split panes

[gelisam]

One feature I really like from iTerm2 is split panes. In particular, I always split my tabs vertically, with my text editor in the left pane, and ghcid in the right pane. I can currently achieve this with two termonad windows side-by-side, but this breaks down when I have multiple tabs, as I would like switching to a different tab to switch both to the source code of the new tab and to the ghcid of the new tab.

How would I go about implementing this?

[cdepillabout]

Do you know of a terminal emulator on Linux that has this same functionality? I'd like to play around with it to get a feel for how it works.

Of course, the easiest way to do what you want to do would be to just use a terminal multiplexer (like tmux or screen) inside a single Termonad tab, but I'm assuming you don't want to do this.

---

Currently Termonad just has a single gi-vte Terminal inside a gtk Notebook page.

The code for creating a new Terminal and Notebook tab is here:

termonad/src/Termonad/Term.hs

Lines 442 to 459 in d817fe4

	-- Create a new terminal and launch a shell in it
	vteTerm <- createAndInitVteTerm tmStateFontDesc (options tmStateConfig)
	maybeCurrDir <- getCWDFromFocusedTab currNote
	termShellPid <- launchShell vteTerm maybeCurrDir
	tmTerm <- newTMTerm vteTerm termShellPid
	-- Create the container add the VTE term in it
	scrolledWin <- createScrolledWin mvarTMState
	containerAdd scrolledWin vteTerm
	-- Create the GTK widget for the Notebook tab
	(tabLabelBox, tabLabel, tabCloseButton) <- createNotebookTabLabel
	-- Create notebook state
	let notebookTab = createTMNotebookTab tabLabel scrolledWin tmTerm
	-- Add the new notebooktab to the notebook.
	addPage mvarTMState notebookTab tabLabelBox

If you wanted to add functionality for having multiple Terminals inside a single Notebook tab, I imagine you'd have to edit the above file to add a function (or modify the linked function) to not add a new Notebook tab, but instead optionally split an existing Notebook tab and add a new Terminal to it.

I imagine GTK provides some sort of widget to allow you to arbitrarily split a layout in half, but I'm not sure what would be the best widget to use. Maybe try taking a look at the following:

• https://hackage.haskell.org/package/gi-gtk-3.0.37/docs/GI-Gtk-Objects-VBox.html
• https://hackage.haskell.org/package/gi-gtk-3.0.37/docs/GI-Gtk-Objects-HBox.html
• https://hackage.haskell.org/package/gi-gtk-3.0.37/docs/GI-Gtk-Objects-Grid.html

There are probably also a few places in the code that make an assumption that there is a one-to-one mapping between Notebook tabs and Terminals. I could help look for these once you have something basically working here.

There are a few other small things you'll have to decide while working on this. The first thing that comes to mind is what to put in the Notebook tab label when you have multiple panes. With a single terminal in a Notebook tab, you can just have either the name of the process running in the terminal (or let the terminal set the title), but with multiple tabs you'll have to decide what makes the most sense here.

You'll likely also need to add a new key command for opening a new pane instead of a new tab. Here's where the New Tab command is defined:

• termonad/src/Termonad/App.hs

  Lines 429 to 432 in d817fe4

  	newTabAction <- simpleActionNew "newtab" Nothing
  	void $ onSimpleActionActivate newTabAction $ \_ -> void $ createTerm handleKeyPress mvarTMState
  	actionMapAddAction app newTabAction
  	applicationSetAccelsForAction app "app.newtab" ["<Shift><Ctrl>T"]

• termonad/src/Termonad/XML.hs

  Lines 57 to 60 in d0f8c9a

  	<item>
  	<attribute name="label" translatable="yes">New _Tab</attribute>
  	<attribute name="action">app.newtab</attribute>
  	</item>

[cdepillabout]

There is also a long-standing issue about the way Termonad makes the Terminal scrollable: #114. The refactoring required to fix this issue (#205) might be a good chance to fix #114 as well.

[gelisam]

Do you know of a terminal emulator on Linux that has this same functionality?

I found one, Konsole.

Of course, the easiest way to do what you want to do would be to just use a terminal multiplexer (like tmux or screen) inside a single Termonad tab, but I'm assuming you don't want to do this.

Yes, that's my fallback plan if adding this feature to Termonad turns out to be too complicated. Thanks for the code pointers, I'll give it a shot!

