Alright, so here are the plausible alternatives

1a. New type-class-based behaviour without extension

1b. New type-class-based behaviour gated by an extension

2. Just a warning (when main isn't at type IO () or IO Void)

3a. A warning + the new type-class-based behaviour gated by an extension. With the extension, types that don't implement the type class raise an error.

3b. A warning + the new type-class-based behaviour gated by an extension. With the extension, types that don't implement the type class raise a warning (which could have a different phrasing than without the extension).

Let's vote!

On Fri, 22 Mar 2024 at 15:30, Malte Ott <malte.ott@maralorn.de> wrote:

On 2024-03-22 08:58, Arnaud Spiwack wrote:
> @Malte, in my opinion, with the extension on, types which are not covered
> by the type class should error out.

Ah, I see. Well, I am fine either way.

I just don’t see much value in deciding for the user which code problems are
unacceptable. Especially since this will make the corresponding language
extension more breaking and thus harder to make the default.
Others have voiced similar opinions in the GitHub thread.

Best
Malte
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Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.

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