Jamison Creek – the latest version

I moved to a new home recently … where “recently” is actually about 6 months ago. I’m using the extra demands on my time involved in buying a house, packing up contents and unpacking at the other end, as an excuse for not making progress on this project for a while. But the excuse if over, and coding has restarted. I, at least, am happy about that.

I’ve just tagged a new version – Jamison Creek – in honour of my new home. The main advance in this release is that types are starting to take shape. I had intended to implement various flavours of numbers – integers and floats of various sizes – when I first did types, but it didn’t turn out that way: they are actually rather complex as they have subtleties that need to be worked out carefully. I needed something simpler to flesh out the infrastructure for types, so I’ve just extended the current built-in types of “number”, “string” and “Boolean” with arrays and structs.

The syntax for declaring an array is different from what I proposed earlier. Had I used `[number:12]` for an array of 12 numbers, then an array of 12 arrays of 3 numbers would have been `[[number:3]:12]` which is ugly. That could be fairly easily abbreviated as `[numer:3:12]` but the indexes are backwards: the last element would be `foo[11][2]`, which would be confusing. So instead, arrays have the base type *after* the element count, and that count is enclosed in brackets, so `[12][3]number` is an array of 12 arrays of 3 numbers, which I think works well.

Structs use the same approach to block structure as complex statements, and they declare fields much like assignment declarations declare variables, so the initial value is optional. A simple example is
struct foo:
name:string="John Doe"

which can also be given as

struct foo { name:string="JohnDoe"; age:number=21;male:Boolean=True; }

These declaration are not part of code – in a `program` – but come as separate declarations. `program` is now also a declaration at the same level. To flesh out these declarations we can also have `const` at the declaration level.

I’ve introduced a few new operators too: `and then` and `or else` are short-circuit Boolean operators which only evaluate the right hand side if the value is necessary to determine an answer. There is also a trinary operator: `if ... else` which effects a conditional expression.

As well as these language enhancements, I’ve added some proper regression testing. There are a collection of small sample programs
together with expected outputs, and a script that not only tests that the output is correct, but also:

  • uses valgrind to detect memory leaks,
  • prints the program and ensures the printed program runs correctly too,
  • ensures the printed program prints exactly the same as the original did, and
  • uses the gcc coverage support to track the fraction of interesting code lines that are being tested, and reports an error if this drops below a set threshold, currently 94%.

Hopefully it will be much less than a year before my next version is tagged. Some things I have planned for the next version are:

  • Improved syntax error handling – particularly better messages and better recovery,
  • further development of ‘struct’, at least anonymous fields,
  • a variety of numeric types,
  • more operators: >> << op= ???,
  • pointers and memory allocation. This will be the big change and will probably get another blog post soon.

For now, you can read the current interpreter as Literate code.

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