Manipulating AST node for code generation

[luan-xiaokun]

Hey guys, I'm trying to implement a code generator for my DSL. Automatons are the fist-class citizens in this language, and they can be nested, for example, you can instantiate one automaton in another one, so I need to unfold all automatons into only one automaton, and then transform it into some target language. (This is very like unfolding every inline functions to get a big function, but it is a bit more complicated).

Therefore, I need to manipulate AST nodes, creating new ones and relate them together. For example, in some cases, I need to take two expressions from two statements, and create a disjunction of them. Then I need to write something like

const disjunct: BinaryExpression = {
    $container: container,
    $type: 'BinaryExpression'
    operator: '||',
    left: expr1,
    right: expr2
}

There are two things bothering me:

1. As both expr1 and expr2 are taken from the parsed AST, their containers are not the node disjunct that I'm creating, so I need to clone them and at the same time change their $container properties, and this process should be recursive.
2. Creating disjunct requires a container, and in many cases, I'm creating disjunct because I need it in a larger container being constructed. We cannot first create disjunct and then use it to construct its constructor, and finally reset the container of disjunct, because $container is a readonly property.

Given enough time, I can find workarounds for these problems, but the progress is very slow because I'm not familiar with TypeScript (nor JavaScript). Do you have any advice on dealing with these inconveniences? Both suggestions on how to do code generation and tips on useful TypeScript features are appreciated.

By the way, I have a prototype of the generator written in Java, it looks like

tnew.setGuard(new BinaryOperatorTerm()
                        .setParent(tnew)
                        .setOpr(EnumBinaryOperator.LAND)
                        .setLeft(
                                new SingleOperatorTerm()
                                        .setOpr(EnumSingleOperator.LNOT)
                                        .setTerm(cond.copy(parent))
                        )
                        .setRight(tnew.getGuard())
                );

which is more intuitive to me (although I'm not familiar with Java either).

[msujew]

Hey @luan-xiaokun,

for cases such as these, we do have a linkContentToContainer function in langium, which automatically sets the $container property on all its direct descendants (not it's transitive descendants!). You can ignore all the $container/$containerProperty/$containerIndex properties during construction and the mentioned function will take care of it.

[luan-xiaokun]

That's a handy utility function, I was using const newNode = Object.assign(node) to change the readonly property.

Is there any way to use something like undefined to first create the node and then call linkContentToContainer? If I don't provide a container with the correct type, TypeScript will complain about the missing property. (I miss python's dynamic types）

[msujew]

Yeah, you can just use $container: undefined! which basically tricks typescript into thinking you've assigned something.

[luan-xiaokun]

Wow, that's a new trick for me, thanks for the help!

[spoenemann]

Nicer alternative:

type Programmatic<T extends AstNode> = Omit<T, '$container'>