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Lab 3: Simple Term Rewriting


This lab is your first encounter with Stratego. You add an outline view and a desugaring transformation to an initial editor provided by us.



  1. Specify rewrite rules to-outline-label which map AST nodes to labels in an outline view. You should include:
    • classes (class name and, if available, parent class name),
    • fields (field name and type),
    • methods (method name, parameter types, return type) and
    • local variables (variable name and type).
  2. Define rewrite rules desugar which desugar
    • unary expressions into terms of the form UnExp(op, exp),
    • binary expressions into terms of the form BinExp(op, exp1, exp2) and
    • octal numbers into decimal numbers.
  3. Integrate desugar into a strategy desugar-all which desugars subtrees in an AST.

For grading, it is required to comply with all constructor names, rule names, and strategy names literally.


You need to submit your MiniJava project with a merge request against branch assignment-3-submission on GitLab. The Git documentation explains how to file such a request.

The deadline for submission is October 5, 2019, 23:59.


  • Outline (55 points)
    • Class (5 points)
    • Field (10 points)
    • Method (20 points)
    • Variable (10 points)
    • Challenge (10 points)
  • Desugar (45 points)
    • Basic (30 points)
    • Strategy (10 points)
    • Challenge (5 points)

Early Feedback

We provide early feedback for the correctness of your outline and desugarings. This feedback is a score out of 90 points; the strategy choice (10 points) is graded manually. You have 3 early feedback attempts.

Detailed Instructions


GitLab Repository

We provide you with a template for this assignment in the assignment-3-template branch. See the Git documentation on how to check out this branch.

Anatomy of a Spoofax Project

Until now, you mainly worked on files in the syntax folder of your project. During this lab you will also edit files in the trans folder. So, this is a good point to talk about the general structure of a Spoofax project.

First of all, every Spoofax project may be part of an Eclipse plug-in project. This allows you to deploy your editor as a plugin using the Eclipse update site mechanism. Users do not need to have Spoofax installed for using your editor.

The actual language definition is spread over three folders:

  • syntax contains all syntax definition files, including the main file minijava.sdf3.
  • trans contains all transformation files, including the main file minijava.str.
  • editor contains editor service definition files, including the main file Main.esv.

In the src-gen folder, you will find files which were generated from your syntax definition. For each file <name>.sdf3, there are generated files:

  • syntax/<name>.sdf: an SDF2 definition which is equivalent to the SDF3 definition.
  • completions/<name>-cp.str: abstract placeholder expansions for syntactic code completion derived from SDF3 templates.
  • completions/colorer/<name>-cc-esv.esv: instructions to color placeholders for syntactic completions in gray.
  • pp/<name>-pp.str: pretty-printing strategies derived from SDF3 templates.
  • signatures/<name>-sig.str: signatures derived from SDF3 templates.

You can find more generated files in src-gen and target/metaborg folders:

  • src-gen/syntax/MiniJava.def: your complete syntax definition in SDF2.
  • src-gen/syntax/MiniJava-Permissive.def: a permissive version of the syntax definition, which supports error recovery.
  • target/metaborg/MiniJava.tbl: the parse table of your language.
  • src-gen/pp/MiniJava-parenthesize.str: strategies to add parentheses to an AST according to the priorities of your language.
  • target/metaborg/stratego.ctree and/or target/metaborg/stratego.jar: compiled Stratego code of your language.

Initial Editor Project

We provide you with an initial MiniJava project in the branch assignment-3-template. This project is a common starting point for all of you. It includes:

  • a parse table reference/sdf.tbl which passes all syntax tests,
  • corresponding signatures reference/src-gen/signatures/*-sig.str,
  • pretty-printing definitions reference/src-gen/pp/*-pp.str, and a
  • content-completion definitions reference/src-gen/completion/*-cp.str (errors in the completion files can be ignored).


Signatures declare sorts and constructors for terms. In Spoofax, terms are used to represent abstract syntax trees. The corresponding signature is generated from the constructors in a syntax definition. You can find the signature for MiniJava in the files reference/src-gen/signatures/*-sig.str. The signature was generated from a syntax definition, which itself is not included in the initial project. If you write your own syntax definition, the generated signatures can be found in src-gen/signatures/.

