If the implementation is that similar to alias, why not make it an argument of alias command？

That's a great question, I was just using fish as a reference point, which has two separate commands.

Alias is syntax sugar for a one-line function more or less, while abbreviation exhibits a more feature set which allows you to be as precise as you'd like to be for when it's actually triggered. The approach that nushell could take is that alias is overloaded to support "inline replacement", and then bring in the rest of these advanced matching arguments?

I could see that as being useful as more idiomatic command builder, where you could define an alias that replaces different things based on the host system for example? But maybe that's a pattern we don't want?

@NotTheDr01ds reminded me that if you're looking for abbreviation expansion, nushell can do already that if setup properly.
https://www.nushell.sh/blog/2024-05-15-top-nushell-hacks.html

Of course it can! Nushell is very robust, and can emulate a lot with some tinkering, including the emulation of a number of other builtins; So I don't think it's a reason to not move forward.

I'm struggling to understand the value with this command and that's not a criticism of your hard work here. I know fish has abbr's and aliases, but I never really used abbr's much. Why is this an important feature that nushell should have?

I'd personally push for these type of changes built into aliases. Aliases need some work (jus search the issues) and it would be nice to clean them up a bit too. Maybe that's something you can help with while adding the abbr features to alias?

re: #16960 (comment)

Thanks for the blog link, @fdncred . Do you happen to know how abbrs can be set up to expand on Space instead of Ctrl+Space?

you could try setting the modifier to none i think

$env.config.keybindings ++= [{
  name: abbr
  modifier: none
  keycode: space
  mode: [emacs, vi_insert]
  event: [
    { send: menu name: abbr_menu }
    { edit: insertchar, value: ' '}
  ]
}]

Added this to the config file, but it's still not similar to fish (e.g. gcm<Space> -> git commit -m). 😓

No one said it worked like fish. If you use the aliases in nu_scripts you'll see what gcm is there.
https://github.com/nushell/nu_scripts/blob/4af008a317185b4e247bdb13002dbc074e98b60c/aliases/git/git-aliases.nu#L73

@fdncred I didn't actually try it with gcm (I said it as an example) but with my custom aliases set in the config file. I also didn't have nu_scripts installed (was not aware it existed, but looks useful; thanks). Regardless, here's what I get if I set fish's abbreviation to be the same as the nushell alias you linked:

• in fish:
  gcm<Space> -> git checkout main (it expands)
• in nushell (with modifier none as mentioned above):
  gcm<Space> -> gcm (it doesn't expand, just puts a literal space)

Basically, I don't need the aliases to mean the same thing as in fish. I just want Space to expand the aliases I define instead of needing a modifier.

EDIT: I think it'd be better not to pollute the PR so I will move my question to a Discussion. Thanks for your help.
UPDATE: There already exists a discussion regarding what I want: #2936