Local variables and scope

While I was writing test code for my first toy language I particularly noticed the lack of interesting data structures:  to write interesting loops you can do a lot more if you can build data structures while doing it.  So my second toy will begin to introduce data types. It won’t be until a later iteration before we get structures though.

The first step to this is knowing  how to declare variables, or local bindings. Then I’ll have something to attach types of data structures too.  So this installment is about local variables and their scope.

I started writing this 3 years ago and am only publishing it now.  Sometimes life works like that.  I had to wait until the code worked and I managed to lose interest for a while and get distracted by other things.  At the recent linux.conf.au I went to a talk about the language “Pony“.  While I’m not thrilled with pony (I don’t currently think that expressions and statements are interchangeable), the talk inspired me to get back to ocean…  So what can we say about scopes?

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Complex documents in edlib

One of my core goals in developing edlib is to allow the display to be programmatically controlled: the content of a document is formatted dynamically as it is displayed, and so can respond to context (e.g. location of “point” can modify appearance more than just by displaying a cursor) and also so that the entire document doesn’t need to be rendered into a buffer, just the parts being displayed.

A natural (for me) extension to this idea was the possibility that the source of the display wasn’t just one single document – multiple documents could be blended. Various examples have occurred to me, though few have been implemented.

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Fewer NULL dereferences in edlib

My recent efforts with edlib have been to get rid of some potential NULL pointer dereference issues.  When I first started writing “commands” I assumed that the correct value would always be passed.  Of course that isn’t very safe unless it is enforced, and is particularly unsafe if I’m going to let users write their own commends in extension languages like python.  So more safety is required.

Auditing the code to ensure I’m always being careful enough is fairly boring and error prone, so I wrote a tool to help me. More accurately I extended some existing tools.  You can read lots more details in my LWN.net article. At the time I wrote that I still had some unresolved issues. I’ve resolved enough of those now that I no longer have warnings. Sometimes that is because I used casts to hide things, but a lot of real issues have been addressed.  The versions of “sparse” and “smatch” that I am using are in the “safe” branch of the copies of these trees on github.com.  So this for smatch and this for sparse.

Doing this involved adding a lot of ‘safe’ annotations throughout edlib.  Hopefully these aren’t too confusing.  It is nice to have the documentation of intent, and nice to know that a whole class of errors is now impossible.

I had to fix a few bugs in smatch/sparse as well as add new functionality to smatch.  I really should post those bug fixes upstream…

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A notmuch based email reader

I’ve been very slack.  Sorry.  I keep thinking “I should write a blog post about that” when I make some progress with edlib.  But I also think “or I could write some more code instead”.  I do enjoy writing blog posts.  But I seem to enjoy the code more.

Consequently, I’ve lost track of what has happened since my last post.  Git tells me that I have made 379 commits since then.  I’ve added “Alt-!” to run a shell command, with a history of recent commands stored in a buffer.  I’ve provided a way for attributes on text to trigger call-backs to display panes, to enabling highlighting of text; and used this to better display matches for the current search.  I’ve broken the “Refresh” event into three separate events, one that updates the sizes of panes,  one that can update the position of a document in a pane, and one that redraws the content.  And I’ve fixed lots of bugs and cleaned up lots of code.  But the big thing that I’ve been working on is a notmuch email client.

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edlib is back on track…

In the weeks leading up toe linux.conf.au 2016 I approached edlib development as a sprint.  I wanted to put together sufficient functionality so that I could present my LCA2016 talk using slides displayed by edlib.  I achieved this goal but at some cost.  A lot of the development that went into that sprint was fairly hackish and not well thought out.  It achieved the immediate goal but wasn’t good long term development.

With that cost came benefits.  Not just working code but a clearer perspective on what I needed to do and what problems would be faced by creating a slide-presentation mode.  I don’t regret the sprint at all but it certainly didn’t result in completed work.  I had to go over all of that development and turn the prototype into well designed code.  This naturally took quite a bit longer but resulted in much more coherent designs and a stronger overall structure.  This is done now and my LCA2016 presentation is now in my mainline of edlib development and works well.

There were a number of structural changes that I made while revising all this functionality, I’ll just address a few of them here.

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LCA-2016 presentation is done

I’ve been busy of the last couple of months.  A number of family and personal things meant I have less time for edlib, but I had a lot to do for edlib too.  I really wanted to use edlib to give my presentation at linux.conf.au 2016 in beautiful Geelong.

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edlib – render-lines, what a good idea.

I had a brain-wave last week.  I thought it would take me about a day to implement but it took 3 to get it working almost reasonably and the rest of the week to polish it and create some proper use-cases to exercise it properly.  But that is done now and I am quite happy with the result. It related to the rendering side of edlib.

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edlib – commands

It been over 3 months – maybe you thought I had given up – but no.  I had some holidays and other distractions, but I’m back and will be working fairly consistently for a while.

The topic for today is “Commands”.  Commands are a standard interface for code in one module of the editor to call code in some other module.  Various objects in the editor have commands attached to perform various tasks specific to that object.  To some extent you can think of commands like object methods in an object-orient system, but that is only part of the story.

In many cases, commands are associated with names in a key-map.  The key maps were hinted at in my previous note about managing input, but they are broader than that and details have changed since last I wrote. So maybe that is a good place to start.

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edlib – managing input

Commands to an editor – at least the sort of editor that edlib is designed for – usually involve single keystrokes or key combinations.  Translating those keystrokes, as well as mouse actions, into commands is the topic of this note.  Many editors also support commands that are written out with words.  Such commands are often introduced with Meta-X in emacs or “colon” in vi.  For the purposes of edlib, such interactions can be managed by creating a simple document in a small window somewhere and accepting key-stroke commands to edit that document.  Some keystrokes will “complete” the command which will make the window disappear and will perform the required action.  So word-based commands are certainly possible, but they happen at a different level.

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edlib – displays and panes

Displaying the document being edited is, of course, very important for any editor.

I have memories of the first editor I used on Unix which was em, rather than the well known “ed”.  It actually allowed a single line to be edited “directly” rather than by using pattern substitution (s/pattern/replace/) commands or rewriting whole lines.  It also had a ‘v’ command to view lines of context around the current line.  That ability to see what has happening helped a lot.

We’ve come a long way of course and today we expect to be able to easily see and move around the context of the place we are editing … though that “context” is usually the very simple “nearby lines of text”.

edlib needs to make it easy to provide a display of context, without mandating what that context might look like. To achieve this it provides “displays” and “panes”.  These are used for directing input from the user to the documents as well as for displaying content from the document to the user, but for now just the latter will be discussed.

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