Braid

Learning to build a language interpreter with RPython and RPly

I don't really know what I'm doing but I'm interested in writing a toy language and interpreter. I've chosen RPython and RPly because I know Python quite well and the RPython EBNF parsing libs were confusing. RPly's interface is a bit higher level.

This project is no longer being actively maintained.

Installing

pip install -r requirements.txt

Running

python braid.py for REPL, python braid.py [filename].bd for interpreting a file

:a gives you the AST of the last statement, :e to list environment variables, :q or Ctrl-C to quit. The REPL now supports multi-line input too — it'll just keep appending code and trying to interpret it until it's valid (eg. you closed the block or whatever), or you break it ;)

Status

Basic arithmetic, floats, integers, booleans, and strings, variable assignment, if expressions, and a print() function.

>>> 5 == 5
= true
>>> 5 != 5
= false
>>> let a = 5
= 5
>>> print(a)
5
>>> print(a + 25)
30
>>> "hi" + 'hi'
= hihi
>>> "hi" * 5 - 1
= hihihihih

# if expressions
>>> if false: print("no") else: print("yes") end
yes
>>> let a = (if true: 1 else: 5 end)
= 1

let a = 50
if a == 50 and true:
  print("doing stuff")
else:
  print("not this though")
end

>>> 5 >= 6
= false

# assignment via if
>>> let a = if true: 5 end
= 5
>>> :a
Program(BinaryOp(Variable('a'), If(Boolean(True))Then(Integer(5))Else(None)))

# arrays
>>> [5, 6, ["hi", 7.0]]
= [5, 6, [hi, 7.0]]

# functions
func a(b):
 b + 1
end

>>> b(1)
= 2

# immutability means loops become recursion
func p_message(msg, n):
  if n > 0:
    print(msg)
    p_message(msg, n - 1)
  end
end

>>> p_message("hellooo",2)
hellooo
hellooo

# functions can be passed around
func a():
  1
end

>>> let b = a
>>> b()
= 1

Compiling

You will need pypy so you can use RPython's compiler. Then, like so:

python path/to/rpython/bin/rpython target.py

This will provide a target-c binary which you can use as a compiled substitute for main.py.

Goals

A language which can do things I find interesting, and the tools necessary to execute it.

• Define the language (ongoing)
• Lexer
• Parser
• Bytecode compiler
• Interpreter/VM
• Compiles to RPython (mostly but sometimes broken)
• JIT
• Immutability (initial support anyway)
• First-class functions (sort of)
• Structs and traits
• FP concepts like map/reduce
• Pattern matching
• Concurrency via message passing
• Standard library

Status updates

I've abandoned this as I'm no longer interested in building a language on RPython. You can follow me on twitter at @joshsharp if you're interested in the other stuff I'm working on.