Outline View

Rewrite Rules

An outline view can be specified by rewrite rules to-outline-label in trans/outline.str. These rules should rewrite AST nodes to their label in an outline view. For example, the following rule rewrites a variable declaration to its name, which will be used as a label.


  to-outline-label: Var(t, v) -> v

On the left-hand side, the rule matches a variable declaration. During the match, variables t and v are bound to actual terms. On the right-hand side, the rule instantiates a label. During the instantiation, variable v is replaced with the term it is bound to. You can extend to-outline-label to provide labels for

  • class declarations,
  • field declarations and
  • method declarations.

When you build the project and open a MiniJava file, you will get an outline of this program in the outline view. In case you do not see any outline view, you can select it in Show View from Eclipse’s Window menu.

Naming Conventions

In Stratego, we use the following naming conventions:

  • constructor and sort names: Pascal case / upper camel case (e.g. Add, BinExp, IntArrayConstructor)
  • rule names, strategy names, variable names: kebab case (e.g. e1, project-path, suggest-completions-left-rec)

String Interpolation

In many cases, you want to provide more information than just the name. For example, you might want to show not only a variable’s name, but also it’s type. The following rule achieves this:

  Var(t, v) -> label
    t'    := <pp-partial-minijava-string> t
  ; label := <concat-strings> [v, ": ", t']

On its right-hand side, it produces a label, which is bound in the where clause. First, the term bound to t is turned into a string bound to t' by applying a strategy pp-partial-minijava-string. This strategy is defined in minijava/trans/pp.str and uses the pretty-printing rules generated from our SDF3 grammar for MiniJava.

Next, the label is bound to the concatenation of the string bound to v, a constant string ": ", and the string bound to t'.

String concatenation is not very intuitive. Instead, you can also use string interpolation:

  Var(t, v) -> $[[v]: [t']]
    t' := <pp-partial-minijava-string> t

String interpolation allows you to combine text with variables. Text is enclosed in $[ and ], while variables inside the text are enclosed in [ and ]. These variables need to be bound to strings.

You should provide the following information in your outline labels:

  • class name and super class name
  • field name and type
  • method name, parameter types (not parameter names), return type
  • variable name and type

For parameter types, you need to turn a list of parameters into a string. You can do this with a recursive strategy:

pp-params: // empty parameter list
  [] -> ...

pp-params: // single parameter
  [Param(t, p)] -> ...

pp-params: // at least two parameters
  [Param(t, p), param | params] -> ...
    // do something on first parameter
    // recursive call on remaining parameters
    ... := <pp-params> [param | params]

For grading, it is required to define a strategy pp-params that always rewrites a list of Params to a string.

Your current outline view is missing a root node. You can add a root node by providing a label for programs.


In Stratego, terms can be annotated with additional information. The Spoofax outline view uses annotations to determine the icon of a node. You can specify the icon to use in an annotation:

  Var(t, v) -> label{icon}
    t'    := <pp-partial-minijava-string> t
  ; label := $[[v]: [t']]
  ; icon  := "icons/var.gif"

We do not require you to use icons and you will not earn any points with them. If you want to use them anyway, you should put the icons into the folder icons and place a proper attribution or license file next to them.


Challenges are meant to distinguish excellent solutions from good solutions. Typically, they are less guided and require more investigation or higher programming skills.

  1. Provide the file name as the root node label. In outline.str: import, command/control click and study libspoofax/term/origin for a suitable strategy.

  2. Outline the main method as a subnode of the main class. You need to change this strategy in the outline.str file:

     outline := <custom-label-outline(to-outline-label, to-outline-node)> ast

    Also import, command/control click, and investigate libspoofax/editor/outline for inspiration.

  3. Use one of the library strategies for folding to implement pp-params. You can find various folding strategies in the API docs.


A uniform representation of unary and binary expressions eases static analysis and code generation. To get such a uniform representation, you need to desugar abstract syntax trees during the analysis phase.


