Improve setting temporary variable for interative use

[xofyarg]

What new feature should Elvish have?

In the follow use case, when running an adhoc command with a certain variable, we got a deprecation warning.

~> E:LS_COLORS= ls
Deprecation: the legacy temporary assignment syntax is deprecated; use "tmp" instead
  [tty 2]:1:1-12: E:LS_COLORS= ls

The current builtin way of doing it is like below, with some extra key presses. Rely on external env is another method.

{ tmp E:LS_COLORS = ''; ls }

It's probably more convenient, if we could improve this a little bit, e.g. wrap user input with a closure automatically.

Output of "elvish -version"

0.21.0-dev.unknown

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[krader1961]

Why ins't the env command (e.g., env LS_COLORS='' ls) a reasonable solution to this problem? The cost is close to zero and adds just a couple of characters when type interactively compared to the deprecated E:LS_COLORS='' ls syntax. If you're on Windows an Elvish env function is reasonably straightforward to implement. Your "wrap user input with a closure automatically" comment suggests you are looking for a solution that is only applicable to interactive user input. I would argue that is an anti-pattern. That is, I think any solution to this problem should work for interactive REPL and non-interactive Elvish command blocks. Special casing interactive REPL usage seems likely to be hard to document, hard to justify, and likely to confuse users.

[xiaq]

I will engage in the technical points later, but here are some comments as a moderator to @krader1961's comment above.

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Why ins't the env command (e.g., env LS_COLORS='' ls) a reasonable solution to this problem? The cost is close to zero and adds just a couple of characters when type interactively compared to the deprecated E:LS_COLORS='' ls syntax. If you're on Windows an Elvish env function is reasonably straightforward to implement.

Please don't be dismissive of other people's preference gratuitously.

Your "wrap user input with a closure automatically" comment suggests you are looking for a solution that is only applicable to interactive user input. I would argue that is an anti-pattern. That is, I think any solution to this problem should work for interactive REPL and non-interactive Elvish command blocks. Special casing interactive REPL usage seems likely to be hard to document, hard to justify, and likely to confuse users.

Please don't use straw-man arguments.

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