportunity to fix Backslashes in `%w()` misbehave · Issue #12277 · crystal-lang/crystal · GitHub so %w and %W behave the same wrt backslashes.

(I hope the alternative way to make them behave the same is not taken )

straight-shoota · March 11, 2026, 4:42pm

Thanks for the pointer to this old issue. I didn’t have that in my sight.
Fixing that seems feasible.
Question is still whether it’s an acceptable change because it might break existing usage. I believe it might be fine, though.

Topic		Replies	Views
Operator % Help & Support	2	303	December 7, 2022
[Question] The key reason not accept single quotes string in Crystal Help & Support	6	615	May 13, 2022
Interpolate string to make a hex value Help & Support	18	752	April 14, 2021
[Propose] Add single quote string(just like most modern language instead of C), use ?A for Char `A` (just like Ruby) Crystal Contrib	12	689	May 15, 2024
RFC: %wt(hey ho) == {"hey, "ho"} Crystal Contrib	5	475	July 20, 2022

RFC 0021: Percent String Array Literal with Interpolation

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