Validating the Model

In the previous lesson, we learned how to add typings to the model, but we didn’t yet use them to provide user feedback.
In this lesson, we’ll use those typings to validate the model and produce type errors.

Adding Type Checking

Freon automatically generates a validator for your DSL that includes several built-in checks.
For example, it verifies that all references point to valid targets and that every property with the type identifier has a valid TypeScript identifier as its value.

However, Freon doesn’t include built-in checks for types—you need to define those yourself.
Let’s take the AndExpression concept. It has both a left-hand and a right-hand side, and both should have the type Boolean.
We can define a similar rule for OrExpression.

Add a file called edu.valid and include the following definitions:

// Education/lesson9-defs/edu.valid#L1-L11

validator for language Education

AndExpression {
    typecheck equalsType( self.left, #PrimitiveType:Boolean );
    typecheck equalsType( self.right, #PrimitiveType:Boolean );
}

OrExpression {
    typecheck equalsType( self.left, #PrimitiveType:Boolean );
    typecheck equalsType( self.right, #PrimitiveType:Boolean );
}

Now, what about instances of LessOrEqualsExpression and the other comparison expressions?
They also have left and right sides, but for them, it’s not about a specific type—it’s about both sides having the same type.
We can define that as follows:

// Education/lesson9-defs/edu.valid#L13-L31

LessOrEqualsExpression {
    typecheck equalsType( self.left, self.right );
}

GreaterOrEqualsExpression {
    typecheck equalsType( self.left, self.right );
}

LessThenExpression {
    typecheck equalsType( self.left, self.right );
}

GreaterThenExpression {
    typecheck equalsType( self.left, self.right );
}

EqualsExpression {
    typecheck equalsType( self.left, self.right );
}

The Result

Once validation is configured, Freon can identify type mismatches and display errors in the editor.

Currently, validation doesn’t run automatically—you need to activate it manually.
Open the Edit menu and select Validate to trigger the checks.
When you do, you’ll see something like this:

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