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In this lesson you’ll learn how to define projections for expressions so you can take full advantage of Freon’s built-in expression support.

From the previous lesson we introduced the GradeScore concept. Let’s make its projection match the requirements:

GradeC: Answer to questionX is correct AND the Number of Correct Answers = 3

GradeD: Answer to questionY is correct OR Answer to questionZ is correct AND the Number of Correct Answers > 2

Define the projection like this:

// Education/lesson5-defs/edu-scoring.edit#L1-L6

editor default

GradeScore {[
    ${self.grade} : ${self.expr}
]}

Next, consider the Answer to questionY is correct part. In the metamodel this is represented by the QuestionReference concept, so the projection becomes:

// Education/lesson5-defs/edu-scoring.edit#L11-L14

QuestionReference {
    [ Answer to ${self.question} is correct ]
    trigger = "question"
    referenceShortcut = ${self.question}

In Lesson 2 (Making an Editor) you saw triggers, which make editing faster. Here we also use a reference shortcut.

When a question reference is added, two nodes are actually created in the AST: an instance of QuestionReference, and its child, a core FreNodeReference. Normally, the user would select both. With a reference shortcut, selecting a reference that fits the question property automatically creates the QuestionReference wrapper—no double selection required.

Add the following projections as well. Nothing new here—just formatting the remaining pieces. Note that for NrOfCorrectAnswers we use a constant string.

// Education/lesson5-defs/edu-scoring.edit#L16-L22


NrOfCorrectAnswers {
    [Number Of Correct Answers]
}

NumberLiteralExpression {
    [${value}]

Projecting binary expressions is straightforward: tell Freon which symbol to display as the operator.

// Education/lesson5-defs/edu-scoring.edit#L24-L53


///////////////////////////////////
/// Boolean AND and OR
//////////////////////////////////

OrExpression {
    symbol = "or"
}
AndExpression {
    symbol = "and"
}

///////////////////////////////////
/// Comparisons: <=, >=, >, <, ===
//////////////////////////////////

LessOrEqualsExpression {
    symbol = "<="
}
GreaterOrEqualsExpression {
    symbol = ">="
}
LessThenExpression {
    symbol = "<"
}
GreaterThenExpression {
    symbol = ">"
}
EqualsExpression {
    symbol = "=="

After generation, you’ll see expressions rendered as proper infix expressions.

Figure 1. Editor after projections for expressions

Binary expressions are shown with brackets to reflect the priorities defined in edu-scoring.ast. If you prefer not to see them, open the View menu and deselect Brackets.

Experiment with the projections so far. You may notice that, in the score for page Theory101, you can select questions from other pages—exactly what we don’t want. Teachers should only grade based on the current page’s questions. That means we need scoping, which we’ll cover in the next lesson.

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