NyanFlip - An extension of BackFlip by Peter Ward (@flowblok). An example program is attached.

nyanflip.py

#!/usr/bin/env python
# NyanFlip - An extension of BackFlip by Peter Ward (flowblok).
# Implemented by David Vo (auscompgeek) and Jonathan Grigg.

import sys

ARROWS = '<V^>'
LEFT, DOWN, UP, RIGHT = ARROWS

def opp(direction):
	if direction == UP:
		return DOWN
	if direction == DOWN:
		return UP
	if direction == LEFT:
		return RIGHT
	if direction == RIGHT:
		return LEFT

def nyanflip(grid):
	height = len(grid)
	width = len(grid[0])

	x, y = 0, 0
	direction = RIGHT
	score = 0

	while (0 <= x < width) and (0 <= y < height):
		tile = grid[y][x]

		if tile in ARROWS:
			grid[y][x], direction = opp(direction), tile

		elif tile == '\\':
			if direction == DOWN:
				direction = RIGHT
			elif direction == UP:
				direction = LEFT
			elif direction == RIGHT:
				direction = DOWN
			elif direction == LEFT:
				direction = UP

			grid[y][x] = '/'

		elif tile == '/':
			if direction == DOWN:
				direction = LEFT
			elif direction == UP:
				direction = RIGHT
			elif direction == RIGHT:
				direction = UP
			elif direction == LEFT:
				direction = DOWN

			grid[y][x] = '\\'

		elif tile == '+':
			score += 1
		elif tile == '-':
			score -= 1
		elif tile == '.':
			print(score)
		elif tile == ',':
			sys.stdout.write(chr(score))

		if direction == UP:
			y -= 1
		elif direction == DOWN:
			y += 1
		elif direction == LEFT:
			x -= 1
		elif direction == RIGHT:
			x += 1

def main():
	if len(sys.argv) > 1:
		with open(sys.argv[1]) as f:
			lines = f.read().split('\n\n')[0].split('\n')
	else:
		read = raw_input if sys.version_info[0] < 3 else input
		try:
			lines, line = [], read()
			while line:
				lines.append(line)
				line = read()
		except EOFError:
			pass

	nyanflip([list(x) for x in lines])

if __name__ == '__main__':
	main()

nyanflip.txt

 \++.+\\ \V         
+ >++ <\ V+>\+.V    
    +VV^  .-   \ V  
    >\ +<+ -   ^-\-<
    >\\<> -\        
>\   ^    >^-    -  
 ^    ^  ^  >   -<  

Version history

This interpreter

v0: initial release
v1: added README and * checkpoint
v2: refactor to allow stepping through the program
v3: added ~ checkpoint

The language

v-1: BackFlip
v0: as described for NCSS 2014 ProgComp
v1: added , checkpoint
v2: added * checkpoint
v3: added ~ checkpoint

read = raw_input if sys.version_info[0] < 3 else input

Many enterprise, such wow.

Apparently this was recycled for this year's NCSS ProgComp. Well.