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In this lesson, you’ll learn how to add types to the concepts in your DSL.

In Freon, types are normal concepts. Typically, they’re not projected—you won’t see them in .edit files.
So, why are they important? Let’s take a look.

Figure 1. Undetected errors in grading expressions

See the grading expression? It compares Number of Correct Answers, which should be an integer, with Answer to lollipops2 is correct, which is a boolean.
We’re effectively comparing an integer to a boolean—clearly an error. In this simple case it’s obvious, but in larger expressions users need help detecting such mistakes.

Let’s introduce a Type concept to handle this.

We’ll keep it simple for now but prepare for future extensions.
We define Type as an interface, and add one implementation: PrimitiveType, which is a limited (enumerated) concept.
We also include an ANY instance to represent a universal type (similar to Object in other languages).

Add the following to your edu-main.ast file:

// Education/lesson8-defs/edu-main.ast#L19-L27

interface Type {
    name: identifier;
}

limited PrimitiveType implements Type {
    Integer;
    Boolean;
    ANY;
}

Now that we’ve defined what types are, Freon needs to know:

1. Which concepts are types, and
2. Which concepts are typed (i.e., have an associated type).

Fortunately, that’s simple!
Create a file called edu-scoring.type, and add:

// Education/lesson8-defs/edu-scoring.type#L1-L5

typer

istype { Type }

hastype { ScoreExpression }

This tells Freon that all instances of Type are valid types, and all instances of ScoreExpression are typed elements.

But what about the NrOfCorrectAnswers concept?
How do we specify which type each expression returns?
For that, we use the infertype directive.

// Education/lesson8-defs/edu-scoring.type#L7-L24

QuestionReference {
    infertype #PrimitiveType:Boolean;
}
NrOfCorrectAnswers {
    infertype #PrimitiveType:Integer;
}
NumberLiteralExpression {
    infertype #PrimitiveType:Integer;
}
OrExpression {
    infertype #PrimitiveType:Boolean;
}
AndExpression {
    infertype #PrimitiveType:Boolean;
}
ComparisonExpression {
    infertype #PrimitiveType:Boolean;
}

Here we declare that:

• Question references and boolean operators evaluate to Boolean.
• Numeric expressions and counts evaluate to Integer.
• Comparison expressions also result in Boolean.

With this setup, Freon can automatically detect and report mismatched types in grading expressions.

In the next lesson, we’ll use these typings to validate expressions and provide meaningful user feedback.

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