No this doesn't work either, unless you sacrifice the functionality of using variables as commands. Suppose $echo is equal to "echo", what is the most reasonable thing one would expect `$echo = $ip` to do?

Also consider the following snippet:

    : ${SSH:=ssh}
    $SSH $host1 'command1'
    $SSH $host2 'command2'

This has at least two use cases: 1) The user may direct the script use an ssh that is installed somewhere not in PATH by overriding SSH; 2) The user may supply extra flags to ssh by overriding SSH.

That is for the traditional shell part, which is still true in elvish (although due to stricter word splitting semantics use case 2 is different in elvish). Also since in elvish closures are first-class values, it's very intuitive to just call them directly:

    var $f = {|$x $who| echo "Hello, "$who"!" }
    $f = world # outputs "Hello, world!"

From the design perspective, it is possible to let `$echo = $ip` stand for assignment and still retain the ability to use variables as commands. If you give special meaning to "=" when it's the second word and alone and introduce a "call" command:

    var $f = {|$x $who| echo "Hello, "$who"!" }
    call $f = world # outputs "Hello, world!"
    $f = world      # assigns "world" to $f

But this introduces quite some ugliness to the language, and I decided that just requiring assignments to use "set" is the best solution.

A relevant note: Coming up with syntax for a shell language is actually very difficult due to the existence of bare words which greatly limit your inventory of potential operators. For instance one would very likely expect "echo user@example.com" to just echo "user@example.com", so if you give special semantics to "@" or "." it causes confusion and inconvenience. The only safe place you can introduce new semantics is the command, and this forces the language to have a prefix structure. If this reminds you of Lisp, you are correct - due to lack of infix operators lisp has very few restrictions on variable names; but the situation in shell languages is the opposite: due to the liberal use of bare words it is very difficult to introduce new infix operators to a shell language.

I agree with all that, and you're right about the syntax being difficult.

My one caveat you can have a little more freedom if you're willing to give up some of the patterns of existing shells, at the cost of unfamiliarity. Of course, you also need to be as expressive as shell so far as possible.

Anyway, "set" isn't a high cost to bear. I don't like it, but that's personal preference, and it clearly helps with the syntax of what you're doing.