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Figwheel REPL provides a stable multiplexing REPL for ClojureScript
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README.md

figwheel-repl

Figwheel-REPL is intended to provide a best of class repl-env for ClojureScript.

Currently a work in progress.

Figwheel-REPL is only a ClojureScript repl-env and doesn't do anything specific to help with automatic file reloading. As such, it is more similar to Weasel in function than to Figwheel.

It is intended to be a single repl-env that will work on as many platforms as possible: including Browser, Node, Worker, ReactNative, etc.

It is also intended to handle multiple clients, think browser tabs, much more gracefully than the current Figwheel REPL.

It is also different in that it only evaluates code on a single client by default. You will still be able to choose to broadcast an eval operation to all connected clients if you prefer. You can also provide a filter function when you create the Figwheel repl-env, to filter the connections to the set of connected clients you want an eval operation to be sent to.

Multiple REPL behavior

The new figwheel.repl namespace currently offers some ClojureScript functions to help you list and choose which connected client to focus on.

The figwheel.repl/conns macro allows you to list the connected clients:

For example:

cljs.user> (figwheel.repl/conns)
Will Eval On:  Darin
Session Name     Age URL
Darin            25m /figwheel-connect
Judson          152m /figwheel-connect
nil

The above figwheel.repl/conns call lists the clients available for the REPL to target.

All connections are given easy to remember session names. The intention is that this will help you easily identify which browser tab your, through the REPL client feedback in the browsers dev-tool console.

The Will Eval On: Darin indicates that the Darin client is where the next eval operation will be sent to because this is currently the youngest connected client.

This youngest client heuristic for choosing which client to evaluate on, allows for a simple understanding of which REPL is the current target of eval operations. Open a new browser tab, or start an new node instance and that becomes the new eval target.

If you want to focus on a specific client,

cljs.user> (figwheel.repl/focus Judson)
Focused On: Judson

From now on all evals will go to Judson unless the connection to Judson is lost in which case the behavior will revert to selecting the youngest connection.

You can confirm that the repl is currently focused with:

cljs.user> (figwheel.repl/conns)
Focused On: Judson
Session Name     Age URL
Darin            28m /figwheel-connect
Judson          155m /figwheel-connect
nil

I think this goes a long way toward solving a problem that has existed since the very beginning of Figwheel.

Attention toward embedding the figwheel-repl endpoint

The other problem that I'm currently trying to work out is how to best support embedding the Figwheel REPL endpoint in your server.

For larger projects it simplest to use figwheel connection as a side-channel, a separate REPL connection, that is distinct from your projects HTTP server. Figwheel's use of Web-sockets and CORS make this side connection a simple matter. But inevitably there are situations where you want to embed the Figwheel endpoint in your server. So I'm giving this some serious attention.

In addition to the Web-socket connection, I have implemented a simple HTTP polling connection which should allow anyone to embed figwheel-repl ring middleware into their stack. (Side note: I'm also looking at long polling).

It is too bad that as a community we haven't landed on an agreed upon Ring web-socket interface, as this makes it much harder to allow simple embedding of a web-socket endpoint into the server of your choice. But I'm going to do my best to facilitate this by making it easier to create a web-socket endpoint from the provided api.

On a side note: I'm also considering making the default server a the ring.jetty.adapter as it is such a common dependency.

License

Copyright © 2018 Bruce Hauman

Distributed under the Eclipse Public License either version 1.0 or any later version.

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