Debug.log for Regexes does not show the regex

[jfmengels]

(I imagine that this fix would likely be made in elm/core, but I thought it would be more appropriate here. Feel free to move the issue)

Problem

I was just in the middle of debugging why a string didn't match a regex, and I tried Debug.logging the regex, which gave me {}, which I found to be very unhelpful.

Creating a regex inside the Elm REPL gives the same behavior:

> import Regex
> Regex.fromString "hello"
Just {} : Maybe Regex.Regex

Expectation

I expected to see some kind of information in the log message to see what the regex is. The Node.js REPL gives the following feedback, which I believe is more helpful:

> /hello/
/hello/
> new RegExp("hello", "ig")
/hello/gi

Suggested solution

I suggest that elm/core's _Debug_toAnsiString function adds a special-case for regexes to print the regex in a useful way.

I feel like the format of the stringified version aims to be somewhat copy-pastable into Elm code (seeing examples like Dict.fromList), which makes this format a tiny bit tricky. Ideally, the format of the regex should probably be the "JavaScript regex format" (/hello/gi), but that wouldn't be valid Elm code.

A second option would be to print the code needed to create the Regex from Elm code (Regex.fromString "hello") but that would have 2 drawbacks:

• The code would not be copy-pastable, because there is no function that directly creates a regex without wrapping it in a Maybe.
• You'd need to add backslashes (/"hello"/ -> Regex.fromString "\"hello\""), which makes the regex harder to read, which might be counterproductive when the intent of the Debug.logging was to make it clearer what the regex is.