Yabil

Yet Another Byte-code Interpreted Language written in C. Yabil is a Lox clone followed from the book Crafting Interpreters by Robert Nystrom with a few extra features. Aside from dynamic typing, classes, higher order functions and closures, basic control flow and string interning, Yabil also supports the following features:

• Dynamic array objects
• Array indexing to get and set
• Array concatenation using the '+' operator
  • [1, 2, 3] + [4, 5] results in [1, 2, 3, 4, 5]
  • [1, 2, 3] + arbitrary_val results in [1, 2, 3, arbitrary_val]
• String indexing to get and set
• Javascript style object field access through string, i.e., obj["field"]
• Ternary operator

Furthermore, Yabil scripts are parsed using a Pratt top-down parser and compiled to a byte-code representation. The Virtual Machine executes the byte-code by which the internal stack gets manipulated. This stack-based approach is very easy to implement and understand.

Interesting concepts

An interesting notion of this C application is the concept of NaN-boxing, where the advantage of NaNs (Not a Number) of the IEEE 754 floating point standard is used to encode more types of values inside a single double type. To encode a value as a NaN value, consider the following:

                         quit bit and Intel bit
                         v v
|?|1|1|1|1|1|1|1|1|1|1|1|1|1|?|?|?|?|...|?|?|?|
   ^^^^^^^^^^^^^^^^^^^^^     ^^^^^^^^^^^^^^^^^
          NaN bits                 50 bits to
 ^                                  encode
 1 bit                        
 to encode

This method allows not only the encoding of every possible 64-bit double precision floating point number, but also the boolean true/false values, nil and generic C pointers that point to values on the heap (because pointers in C are 64 bit values but only contain 48 bits of memory, which fits our total of 51 bits to encode).

Quick Start

$ make
$ ./yabil <filename>

Provide filename to execute script or run without filename to run the REPL (Run Execute Print Loop) environment