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Before explaining the interpreter framework, we introduce a simple DSL that will serve as our running example. This DSL will serve as the foundation for illustrating how Freon’s interpreter framework processes and evaluates expressions. The language defines a single model unit, ExpressionUnit, which contains both function definitions and expressions. You can explore this language by creating a new Freon project using npm create freon, then selecting the Expressions template.

// Expressions/src/defs/Expressions.ast#L1-L11

language Expressions

model Expressions {
    name: identifier;
    units: ExpressionUnit[];
}

modelunit ExpressionUnit {
    functions: Function[];
    expressions: Expression[];
}

We add several basic expression concepts to represent numeric operations: NumberLiteralExpression and standard binary expressions. Operator priorities are specified to support straightforward editing (See Binary Expressions for details.):

// Expressions/src/defs/Expressions.ast#L15-L40

abstract expression Expression { }

expression NumberLiteralExpression base Expression {
    value: number;
}

abstract binary expression BinaryExpression base Expression {
    left: Expression;
    right: Expression;
}

binary expression MultiplyExpression base BinaryExpression {
    priority = 8;
}

binary expression PlusExpression base BinaryExpression {
    priority = 4;
}

binary expression MinusExpression base BinaryExpression {
    priority = 4;
}

binary expression DivideExpression base BinaryExpression {
    priority = 8;
}

We also define concepts to represent functions — both their definitions and calls:

// Expressions/src/defs/Expressions.ast#L42-L61

concept Function {
    name: identifier;
    body: Expression;
    parameters: Parameter[];
    reference declaredType : Type;
}

concept Parameter {
    name: identifier;
    reference declaredType: Type;
}

expression ParameterRef base Expression {
    reference parameter: Parameter;
}

expression FunctionCallExpression base Expression {
    reference calledFunction: Function;
    arguments: Expression[];
}

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