def scheme_read(src):
"""Read the next expression from SRC, a Buffer of tokens.
>>> lines = ["(+ 1 ", "(+ 23 4)) ("]
>>> src = Buffer(tokenize_lines(lines))
>>> print(scheme_read(src))
(+ 1 (+ 23 4))
>>> read_line("'hello")
Pair('quote', Pair('hello', nil))
>>> print(read_line("(car '(1 2))"))
(car (quote (1 2)))
"""
if src.current() is None:
raise EOFError
val = src.pop()
if val == "nil":
return nil
elif type(val) is int or type(val) is float:
return scnum(val)
elif type(val) is bool:
return scbool(val)
elif val not in DELIMITERS:
if val[0] == '"':
return scstr(eval(val))
else:
return intern(val)
elif val == "'":
return Pair(intern('quote'), Pair(scheme_read(src), nil))
elif val == "(":
return read_tail(src)
else:
raise SyntaxError("unexpected token: {0}".format(val))
def read_tail(src):
"""Return the remainder of a list in SRC, starting before an element or ).
>>> read_tail(Buffer(tokenize_lines([")"])))
nil
>>> read_tail(Buffer(tokenize_lines(["2 3)"])))
Pair(2, Pair(3, nil))
>>> read_tail(Buffer(tokenize_lines(["2 (3 4))"])))
Pair(2, Pair(Pair(3, Pair(4, nil)), nil))
>>> read_line("(1 . 2)")
Pair(1, 2)
>>> read_line("(1 2 . 3)")
Pair(1, Pair(2, 3))
>>> read_line("(1 . 2 3)")
Traceback (most recent call last):
...
SyntaxError: Expected one element after .
>>> scheme_read(Buffer(tokenize_lines(["(1", "2 .", "'(3 4))", "4"])))
Pair(1, Pair(2, Pair('quote', Pair(Pair(3, Pair(4, nil)), nil))))
"'"
"""
try:
if src.current() is None:
raise SyntaxError("unexpected end of file")
if src.current() == ")":
src.pop()
return nil
"*** YOUR CODE HERE ***"
first = scheme_read(src)
rest = read_tail(src)
return Pair(first, rest)
except EOFError:
raise SyntaxError("unexpected end of file")
def read_tail(src):
"""Return the remainder of a list in SRC, starting before an element or ).
>>> read_tail(Buffer(tokenize_lines([")"])))
nil
>>> read_tail(Buffer(tokenize_lines(["2 3)"])))
Pair(2, Pair(3, nil))
>>> read_tail(Buffer(tokenize_lines(["2 (3 4))"])))
Pair(2, Pair(Pair(3, Pair(4, nil)), nil))
>>> read_line("(1 . 2)")
Pair(1, 2)
>>> read_line("(1 . (2 3))")
Pair(1, Pair(2, Pair(3, nil)))
>>> read_line("(1 2 . 3)")
Pair(1, Pair(2, 3))
>>> read_line("(1 . 2 3)")
Traceback (most recent call last):
...
SyntaxError: Expected one element after .
>>> read_line("(2 (3 . 4) 5)")
Pair(2, Pair(Pair(3, 4), Pair(5, nil)))
>>> scheme_read(Buffer(tokenize_lines(["(1", "2 .", "'(3 4))", "4"])))
Pair(1, Pair(2, Pair('quote', Pair(Pair(3, Pair(4, nil)), nil))))
"'"
"""
try:
if src.current() is None:
raise SyntaxError("uexpected end of file")
# base case for recursion on lists of Pairs. Last element is "nil"
if src.current() == ")":
src.pop()
return nil
"*** YOUR CODE HERE ***"
if src.current() == ".":
src.pop()
# recursivly get second element
second_element = scheme_read(src)
# now we should be almost done if there is no third element but
# the closing bracket
if src.current() is not ')':
raise SyntaxError("Expected one element after .")
# "eat" the ')'
src.pop()
return second_element
first = scheme_read(src)
rest = read_tail(src)
return Pair(first, rest)
except EOFError:
raise SyntaxError("unexpected end of file")
def scheme_read(src):
# import pdb; pdb.set_trace()
"""Read the next expression from SRC, a Buffer of tokens.
>>> lines = ["(+ 1 ", "(+ 23 4)) ("]
>>> src = Buffer(tokenize_lines(lines))
>>> print(scheme_read(src))
(+ 1 (+ 23 4))
>>> read_line("(+ 1 2)")
Pair('+', Pair(1, Pair(2, nil)))
>>> read_line("'hello")
Pair('quote', Pair('hello', nil))
>>> print(read_line("(car '(1 2))"))
(car (quote (1 2)))
"""
if src.current() is None:
raise EOFError
val = src.pop()
if val == "nil":
return nil
elif type(val) is int or type(val) is float:
return scnum(val)
elif type(val) is bool:
return scbool(val)
# self-evaluating
elif val not in DELIMITERS:
if val[0] == '"':
return scstr(eval(val))
else:
return intern(val)
elif val == "'":
"*** YOUR CODE HERE ***"
# nested Pair eventually matches the required structure
rest = scheme_read(src)
return Pair("quote", Pair(rest, nil))
elif val == "(":
return read_tail(src)
else:
raise SyntaxError("unexpected token: {0}".format(val))