scheme_reader.py

"""This module implements the built-in data types of the Scheme language, along
with a parser for Scheme expressions.

In addition to the types defined in this file, some data types in Scheme are
represented by their corresponding type in Python:
    number:       int or float
    symbol:       string
    boolean:      bool
    unspecified:  None

The __repr__ method of a Scheme value will return a Python expression that
would be evaluated to the value, where possible.

The __str__ method of a Scheme value will return a Scheme expression that
would be read to the value, where possible.
"""

from ucb import main, trace, interact
from scheme_tokens import tokenize_lines, DELIMITERS
from buffer import Buffer, InputReader, LineReader

# Pairs and Scheme lists

class Pair(object):
    """A pair has two instance attributes: first and second.  For a Pair to be
    a well-formed list, second is either a well-formed list or nil.  Some
    methods only apply to well-formed lists.

    >>> s = Pair(1, Pair(2, nil))
    >>> s
    Pair(1, Pair(2, nil))
    >>> print(s)
    (1 2)
    >>> print(s.map(lambda x: x+4))
    (5 6)
    """
    def __init__(self, first, second):
        self.first = first
        self.second = second

    def __repr__(self):
        return "Pair({0}, {1})".format(repr(self.first), repr(self.second))

    def __str__(self):
        s = "(" + str(self.first)
        second = self.second
        while isinstance(second, Pair):
            s += " " + str(second.first)
            second = second.second
        if second is not nil:
            s += " . " + str(second)
        return s + ")"

    def __len__(self):
        n, second = 1, self.second
        while isinstance(second, Pair):
            n += 1
            second = second.second
        if second is not nil:
            raise TypeError("length attempted on improper list")
        return n

    def __eq__(self, p):
        if not isinstance(p, Pair):
            return False
        return self.first == p.first and self.second == p.second

    def map(self, fn):
        """Return a Scheme list after mapping Python function FN to SELF."""
        mapped = fn(self.first)
        if self.second is nil or isinstance(self.second, Pair):
            return Pair(mapped, self.second.map(fn))
        else:
            raise TypeError("ill-formed list")

class nil(object):
    """The empty list"""

    def __repr__(self):
        return "nil"

    def __str__(self):
        return "()"

    def __len__(self):
        return 0

    def map(self, fn):
        return self

nil = nil() # Assignment hides the nil class; there is only one instance

# Scheme list parser


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 val not in DELIMITERS:
        return val
    elif val == "'":
        # BEGIN Question 1
        return Pair('quote', Pair(scheme_read(src),nil))
        # END Question 1
    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")
        elif src.current() == ")":
            src.pop()
            return nil
        elif src.current() == ".":
            # BEGIN Question 2
            src.pop()
            temp = scheme_read(src)
            if src.pop()  == ")":
                return temp
            else:
                raise SyntaxError("Expected one element after .")
            # END Question 2
        else:
            first = scheme_read(src)
            rest = read_tail(src)
            return Pair(first, rest)
    except EOFError:
        raise SyntaxError("unexpected end of file")

# Convenience methods

def buffer_input(prompt="scm> "):
    """Return a Buffer instance containing interactive input."""
    return Buffer(tokenize_lines(InputReader(prompt)))

def buffer_lines(lines, prompt="scm> ", show_prompt=False):
    """Return a Buffer instance iterating through LINES."""
    if show_prompt:
        input_lines = lines
    else:
        input_lines = LineReader(lines, prompt)
    return Buffer(tokenize_lines(input_lines))

def read_line(line):
    """Read a single string LINE as a Scheme expression."""
    return scheme_read(Buffer(tokenize_lines([line])))

# Interactive loop

@main
def read_print_loop():
    """Run a read-print loop for Scheme expressions."""
    while True:
        try:
            src = buffer_input("read> ")
            while src.more_on_line:
                expression = scheme_read(src)
                print("str :", expression)
                print("repr:", repr(expression))
        except (SyntaxError, ValueError) as err:
            print(type(err).__name__ + ":", err)
        except (KeyboardInterrupt, EOFError):  # <Control>-D, etc.
            print()
            return