"The preview server is not enabled in the version of Hakyll"

[tomjaguarpaw]

When using site --watch under hakyll==4.16.2.0 I am getting

The preview server is not enabled in the version of Hakyll. To
enable it, set the flag to True and recompile Hakyll.
Alternatively, use an external tool to serve your site directory.

As a workaround I can set the previewserver flag explicitly, for example

cabal run --constraint="hakyll +previewserver" -- site watch

Firstly, according to #298 (comment) I shouldn't need to do this, it should be the default. Does anyone know what's going on?

Secondly, it was really hard to discover the above workaround! Could it be explicitly be added to the error message to give users a clue about what they should do? "set the flag to True and recompile Hakyll" doesn't explain much at all.

[Minoru]

Firstly, according to #298 (comment) I shouldn't need to do this, it should be the default. Does anyone know what's going on?

I don't. Is your site's code available anywhere so I can try to reproduce this locally?

Secondly, it was really hard to discover the above workaround! Could it be explicitly be added to the error message to give users a clue about what they should do? "set the flag to True and recompile Hakyll" doesn't explain much at all.

Agreed on that. Do you have a specific wording in mind? I'd review a PR that improves this message.

[tomjaguarpaw]

Is your site's code available anywhere so I can try to reproduce this locally?

Yes, it's https://github.com/tomjaguarpaw/H2, but if you prefer to wait until I submit a minimal example of the problem that's fine too.

Do you have a specific wording in mind?

At the very least I suggest adding this message or linking to one of these GitHub issues that mentions the problem and the fix.

You can set the preview server by running cabal with, e.g. --constraint="hakyll +previewserver"

[jaspervdj]

I also think this is confusing -- I guess the problem is that cabal toggles the flags when it can't find a resolution. Should we set previewServer (and probably watchServer) to Manual: True in the cabal file?

The disadvantage is that instead of seeing this error, the user will see an error that cabal failed to resolve the dependencies...

[tomjaguarpaw]

But in this case cabal can find a resolution, because if I manually set +previewserver then it succeeds.

[jstolarek]

I've run into the same problem - preview server stopped working after rebuilding my website dependencies. Workaround from the first post works, but it isn't an easy thing to figure out. Having this documented would be helpful.