Before you can implement a desugaring, you need to define a signature for the uniform representation of expressions in trans/desugar.str:

  1. Identify unary and binary expressions in MiniJava. A unary expression has one subexpression and an operator. A binary expression has two subexpressions and an operator. There are more than one kind of unary expressions and more than three kinds of binary expressions in MiniJava.
  2. Specify new constants for unary and binary operators in a signature. Use UnOp and BinOp as types of these operators. Again, you should use names based on the semantics of an operator, not on its syntax. Reading an expression aloud might help you to find suitable constructor names.
  3. Define constructors UnExp and BinExp, which combine an operator and an expression (respectively two expressions) to an expression.

Rewrite Rules

The following rewrite rule defines a rule to desugar an addition:


  desugar: Add(e1, e2) -> BinExp(Plus(), e1, e2)

This rewrite rule is named desugar. On the left-hand side, the rule matches an addition. During the match, variables e1 and e2 are bound to actual terms. On the right-hand side, the rule instantiates a binary expression (in a uniform representation). During the instantiation, variables e1 and e2 are replaced with the terms they are bound to. You can extend desugar to replace the different unary and binary expressions in the abstract syntax tree with a uniform representation of these expressions. Define a rewrite rule desugar in trans/desugar.str for every unary or binary operator, which transforms the original expression into a uniform representation.

Editor Integration

To test your transformation, you need to define a builder. This is done similar to the builder for pretty-printing. Add the following rewrite rule to trans/minijava.str:

  (_, _, ast, path, project-path) -> (filename, text)
    filename := <guarantee-extension(|"desugared.aterm")> path ;
    text     := <desugar-all> ast

This rule follows Spoofax’ convention for strategies which implement editor services. On the left-hand site, it matches a tuple of

  • the first two parts are ignored
  • the path of the current file and
  • the project path.

On the right-hand site, it instantiates a pair, consisting of a filename and the designated text of the file. Both variables are bound in the where clause. The file name is derived from the path of the current file, while the content of the file is a desugared version of the selected AST node. You also need to hook your strategy into the editor, making desugaring available in the Syntax menu. You can do this in editor/Syntax.esv under the Show Parsed AST action:

action : "Show desugared syntax" = editor-desugar (source)

This rule defines

  • a builder,
  • its label in the Syntax menu, and
  • its implementation strategy editor-desugar.

Annotations can be used for different variants of builders:

  • (openeditor) from the Syntax menu ensures that a new editor window is opened for the result.
  • Finally, (source) tells Spoofax to run the builder on an unanalysed (and also not desugared) AST.

Note that the editor-desugar rule uses desugar-all. You should implement this strategy as explained in the next section. Until you do, the menu option will not work.


Rewrite rules typically define local transformations inside an AST. Rewrite rules with the same name define a strategy of this name. Furthermore, strategies can be defined to orchestrate rewrite rules to complex transformations of complete ASTs. A strategy consists of a name and a definition, which is typically a combination of strategy applications. For example, the following strategy orchestrates local desugarings to a desugaring of complete ASTs:


  desugar-all = innermost(desugar)

This strategy is named desugar-all. It applies local desugar rules. The application is guided by a generic traversal strategy innermost, which tries to apply its parameter inside a tree, starting at the leaves (bottom-up, left-to-right). Whenever an application is successful, the result is traversed again.

Same results can be achieved with different generic traversals. You should try different traversals in trans/desugar.str:

  • desugar-all = innermost(desugar)
  • desugar-all = topdown(desugar)
  • desugar-all = topdown(try(desugar))
  • desugar-all = bottomup(desugar)
  • desugar-all = bottomup(try(desugar))
  • desugar-all = alltd(desugar)

Try to understand what is going on and decide for a suitable one. You can use the library strategy debug to print the currently visited node. For example, innermost(debug; desugar) will debug all nodes before it tries to desugar them.

For the submission, you need to provide an explanation (1 paragraph in weblab) of

  1. the choice you made,
  2. why this choice is suitable for this project, and
  3. why other choices would be less suitable.

Try changing the editor-desugar rule to use desugar-all instead of desugar. If you chose a suitable strategy, the builder should succeed even when no node is selected in the MiniJava program.


Define a desugaring for octal numbers. In Java, octal numbers start with leading zeros. Define a rewrite rule which matches such numbers and transforms them to decimal integers. If the number with leading zero is not octal, do not transform it. See the API docs for useful helper strategies.