Add a simple example without Langium

[Lotes]

I wrote a simple handwritten expression parser.

It has:

• print outs like print 123;
• variable declaration like var index = 123;
• variable usage like print index+1;
• binary expressions, like +, -, *, / and %
• unary expressions, like + and -

It worked good so far, even the desired validations I get for free :-) .
Currently there is not much, that can go wrong. Only operators that are not overloaded like string * string cause troubles.

Maybe one of you has more ideas, what to add on top for testing or for language features.

For sure I can add parentheses expressions or assignments in order to make the language "round & sound".

In the end I just wanted to have something framework-agnostic and I think this was proven.

[JohannesMeierSE]

Thank you @Lotes for this nice application example without Langium!

A general remark: Since this example is for demonstration, I suggest to add more comments:

• Add a README.md which describes the purpose and the interesting parts of this application of Typir.
• Document the features/concepts of your DSL (at the moment you need to look into the implementation
• Give readers a hint, how they can experiment with your DSL, e.g. point them to your test cases.
• Add one or two sentences as comments into each of the five src/*.ts files to describe their purpose. That will help people who never wrote a parser themselves to get the ideas of the files.

Ideas for more validations:

• VAR X = 2 * "hello"; should fail
• Introduce an AssignmentStatement and check asignability of left- and right-hand side

If you plan to add a generator, maybe these ideas are helpful:

• Keep it as simple as possible, I like the slim design of the current state of your example!
• Make the implicit conversions (number as string) in the generated code explicit, since here the type system is very helpful (the assignability service returns the conversion path now!)

[JohannesMeierSE]

Thanks a lot @Lotes for your example! The additional comments, diagrams and test cases are really helpful!

[JohannesMeierSE]

Very nice graphics, thanks a lot!

[JohannesMeierSE]

You could write this shorter in this way:

return typir.validation.Constraints.ensureNodeIsAssignable(node.value, node.variable, (actual, expected) => <ValidationMessageDetails>{
    languageNode: node, severity: 'error', message: `'${actual.name}' is not assignable to '${expected.name}'.`,
});

But it is also interesting to the see the implementation behind 🙂

[JohannesMeierSE]

An idea: For the examples OX and LOX, we use the prefixes ox- and lox- for the typescript files. Does it makes sense to have a similar prefix (maybe expr- or expressions-) for your expressions examples as well?