chapter2.py

#!/usr/bin/python3

line = ""
cursor = 0 # Index of current reading position
token = None # The last token matched, if any

def skip_whitespace():
	global cursor
	while cursor < len(line) and line[cursor].isspace():
		cursor += 1

def match_keyword():
	global cursor, token
	
	skip_whitespace()
	if cursor >= len(line) or not line[cursor].isalpha():
		return False
		
	mark = cursor
	while cursor < len(line) and line[cursor].isalpha():
		cursor += 1
	token = line[mark:cursor]
	return True

def match_number():
	global cursor, token
	
	skip_whitespace()
	if cursor >= len(line) or not line[cursor].isdigit():
		return False
		
	mark = cursor
	while cursor < len(line) and line[cursor].isdigit():
		cursor += 1
	token = line[mark:cursor]
	return True

def match_varname():
	global cursor, token
	
	skip_whitespace()
	if cursor >= len(line) or not line[cursor].isalpha():
		return False
		
	mark = cursor
	while cursor < len(line) and line[cursor].isalnum():
		cursor += 1
	token = line[mark:cursor]
	return True

def match_string():
	global cursor, token
	
	skip_whitespace()
	if cursor >= len(line) or line[cursor] != '"':
		return False
		
	mark = cursor
	cursor += 1 # Skip the opening double quote.
	while line[cursor] != '"':
		cursor += 1
		if cursor >= len(line):
			raise IndexError("Unclosed string")
	cursor += 1 # Skip the closing double quote.
	
	# Save string value without the double quotes.
	token = line[mark + 1:cursor - 1]
	return True

def parse_statement():
	if not match_keyword():
		raise SyntaxError("Statement expected")
	
	stmt = token.lower()
		
	if stmt == "let":
		parse_let()
	elif stmt == "print":
		parse_print()
	else:
		raise SyntaxError("Unknown statement")

variables = {}

def parse_let():
	if not match_varname():
		raise SyntaxError("Variable expected")
		
	var_name = token.lower()
	
	if not match("="):
		raise SyntaxError("'=' expected")
	elif not match_number():
		raise SyntaxError("Variable expected")
	elif not match_eol():
		raise SyntaxError("End of line expected")
		
	variables[var_name] = int(token)

def match(text):
	global cursor
	skip_whitespace()
	if line.startswith(text, cursor):
		cursor += len(text)
		return True
	else:
		return False

def match_eol():
	skip_whitespace()
	return cursor >= len(line)

def parse_print():
	if match_eol():
		print()
		return
	value = str(parse_value())
	while match(","):
		value += str(parse_value())
	print(value)
	if not match_eol():
		raise SyntaxError("End of line expected")

def parse_value():
	global token
	if match_string():
		return token
	elif match_number():
		return int(token)
	elif match_varname():
		token = token.lower()
		if token in variables:
			return variables[token]
		else:
			raise NameError("Var not found")
	else:
		raise SyntaxError("Value expected")

#line = "let a = 42"
#cursor = 0
#parse_statement()
#line = 'print "a = ", a'
#cursor = 0
#parse_statement()

print("Basic v0.1 READY\n")
done = False
while not done:
	line = input("> ")
	if line.lower() == "bye":
		done = True
	else:
		cursor = 0
		try:
			parse_statement()
		except Exception as e:
			print(str(e) +
				" in column " +
				str(cursor))