Wednesday, September 28, 2011

The dynamic interpreter, your code generation companion

The latest stable release of Acceleo, version 3.1.1 included in Indigo SR1, is fresh from the oven; but development on the next version is already well underway. The upcoming version 3.2 already includes major improvements of the tooling performance (compilation time, completion proposal computation, memory management...), making the everyday use of the Acceleo editor much more appealing; and some of the planned features for this version are already in their finalization stage.

The one feature I'll linger on here is the dynamic interpreter for Acceleo and OCL expressions that we implemented for this version. Typically, you develop your code generators within the Acceleo editor, making use of the advanced edition features such as completion proposals, live compilation and syntax error reporting, syntax highlighting ...

However the editor itself cannot help you determine what a given expression will return as its result. When you are in the middle of a complex OCL expression, it is sometimes difficult to say "this expression will output that classifier". And here is where the live interpreter comes into play!

The image above displays the "interpreter" view and the use we can make of it : We opened the UML metamodel and selected "Class" in it. This makes "Class" the context of any evaluation entered in the interpreter (that would work with anything that can be adapted to an EMF object : selection in a model editor, selection in a graphical editor (GMF, Obeo Designer... even the "variable" view when debugging Acceleo generations!). We entered an expression... and the view made the rest, evaluating that expression on-the fly and displaying its result (here, all of the super-types of the UML "Class").

Using the Interpreter during debug

The Interpreter can be used as is, defining your own variables as you need them and entering the expression manually... but it can also be used in conjunction with the debugger. For example, let's say I want to know what happens within the 'UML 2 Java' example that is provided with Acceleo.
  • I open the "classBody" module of that example, and set a breakpoint somewhere within the "generateClassBody" template :
  • Then, run the example in debug mode until the breakpoint is hit :
  • And from there, I can either copy/paste whole expressions in the interpreter :
  • Or "link" the interpreter with the current module context...
  • And directly call its templates :
  • Of course, the result of this last action is lengthier than a single line, double clicking the result will open a popup in which it will be more readable :

The view features a number of possibilities, creating and assigning variables to be used in the expression, real-time evaluation, linking with an Acceleo editor's context, saving the expression as a new query or template in a given Acceleo module... The view itself can be used without Acceleo, and accepts any other languages through extension points. I can't detail all of its features and extension possibilities here, see the wiki page for more (still a work in progress, yet it does describe the view in little more details than here) :).