[gelisam]

I imagine GTK provides some sort of widget to allow you to arbitrarily split a layout in half, but I'm not sure what would be the best widget to use. Maybe try taking a look at the following: [VBox, HBox, Grid]

Nah, all three of those are for programmatically laying out components next to each other, e.g. a Cancel button next to an Ok button. What I had in mind is Paned, which gives the user a draggable divider for resizing.

[Minda1975]

Hello. Also, terminator and kitty have this features.

[cdepillabout]

@gelisam Oh nice, Paned looks like it might work well.

@Minda1975 thanks for the pointers!

[gelisam]

There are probably also a few places in the code that make an assumption that there is a one-to-one mapping between Notebook tabs and Terminals

My attempts so far have been caught by assertInvariantTMState. It's refreshing to have such a formally-specified list of assumptions!

[gelisam]

Any particular reason why assertInvariantTMState is returning a list of errors instead of using a Validation monad, or even just Writer or ExceptT?

[cdepillabout]

Any particular reason why assertInvariantTMState is returning a list of errors instead of using a Validation monad, or even just Writer or ExceptT?

No good reason.

Feel free to refactor that function (or, really, any of the Termonad code) if it makes the implementation easier for you!

[gelisam]

All right, I finally have something to show! On my always-split branch, I have a version of Termonad in which every tab is split into two panes. I currently have a bug where exiting the shell of either pane will cause the tab to close, but won't cause the other pane's shell to exit, leaving orphan processes. But that seems minor compared to the bigger issue which follows.

While always having two panes is actually what I want, that's probably not what most users want, nor is it the feature I had in mind when I opened this issue. A more typical split-pane feature would start each tab with a single shell, and allow the user to recursively subdivide the area into more and more (and smaller and smaller) shells, and to close each pane independently. I don't have those features yet, but I do have a good idea of how to implement them.

Before I do, however, I think it's worth having a discussion about the split-paned API. Termonad boasts about being "extremely customizable", so after implementing proper split panes, it would be nice if I was then able to customize Termonad so that each tab starts with two panes rather than one. But (1) I am not very familiar with the way in which Termonad makes itself customizable, and (2) I worry about accidentally making a customization API which is too focused on my own use case. Any guidance?

[cdepillabout]

That all sounds really good. Definitely feel free to open a PR with what you currently have (or a draft PR if that makes more sense).

Termonad boasts about being "extremely customizable", so after implementing proper split panes, it would be nice if I was then able to customize Termonad so that each tab starts with two panes rather than one. But (1) I am not very familiar with the way in which Termonad makes itself customizable, and (2) I worry about accidentally making a customization API which is too focused on my own use case.

The customization features of Termonad aren't currently super well thought-out. Here's the config module: https://hackage.haskell.org/package/termonad-4.2.0.0/docs/Termonad-Config.html

You can see most things are just dumb, toggleable options. However, there is also a "hooks" feature that lets end users hook into different points in the execution of Termonad and do arbitrary actions. This is mostly based on the same ideas from XMonad:

https://hackage.haskell.org/package/termonad-4.2.0.0/docs/Termonad-Config.html#t:ConfigHooks

You can see that there is currently only one hook. There really haven't been many users asking for additional hooks, so I haven't added any others yet. Although it is something I'd like to do at some point.

So I think you basically have the following two choices for how to go about customizing the split pane stuff:

1. Add a "dumb" configuration value to https://hackage.haskell.org/package/termonad-4.2.0.0/docs/Termonad-Config.html#t:ConfigOptions. This might be something like how many split panes to start by default, or what layout to have the split panes in that start by default (like two vertical, two horizontal, etc).

2. Use the existing createTermHook hook for adding new panes upon tab creation in your own code. Or possibly adding a new hook that makes more sense for your use-case.

   It would probably be nice to create some high-level functions that other users could easily use if they also wanted to manipulate panes from their own hooks.

It sounds like you're thinking something like (2) makes more sense. I agree that seems reasonable.

[gelisam]

Definitely feel free to open a PR with what you currently have (or a draft PR if that makes more sense).

Ah, I forgot about draft PRs! Good idea, better start to discuss the changes early. Here is a draft PR.

[craigem]

Loving following this feature request.

When it was opened it did not hold much interest for me as I was working with termonad + tmux for years but I've recently become rather frustrated with how tmux mis-represents colours for tmux to be my all-day driver.

I'll be testing this pretty hard soon.

Thanks for making this happen! 😃

[cdepillabout]

@craigem I haven't been able to take a look at this (hopefully I can get to it in the next week or so), but any help testing would be really great!

[formula-spectre]

i would like to be a tester too, but I don't quite know how to; do I simply clone his branch, install and test split panes?