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C'est la Z

Learning Elisp 9

In this video, we're finally putting everything together for the function-header project.

for the most part, it's just putting a function wrapper (defun) around code we've already written but there area a few new things.

In order to parse the parameter string we use a few interesting functions. The parameter string might look something like this "int a, double d, int something, char c" - type name pairs separate by a comma and space. To parse this into a list, we can use string-split. We use ~(string-split param-string ", ") which splits around the comma/space separators. This gives us a list ("int a" "double d" "int something" "char c") In elisp, stuff in parens is a list. We've usually been interpreting lists as functions where the first item is the name and the rest the parameters but we can also use them strictly as data.

We then revisit the mapcar function that we used in rot13. Mapcar takes a function and a list and then returns a new list having applied the function to each element. In our case, we use the format function we covered last time to reformat each paramter into a javadoc string.

Finally, we use string-join to join all the parameter string elements in our list back into one string.

The other new elisp function we look at is save-excursion. It's pretty simple but very useful for editor functions. It basically saves the location of the point, does everything in its body and then restores the point to where it was.

So, if you had this code block:

  (next-line 5)
  (forward-char 3)
  (do some stuff)

Your cursor (point) would have moved down 5 and 3 across and the stuff would be done. When the block ends, the point would be wherever it was moved to.

If, instead we do this:

    (next-line 5)
    (forward-char 3)
    (do some stuff))

Then the same thing happens but at the end, the point is restored to where it was when you started.

I also started using eros mode which temprarily overlays the results of elisp forms when you execute them. If you use Cider for Clojure development it looks a lot like that. It's easy and very useful for interactive development.

Finally, as I mention in the video, I'll be away for a couple of weeks at the end of September and into October. Visiting Utrecht, Brussels, Antwerp, and Amsterdam so if you've got any hidden gems for us to visit please let us know. I really mention this though because it will likely affect my video making schedule. Our next project - displaying emojis automatically, is going to be three or four videos. Maybe more. I don't know if it makes sense to start 1 and then take the big break so I might pause this project until I'm back in mid October or maybe just do a one off before I go away. We'll seen.

In the meantime, enjoy this one.


The code for the series is still up here:

but this episode doesn't have any specific code.

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