Negell

A Haskell Negatron interpreter

Negatron

Negatron is a somewhat esoteric language from my compilers class that is essentially a subset of C except it doesn't support addition or support the use of true. Instead of defining the full spec of the language here, I will instead give a few of the important features of Negatron. Negatron is statically typed, statically scoped, has pass by value semantics, and only supports the types of int and bool for variables (output statements accept strings enclosed in double quotes e.g. "Hello, world"). Negatron programs start with an invocation of a main function. Additionally, Negatron has builtins for reading from and writing to the console thorough output and input respectively.

Example Negatron Program

Below is Hello World in Negatron, for more examples look at the .nega files in the tests folder.

void main() {
    output "Hello, world!";
}

The Interpreter

Negell itself uses Alex and Happy to lex and parse input programs into an AST. In the future this may be swapped out for something like parsec, but I initially wanted to focus more on the evaluation phase of the interpreter and I was already familiar with flex and bison so Alex and Happy were easy to pick up. Evaluation is accomplished by a single pass over the AST initially starting with the main function and collecting state changes from there.

Usage

This project uses cabal for project management. TL;DR is use cabal run to build and run Negell against test.nega and cabal build to only build the Negell binary.

Running Negell

Currently Negell optionally takes one command line argument which is the path of the Negatron source file to execute, if no filename is provided it's assumed to be test.nega.

Testing Negell

Testing is currently accomplished by using a built version of Negell and comparing program output with expected output. To run the full suite of tests use the included shell script by running ./test.sh.

Known Issues and Todos

• There is little semantic checking of any kind. Badly typed or named programs may either crash or have undefined behavior
• Eval needs to be cleaned up as it's currently very messy
• Remove use of unsafePerformIO in Eval and instead trickle IO back up to main
• Tests are currently run outside of cabal and it would be nice if they used cabal test instead