Move default statusline from C implementation to 'stl' expression

[yochem]

Partly fixes: #28809. After this is merged, we can look into implementing a Lua/Vimscript version of a more modern default statusline.

Let me know if/what docs I need to update. The current default is taken from :h 'statusline', where they describe it as Emulate standard status line with 'ruler'. I can change this to Default statusline with 'ruler' set. Should there also be a news entry?

[famiu]

I cannot check to make sure as I do not have access to my PC right now, but this does not seem like the correct way of setting the new default to me. This only sets the initial default value instead of a fallback value. E.g. if you did vim.o.statusline = "", I assume it wouldn't use the default statusline anymore, which breaks old behavior. Do correct me if I'm wrong.

A better approach may be to take a statusline string value as a parameter in redraw_custom_statusline. Then you could pass the custom statusline string when it exists, and the default value otherwise.

[yochem]

Yes, you're right. I forgot that setting strings option to an empty string usually reverts it to the default value. I'll look into your proposed approach.

[yochem]

The easiest way to set the default statusline would be to modify the p_stl or wp->w_p_stl when NUL:

  } else if (*p_stl != NUL || *wp->w_p_stl != NUL) {
    // redraw custom status line
    redraw_custom_statusline(wp);
  } else {
    // default statusline case

    // define default statusline
    char *default_stl = (char*)xmalloc(sizeof("hallo"));
    strcpy(default_stl, "hallo");
    p_stl = default_stl;

    redraw_custom_statusline(wp);
  }

(excuse the probably really bad C code)

But this seems like an antipattern to me:

vim.o.statusline = ""
print(vim.o.statusline)
-- prints "hallo" when built with the C code above

As @famiu suggested, another option would be to modify the functions that draw the stl (win_redr_status, redraw_custom_statusline, win_redr_custom, build_stl_str_hl) to accept a string value for the statusline, which could be set to the default in the else clause above.

The drawback here is that the default statusbar format is then still defined in statusbar.c, and I can imagine it not being ideal if we later want to define the default statusline from Lua/Vimscript.

Also, how would nvim_eval_statusline then be able to evaluate the statusline? Either build_stl_str_hl should know the default value of the statusline, or maybe a function in statusline.c default_stl_str, that returns the default statusline format? nvim_eval_statusline can then call this function to get the default.

@justinmk What do you think would be the right approach here?

[famiu]

The easiest way to set the default statusline would be to modify the p_stl or wp->w_p_stl when NUL:

  } else if (*p_stl != NUL || *wp->w_p_stl != NUL) {
    // redraw custom status line
    redraw_custom_statusline(wp);
  } else {
    // default statusline case

    // define default statusline
    char *default_stl = (char*)xmalloc(sizeof("hallo"));
    strcpy(default_stl, "hallo");
    p_stl = default_stl;

    redraw_custom_statusline(wp);
  }

(excuse the probably really bad C code)

But this seems like an antipattern to me:

vim.o.statusline = ""
print(vim.o.statusline)
-- prints "hallo" when built with the C code above

As @famiu suggested, another option would be to modify the functions that draw the stl (win_redr_status, redraw_custom_statusline, win_redr_custom, build_stl_str_hl) to accept a string value for the statusline, which could be set to the default in the else clause above.

The drawback here is that the default statusbar format is then still defined in statusbar.c, and I can imagine it not being ideal if we later want to define the default statusline from Lua/Vimscript.

Also, how would nvim_eval_statusline then be able to evaluate the statusline? Either build_stl_str_hl should know the default value of the statusline, or maybe a function in statusline.c default_stl_str, that returns the default statusline format? nvim_eval_statusline can then call this function to get the default.

I don't think we should change option values as a side effect when building the statusline, that sounds like a recipe for disaster.

Instead, we can define a function that returns the default value, and use that in relevant places. This keeps the default value contained to a single place while avoiding any unnecessary hacks