I have the following front matter in YAML, but I would like express the same in TOML and JSON. I can’t seem to get either to work (hugo serve throws an error):
unable to process menus for "": unable to cast []map[string]interface {}{map[string]interface {}{"identifier":"about", "name":"about hugo", "pre":"<i class='fa fa-heart'></i>", "url":"/about/", "weight":-110}} of type []map[string]interface {} to map[string]interface{}
I believe that notation is for the config.toml file. Have a look at hugo convert toTOML - maybe create the frontmatter once in yaml and then convert to see what hugo is missing.
And try without the first [menu] and/or [menu.main] (one bracket).
Interesting that there seems to be NOWHERE with an example of frontmatter menus in TOML…