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_valresults 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.
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).
$ make
$ ./yabil <filename>
Provide filename to execute script or run without filename to run the REPL (Run Execute Print Loop) environment