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How to write a programming language
Part 2: The Parser
Andy Balaamartificialworlds.net/blog
This work is licensed under a
Creative Commons
foo = "bar";
becomes:
("symbol", "foo")
("=" , "" )
("string", "bar")
(";" , "" )
print( x + 2 );
becomes:
("symbol" , "print")
("(" , "" )
("symbol" , "x" )
("operator", "+" )
("number" , "2" )
(")" , "" )
(";" , "" )
x = 3 + 4;
becomes:
Assignment:
Symbol: x
Value:
Operation:
Type: +
Arguments:
3
4
print( x + 2 );
becomes:
FunctionCall:
Function: print
Arguments:
Operation:
Type: +
Arguments:
x
2
print( x + 2 );
becomes:
("call",
("symbol", "print"),
[
("operation",
"+",
("symbol", "x"),
("number", "2")
)
]
)
Cell has four expression types:
The coolest thing about Cell is:
More at: github.com/andybalaam/cell
if(
is_even( 2 ),
{ print "Even!"; },
{ print "Odd."; }
);
More at: github.com/andybalaam/cell
def parse(tokens):
parser = Parser(tokens, ";")
while parser.tokens.next is not None:
p = parser.next_expr(None)
if p is not None:
yield p
parser.tokens.move_next()
class Parser:
def __init__(self, tokens, stop_at):
self.tokens = tokens
self.stop_at = stop_at
def next_expr(self, prev):
self.fail_if_at_end(";")
typ, value = self.tokens.next
if typ in self.stop_at:
return prev
self.tokens.move_next()
# ...
def next_expr(self, prev):
# ...
if prev is None and typ in (# ...
elif typ == "operation": # ...
elif typ == "(": # ...
elif typ == "{": # ...
elif typ == "=": # ...
else: # ...
def next_expr(self, prev):
# ...
if prev is None and typ in (
"number", "string", "symbol"):
return self.next_expr((typ, value))
elif typ == "operation": # ...
elif typ == "(": # ...
elif typ == "{": # ...
elif typ == "=": # ...
else: # ...
def next_expr(self, prev):
# ...
if prev is None and typ in (...
elif typ == "operation":
nxt = self.next_expr(None)
return self.next_expr(
("operation", value, prev, nxt))
elif typ == "(": # ...
elif typ == "{": # ...
elif typ == "=": # ...
else: # ...
def next_expr(self, prev):
# ...
if prev is None and typ in (...
elif typ == "operation": # ...
elif typ == "(":
args = self.multi_exprs(",", ")")
return self.next_expr(("call", prev, args))
elif typ == "{": # ...
elif typ == "=": # ...
else: # ...
def multi_exprs(self, sep, end):
ret = []
self.fail_if_at_end(end)
typ = self.tokens.next[0]
if typ == end:
self.tokens.move_next()
else:
# ...
return ret
def multi_exprs(self, sep, end):
# ...
else:
arg_parser = Parser(self.tokens, (sep, end))
while typ != end:
p = arg_parser.next_expr(None)
if p is not None:
ret.append(p)
typ = self.tokens.next[0]
self.tokens.move_next()
self.fail_if_at_end(end)
return ret
def next_expr(self, prev):
# ...
elif typ == "(": # ...
elif typ == "{":
params = self.parameters_list()
body = self.multi_exprs(";", "}")
return self.next_expr(("function", params, body))
elif typ == "=": # ...
else: # ...
def next_expr(self, prev):
# ...
elif typ == "(": # ...
elif typ == "{": # ...
elif typ == "=":
if prev[0] != "symbol":
raise Exception(...)
nxt = self.next_expr(None)
return self.next_expr(("assignment", prev, nxt))
else: # ...
def next_expr(self, prev):
# ...
elif typ == "(": # ...
elif typ == "{": # ...
elif typ == "=": # ...
else:
raise Exception("Unexpected token: " ...
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