68 lines
3.0 KiB
Plaintext
68 lines
3.0 KiB
Plaintext
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What is it with pointers?
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Listen, if I had to pick a language to write a decent program in, I'd want to
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use something like Rust. It behaves in more or less all the sane ways I want a
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language to behave. Python's nice and quick for Python-y stuff, and Javascript
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is Javascript, etc. But for some reason, I just <i>like</i> C.
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I've written something of an implementation of Lisp for my Pebble smartwatch,
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which I named PebbLisp. I'd love to have written it in Rust, to get some more
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practice with it, but it was a real pain in the tookus to get compiling for the
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watch, and I gave up. So, C.
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I'll never act like C is a perfect language. Many, many, stupid memory leaks
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could've been avoided with something smarter. For some reason, though, it's
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just fun to work with. PebbLisp involves some dancing about with pointers, and
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includes a homegrown tagged-union setup. There are parts that are probably very
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ugly to more practiced eyes, and correcting unexpected behavior is a nightmare,
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but coding it leaves me downright giggly sometimes.
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The Object struct in PebbLisp has three parts:
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<code class="codeblock">struct Object {<br>
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Type type;<br>
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union {<br>
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...<br>
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};<br>
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Object *forward;<br>
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}</code>
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<h2>The Tag</h2>
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The first part is the `Type` of the `Object`. This is just an enumeration with
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all the possible types that a PebbLisp object can be. It's the tag of our
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tagged union. Already I like it far more than is warranted. One number is a
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front for every possible type? Incredible!
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<h2>The Union</h2>
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The second part is, of course, the union itself. I've cut it out here, but
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there are members of the union for integers, pointers to strings, function
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pointers, etc. Unions are also something I treasure. The famous issue with
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unions is trying to keep track of what state any given union is actually in,
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and god help you should it ever be interpreted incorrectly. That's why it's
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fun, though. Coding a consistent way to access a union feels like cracking open
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the secret door of programming. The Pointer
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Finally, `Object *forward`: something that traditionally might be named `cdr`
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(which I had to look up, because I'm really not enough of a Lisp guy). In Lisp,
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everything is a list. An object with `forward == NULL` is a list of length 1.
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An object that points to another object is a list of length `1 +
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listLength(forward)`, aka itself plus the rest of the list.
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So much code in PebbLisp is dedicated to making sure that this value is never
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incorrect, duplicate, or useless. A `cloneObject()` function exists largely
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because copies of individual objects can't keep this address. I'd love to use
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our friendly neighborhood assignment operator: `=`, but C doesn't allow operator
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overloading. But `cloneObject()` was fun to write, somehow!
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<h2>C</h2>
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Anyway, this has already run too long. Point is, C doesn't care what you do
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with data. And for some reason, it's fun to redo decades of other people's work
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in a project no one asked for. I highly recommend it.
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#100DaysToOffload
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2020-09-01
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