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Rascal DSL crash course

In this crash course on DSL engineering you will learn the basics of implementing a DSL in the Rascal Language Workbench:

Topics covered by the course:

  • Syntax definition
  • Consistency checking
  • Dynamic interpretation
  • IDE support

Preliminaries

Please install VS Code and then the Rascal VS Code extension (you can also find Rascal in the extension browser).

Git clone this repository (NB: don't rename the folder!). Finally, go to the File menu of VS Code and select "Add Folder to workspace", navigate to where you've cloned the repo, and select that directory.

The course project comes with pre-wired IDE support for QL: as you progress through the exercises, you will notice this in the editors of the QL programs. As soon as your grammar is correct, you will see syntax highlighting. Type checking will show errors as squiglies in the editor. As soon as your interpreter works, the code lens "Run..." (link at top of the form) will trigger execution of the form.

QL

The course is based on a DSL for questionnaires, called QL. QL allows you to define simple forms with conditions and computed values. A QL program consists of a form, containing questions. A question can be a normal question, or a computed question. A computed question has an associated expression which defines its value. Both kinds of questions have a prompt (to show to the user), an identifier (its name), and a type. The conditional construct comes in two variants if and if-else. A block construct using {} can be used to group questions.

Questions are enabled and disabled when different values are entered, depending on their conditional context.

A full type checker of QL detects:

  • references to undefined questions
  • duplicate question declarations with different types
  • conditions that are not of the type boolean
  • operands of invalid type to operators
  • duplicate labels (warning)
  • cyclic data and control dependencies

The language supports booleans, integers and string types.

Different data types in QL map to different (default) GUI widgets. For instance, boolean would be represented as checkboxes, integers as numeric sliders, and strings as text fields.

Here’s a simple questionnaire in QL from the domain of tax filing:

form taxOfficeExample { 
  "Did you sell a house in 2010?"
    hasSoldHouse: boolean
  "Did you buy a house in 2010?"
    hasBoughtHouse: boolean
  "Did you enter a loan?"
    hasMaintLoan: boolean
    
  if (hasSoldHouse) {
    "What was the selling price?"
      sellingPrice: integer
    "Private debts for the sold house:"
      privateDebt: integer
    "Value residue:"
      valueResidue: integer = 
        (sellingPrice - privateDebt)
  }
}

See the folder examples/ for example QL programs.

Tutorial (1 hour)

Open tutorial/Series1.rsc, click on Run in new Rascal terminal (top of editor). Fill in the blanks of the functions, and execute in the terminal. If you have time left, move on to Series2.rsc. Documentation can be found here. See also the cheat-sheet in the slides folder.

QL Exercises (3 hours total)

Each task corresponds to a Rascal module, see each module for an explanation:

  • Syntax.rsc: syntax definition (grammars and parsing)
  • Check.rsc: type checking
  • Eval.rsc: semantics
  • App.rsc: execution (run QL as a Salix web app)

Optional (if there's time left): extend the language with a date question type and revisit all the modules to adapt type checking, interpretation, and execution to support it.