craftr-dsl

The Craftr DSL is a transpiler for the Python language that introduces the concept of closures an function calls without parentheses into the language.

Getting started

Installation

From Pip:

$ pip install craftr-dsl

Latest from GitHub:

$ pip install git+https://github.com/craftr-build/craftr-dsl

Requirements: Python 3.8 or newer

Hello, World!

A convoluted Hello, World! example in Craftr DSL might look like this:

# hello.craftr
world = { self('World!') }
world { print('Hello,', self) }

This is transpiled to

# $ python -m craftr.dsl hello.craftr -E | grep -v -e '^$'
def _closure_1(self, *arguments, **kwarguments):
    self('World!')
world = _closure_1
def _closure_2(self, *arguments, **kwarguments):
    print('Hello,', self)
world(_closure_2)

And evaluates to

# $ python -m craftr.dsl hello.craftr
Hello, World!

Language features

Closures

Closures are formed with the following syntax: [ arg -> | (arg1, arg2, ...) -> ] { body }. A closure without an argument list automatically has the signature (self, *argnames, **kwargnames).

filter({ self % 2 }, range(5))

def _closure_1(self, *argnames, **kwargnames):
    self % 2
filter(_closure_1, range(5))

filter(x -> x % 2, range(5))

def _closure_1(x):
    return x % 2
filter(_closure_1, range(5))

reduce((a, b) -> {
  a.append(b * 2)
  return a
}, [1, 2, 3], [])

def _closure_1(a, b):
    a.append(b * 2)
    return a
reduce(_closure_1, [1, 2, 3], [])

Function calls without parentheses

Such function calls are only supported at the statement level. A function can be called without parentheses by simply omitting them. Variadic and keyword arguments are supported as expected. Applying a closure on an object is basically the same as calling that object with the function, and arguments following the closure are still supported.

print 'Hello, World!', file=sys.stderr

print('Hello, World!', file=sys.stderr)

map {
  print('Hello,', self)
}, ['John', 'World']

def _closure_1(self):
    print('Hello,', self)
map(_closure_1, ['John', 'World'])

list(map {  # Not allowed inside an expression
  print('Hello,', self)
}, ['John', 'World'])

craftr.dsl.rewrite.SyntaxError: 
  in <stdin> at line 1: expected ) but got TokenProxy(Token(type=<Token.Control: 8>, value='{', pos=Cursor(offset=9, line=1, column=9)))
  |list(map {
  |~~~~~~~~~^

Limitations

Craftr DSL is intended to behave as a complete syntactic superset of standard Python. However there are currently some limitations, namely:

• Literal sets cannot be expressed due to the grammar conflict with parameter-less closures
• Type annotations are not currently supported
• The walrus operator is not currently supported

Copyright © 2021 Niklas Rosenstein