repl: Proposal for better repl

[princejwesley]

This is my idea/wish to address Node.js repl related issues.

• Automatic multiline command mode has serious issues.

  • Recoverable error guess is based on error message emitted by v8(vm specific) engine
    (we may have to fix the code if vm emits different error string in next release)
  • False positives. We have to .break/^C the expression to continue.
  • We can't reliably deduce the end of expression even if we build an incremental JS parser.

  let echo = e => e;
  let x;
  x = echo(2); // 2
  x = echo // both echo and echo\n(2) are completely valid expression 
  (2) // 2

• No welcome message (it helps to see version number)

Proposal

• Welcome message with version and help guide
• Differentiate execute & continue actions
  • ^M or enter key to execute the command (current behavior)
  • ^J to continue (Hand over the problem to user, let user decides it) building multiline expression.

Welcome message template

> node
Welcome to Node.js <<version>> (<<vm name>> VM, <<vm version>>)
Type ^M or enter to execute, ^J to continue, ^C to exit
Or try .help for help, more at https://nodejs.org/dist/v<<version>>/docs/api/repl.html
>

e.g.

> node
Welcome to Node.js 6.4.0 (v8 VM, 5.0.71.60)
Type ^M or enter to execute, ^J to continue, ^C to exit
Or try .help for help, more at https://nodejs.org/dist/v6.4.0/docs/api/repl.html
>

@nodejs/collaborators, I am happy to implement this if there is no objection

Note : Current system executes code when ^M, ^J or enter pressed. ^I or tab for code completion

[yorkie]

There are a consideration: introducing the combining types like ^M or ^J might be unreasonable conflict with the shell itself.

For example, in my iTerm.app, ^M would close the current window.

[princejwesley]

@yorkie Tried cmd + M or ctrl + M(^M) ? if we are in repl context, I don't see any conflict. ^M for execute command is an existing behaviour.

[yorkie]

@princejwesley it depends on what shell you are using IMO, it doesn't work at iTerm. Let me verify it again later :)

[mscdex]

What about just enter and shift+enter or similar for execute and continue respectively?

[princejwesley]

@yorkie I use same iTerm 😄 . See control characters and here

[princejwesley]

@mscdex Terminal emits same value for both shift + enter and enter.

[yorkie]

Sorry @princejwesley you are right, it works :-)

[jasnell]

This makes sense. I do like the idea of ^J or Shift-Enter to continue. That would pair nicely with ^M or Enter to execute.

[princejwesley]

@jasnell We can't capture Shift-Enter. Terminal emits same ^M for Enter and Shift-Enter(terminal ignores ctrl, shift or meta keys when paired with Enter)

~ 🙈 ₹^M (ctrl+v, enter)
~ 🙈 ₹^M (ctrl+v, shift+enter)

For me, ^J sounds like jump to next line 😄 .

[jasnell]

Ok, I'm good with that then.

[princejwesley]

@nodejs/collaborators if there is no objection, I'll start work on it