Use of the generated isTYPE() function

[goto40]

I am using langium 0.4.0.

I created a minimal language which models structs, which contain a list of attributes. These attributes can be of two types: arrays and scalar (https://github.com/goto40/msg-dsl-mixed-lookup-and-path):

//...
MsgStruct:
    'struct' name=ID '{'
        (msg_attributes+=MsgAttribute)*
    '}';

MsgAttribute: MsgScalarAttribute|MsgArrayAttribute;

MsgScalarAttribute: 'scalar' name=ID ':' msg_type=[MsgType:FQN];
MsgArrayAttribute: 'array' name=ID ':' msg_type=[MsgType:FQN] '[' dim=[MsgArrayDim:FQN] ']';
//...

Generating code from the list of attributes of a struct
implies distinguishing the type of attribute (e.g. MsgScalarAttribute
or MsgArrayAttribute):

When this is done with the generated isMsgArrayAttribute
and isMsgScalarAttribute I found that (obviously) the MsgArrayAttribute
is-a MsgScalarAttribute... That confused me (is it a structural type check?).

The following code "works":

            if (isMsgArrayAttribute(attr)) {
              // process array attribute
            }
            else if (isMsgScalarAttribute(attr)) {
              // process scalar attribute
            }

The following code generates a problem for the else path:

            if (isMsgScalarAttribute(attr)) {
              // process scalar attribute
              // process array attribute (is-a MsgScalarAttribute???)
            }
            else if (isMsgArrayAttribute(attr)) {
              // never happens (impossible)
            }

Is this the intended way? Should I check for the $type-field to identify the underlying model type?

[msujew]

Hey @goto40,

I'm a bit confused, since I couldn't reproduce the problem that you experience with your repository. In general, the langium-cli will generate code (the AstReflection) that allows to perform Class-like inheritance checking based on the $type attribute and nothing else.

I've used a small test script:

const ast = require('./language-server/generated/ast');
const isScalar = ast.isMsgScalarAttribute({ $type: 'MsgScalarAttribute' });
const isArray = ast.isMsgScalarAttribute({ $type: 'MsgArrayAttribute' });
console.log('scalar: ' + isScalar);
console.log('array: ' + isArray);

However, it prints that MsgArrayAttribute isn't a MsgScalarAttribute as expected. Can you provide me with a test case where we can clearly see the unexpected behavior?

[goto40]

You are right. I was confused by my vscode indicating that attr will have type never.

However, compiling and executing the code generated behavior as expected!

[msujew]

Interesting. I guess that TypeScript knows that MsgScalarAttribute is a structural subtype of MsgArrayAttribute, which confuses it, since that would mean that isMsgScalarAttribute has to return true. If it returns false, it assumes that it also can't be a MsgArrayAttribute, but rather never.