[RFC]: add support for eager evaluation of side-effect free code in the REPL

[kgryte]

Description

This RFC proposes adding support for eager evaluation of side-effect free code in the REPL. E.g.,

In [1]: [ 'foo', 'bar', 'beep' ].sort()<|>
[ 'bar', 'beep', 'foo' ]

would trigger a preview below the input command, where <|> represents the cursor. Upon hitting ENTER, the command would execute with the result displayed after the output prompt, as normal.

This feature is now implemented in browser devtools, such as Chrome (see https://developer.chrome.com/blog/new-in-devtools-68/#eagerevaluation).

The tricky bit with implementing this is detecting side effects. And we'd likely want to avoid eagerly evaluating things like HTTP requests. Additionally, we'd like want to exclude things which are not idempotent (e.g., PRNGs, Date constructor, etc).

There is also the concern as to how to handle eager evaluation of computationally expensive commands or commands returning large outputs.

As this feature may not be desired by all users, we should presumably support an option for enabling/disabling this on REPL instantiation. And we should disable eager evaluation when not in TTY mode.

Related Issues

• object completion: [RFC]: add object inspection on the fly in REPL #1794

Questions

No.

Other

Resources:

• https://developer.chrome.com/blog/new-in-devtools-68/#eagerevaluation
• https://github.com/pugjs/constantinople/tree/master

Checklist

• I have read and understood the Code of Conduct.
• Searched for existing issues and pull requests.
• The issue name begins with RFC:.