A lisp REPL interpreter made in Haskell.

Language Features

• Numbers (Float)
• Addition (+)
• Subtraction (-)
• Booleans (true, false)
• Comparison operators (=, >, <, >=, <=)
• def
• if
• lambdas

How to run

With Docker

Just run:

docker build . -t hisp
docker run -it hisp

Observation: the build tools of Haskell themselves seem to be very heavy, I've got a 1GB Docker image on my computer just with a 3 line Dockerfile 😨

Without Docker

hisp can be compiled/executed using either using stack or just by using ghc.

Using Stack

You will need stack installed on your computer, which is a tool for developing Haskell projects.

To compile and run:

stack run

Using just GHC

You will need ghc installed on your computer, which is the Haskell compiler.

This repository has a shell script (already with chmod +x, to run like a binary) called compile_and_run.sh. Just run it like this:

./compile_and_run.sh

Project structure

Since hisp is built with stack, the folders follow the standard of it.

hisp
│   README.md
│   ...
└─── app
│   └─── Main.hs
└─── src
│   └─── ...
└─── test
    └─── ...

• The app folder contains the main function that starts the REPL.
• The src folder contains the code that is consumed by the main function, so it is much like a library folder.
• The test folder, well, as the name suggests, it has the code for the tests.

Tests

Tests are run using stack, you should install it first. To run them, just use:

stack test

Lint

To run the linter, you will have to install first hlint. To check if everything is correct:

hlint .

How it works

Well it has 3 main steps, like many interpreters:

1. Tokenize

Considering someone wants to see the result of the following lisp code:

First we take it as raw input (String) and divide it into tokens, which is a list of values ([String]):

2. Parse

Now that we have those tokens, we can convert them to something meaningful to us, that we can interpret.

enum Expression {
  Symbol(String),
  Number(Float),
  List([Expression])
  Function(([Expression]) -> Expression),
}

The code above is a pseudocode with a Rust-like syntax. The idea is that you can have a value that is either a Symbol (holding a String), a Number (holding a Float), a List (holding a list of Expression) or a Function (holding a function that maps a list of Expression to a single Expression).

let four = Expression::Number(4.0);
let plus_sign = Expression::Symbol("+");

3. Evaluate

After we have the values that represent the code we need to execute, we will interpret it!

To actually get to that 5.0 in the end, we must have somewhere defined that + receives a bunch of Numbers and then sum them. We could have an Environment that contains the standard functions/operators.

struct Environment {
  data: HashMap<String, Expression>// map of keys and values
}

Imagine this like a class that has a map of things like +, -, =, ... to values like Expression::Function(implementation).

function sum_implementation (values) {
  return values.reduce((acc, curr) => acc + curr, 0)
}

environment = {
  "+": sum_implementation,
  "-": subtract_implementation,
  ...
}

Above there is a JSON/JavaScript-like visualization.

That is it! It was just an overview, but you can see all of this on this repository, just a bit more complex to make things like defs, ifs and lambdas available to the language.

If you really want to understand more, you can follow some tutorials that are below 🙂

Notes

This project is heavly inspired by:

• Rust Lisp REPL interpreter
• Python Lisp REPL interpreter

Also, I don't really know Haskell well, so the code probably could be a lot better. I did my best with what I know about the language at the time.

License

You can check out the full license here

This project is licensed under the terms of the WTFPL license. You just DO WHAT THE FUCK YOU WANT TO.