Overview

A C++ implementation of an interpreter for the A++ Programming Language (variant of Delta). Developed as the main project of the graduate course "Advanced Topics in Programming Language Development" (Department of Computer Science, University of Crete), by Emmanouil Giortamis, Anastasios Livanidis and Georgios Lydakis.

The project showcases metaprogramming capabilities including runtime code modifications and dynamic execution of code strings, along with a functional visual debugger that supports breakpoints and dynamic code/variable inspection.

Notable language features

The A++ language is a variant of the Delta programming language, developed in ICS-FORTH. The syntax is in general similar to Javascript, with the addition of metaprogramming features that allow dynamic changes to the code being executed. The following metaprogramming features are available:

• eval("string"): parses and executes strings as code.

  Example:

  eval("print(2 + 3);");
  

  Prints 5.

• <<expr>> (quasi-quotes): returns the enclosed expression as a syntax tree.

  Example:

  tree = <<x = 1; y = 2; print(x + y);>>;
  

  Produces a syntax tree whose execution would assign the variables x, y and print "3".

• ~(expr) (escape): evaluates the expression normally and inserts its value in the syntax tree.

  Example: if z = 9 then:

  tree = <<x = ~(sqrt(z)); y = 2; print(x + y);>>;
  

  returns a syntax tree whose execution would assign the variables x, y and print "5". Only usable inside inside quasi-quotes.

• !(syntax_tree) (inline): executes a syntax tree (usually created with quasi-quotes). Any of the two example trees from above can be executed by !(tree).

• @("string") (compile string): parses a string into a syntax tree. Similar to eval, except for the fact that the code is not immediately executed..

  Example:

  tree = @("print(2 + 3);");
  !(tree);
  

  Prints 5, i.e., has the same effect as eval("print(2 + 3);").

• #(syntax_tree) (unparse): converts a syntax tree to source code.

  Example:

  z = <<x = 1; y = 2; print(x + y);>>;
  w = #(z);
  

  w is assigned the string "x = 1;\n y = 2\n; print(x + y);". Useful for debugging meta-programs by showing the generated code.

The metaprogrammingTests directory contains several examples for these features: each .asc file contains A++ code, and the corresponding .res file contains the expected output of the program.

Installation & usage

You'll need the gtkmm3 library in order to use the visual debugger. Installation instructions can be found here.

After installing gktmm3, from the project root, run

make -C src release

This will create a directory named build next to src, containing three executables:

• app.out <sourceFile>: the A++ interpreter.
• appGUIDebug.out <sourceFile>: the visual debugger for A++.
• appCLIDebug.out <sourceFile>: a simple CLI debugger; functional but less refined than the visual debugger.