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I'm trying to get familiar with Emacs Lisp, its syntax, data structures etc, so I was wondering what are some built-in packages whose source could help me better understand how to use the language and learn its idioms?

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    Be aware that the built-in packages are full of legacy code and therefore less suitable for learning idioms. Have you considered using external packages and libraries instead?
    – wasamasa
    Sep 24, 2014 at 9:27
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    Have you considered using the emacs tutorial (available as info file ? Some built-in functions are explained and detailed.
    – kiddouk
    Sep 24, 2014 at 9:48
  • @wasamasa I see, I will look at more recent packages.
    – caisah
    Sep 24, 2014 at 11:14
  • @kiddouk Yes I did but I can't cope with the writing/explaining style.
    – caisah
    Sep 24, 2014 at 11:15

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I can recommend Ruby Mode in ruby-mode.el and Python Mode in python.el, for some aspects of their implementation. The former is a particularly great source for advanced indentation and navigation code based on the new SMIE engine. However, neither uses data structures in any particularly clever or advanced way.

I would also recommend some parts of Org Mode 8, for its design. Notably the exporter framework of Org 8 shows how to create an generic, extensible, object-like framework in plain Emacs Lisp. However, I'd recommend against most other parts of Org, for reasons already stated in comments: It's old, and full of legacy and harmful practices (e.g. find-file-noselect to read files non-interactively).

Gnus is also said to have good code, but again I'd be careful to determine how well-maintained the particular piece of code is that you are looking at.

Since Emacs itself is very old, and only superficially maintained in many places, the average quality of its code is actually rather low by modern standards. There's many legacy stuff in there. For instance, even in core parts such as the byte compiler you'll find “TODO”s and “DUNNO”s dating back years, if not decades. Also, since Emacs is very slow to take code and developers from the “outside” community due to the restrictive policies established by the FSF, the overall quality increases only slowly.

All in all, you are more likely to find good code in popular 3rd party packages maintained by high-profile community members, e.g. Magit, Projectile, etc.

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    Oh I see, they changed it for the latest trunk.
    – shosti
    Sep 24, 2014 at 15:38
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    Those planning on taking a dive into gnus' code: Please do take care. Those are deep waters and we wouldn't want to loose anyone.
    – Malabarba
    Sep 25, 2014 at 13:27
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    If you look at Magit then I recommend looking at its next branch.
    – tarsius
    Sep 25, 2014 at 13:38
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    Do you have anything to backup your last claim? I would strongly expect the opposite...
    – rasmus
    Nov 19, 2014 at 23:32
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    What I had in mind was: "[...] Emacs is [...] superficially maintained in many places, the average quality of its code is actually rather low" versus "you are more likely to find good code in popular 3rd party packages". But "Emacs is very slow to take code and developers from the “outside”" also seems like nonsense IMO (do you read the debbugs?)...
    – rasmus
    Nov 20, 2014 at 0:42
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For basic idioms look into simple.el

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