"""This module implements the core Scheme interpreter functions, including the eval/apply mutual recurrence, environment model, and read-eval-print loop. """ from scheme_primitives import * from scheme_reader import * from ucb import main, trace ############## # Eval/Apply # ############## def scheme_eval(expr, env): """Evaluate Scheme expression EXPR in environment ENV. If ENV is None, simply returns EXPR as its value without further evaluation. >>> expr = read_line("(+ 2 2)") >>> expr Pair('+', Pair(2, Pair(2, nil))) >>> scheme_eval(expr, create_global_frame()) scnum(4) """ while env is not None: # Note: until extra-credit problem 22 is complete, env will # always be None on the second iteration of the loop, so that # the value of EXPR is returned at that point. if expr is None: raise SchemeError("Cannot evaluate an undefined expression.") # Evaluate Atoms if scheme_symbolp(expr): expr, env = env.lookup(expr).get_actual_value(), None elif scheme_atomp(expr): env = None # All non-atomic expressions are lists. elif not scheme_listp(expr): raise SchemeError("malformed list: {0}".format(str(expr))) else: first, rest = scheme_car(expr), scheme_cdr(expr) # Evaluate Combinations if (scheme_symbolp(first) # first might be unhashable and first in SPECIAL_FORMS): if proper_tail_recursion: "*** YOUR CODE HERE ***" else: expr, env = SPECIAL_FORMS[first](rest, env) expr, env = scheme_eval(expr, env), None else: procedure = scheme_eval(first, env) args = procedure.evaluate_arguments(rest, env) if proper_tail_recursion: "*** YOUR CODE HERE ***" else: # UPDATED 4/14/2014 @ 19:08 expr, env = scheme_apply(procedure, args, env), None return expr proper_tail_recursion = False ################################################################ # Uncomment the following line to apply tail call optimization # ################################################################ # proper_tail_recursion = True def scheme_apply(procedure, args, env): """Apply PROCEDURE (type Procedure) to argument values ARGS in environment ENV. Returns the resulting Scheme value.""" # UPDATED 4/14/2014 @ 19:08 # Since .apply is allowed to do a partial evaluation, we finish up # with a call to scheme_eval to complete the evaluation. scheme_eval # will simply return expr if its env argument is None. expr, env = procedure.apply(args, env) return scheme_eval(expr, env) ################ # Environments # ################ class Frame: """An environment frame binds Scheme symbols to Scheme values.""" def __init__(self, parent): """An empty frame with a PARENT frame (that may be None).""" self.bindings = {} self.parent = parent def __repr__(self): if self.parent is None: return "<Global Frame>" else: s = sorted('{0}: {1}'.format(k,v) for k,v in self.bindings.items()) return "<{{{0}}} -> {1}>".format(', '.join(s), repr(self.parent)) def __eq__(self, other): return isinstance(other, Frame) and \ self.parent == other.parent def lookup(self, symbol): """Return the value bound to SYMBOL. Errors if SYMBOL is not found. As a convenience, also accepts Python strings, which it turns into symbols.""" if type(symbol) is str: symbol = intern(symbol) "*** YOUR CODE HERE ***" raise SchemeError("unknown identifier: {0}".format(str(symbol))) def global_frame(self): """The global environment at the root of the parent chain.""" e = self while e.parent is not None: e = e.parent return e def make_call_frame(self, formals, vals): """Return a new local frame whose parent is SELF, in which the symbols in the Scheme formal parameter list FORMALS are bound to the Scheme values in the Scheme value list VALS. Raise an error if too many or too few arguments are given. >>> env = create_global_frame() >>> formals, vals = read_line("(a b c)"), read_line("(1 2 3)") >>> env.make_call_frame(formals, vals) <{a: 1, b: 2, c: 3} -> <Global Frame>> """ frame = Frame(self) "*** YOUR CODE HERE ***" return frame def define(self, sym, val): """Define Scheme symbol SYM to have value VAL in SELF. As a convenience, SYM may be Python string, which is converted first to a Scheme symbol. VAL must be a SchemeValue.""" assert isinstance(val, SchemeValue), "values must be SchemeValues" if type(sym) is str: sym = intern(sym) self.bindings[sym] = val ##################### # Procedures # ##################### class Procedure(SchemeValue): """The superclass of all kinds of procedure in Scheme.""" # Arcane Technical Note: The odd placement of the import from scheme in # evaluate_arguments is necessary because it introduces mutually recursive # imports between this file and scheme.py. The effect of putting it # here is that we delay attempting to access scheme.scheme_eval until # after the scheme module's initialization is finished. def evaluate_arguments(self, arg_list, env): """Evaluate the expressions in ARG_LIST in ENV to produce arguments for this procedure. Default definition for procedures.""" from scheme import scheme_eval return arg_list.map(lambda operand: scheme_eval(operand, env)) class PrimitiveProcedure(Procedure): """A Scheme procedure defined as a Python function.""" def __init__(self, fn, use_env=False): self.fn = fn self.use_env = use_env def __str__(self): return '#[primitive]' def __repr__(self): return "PrimitiveProcedure({})".format(str(self)) def apply(self, args, env): """Apply a primitive procedure to ARGS in ENV. Returns a pair (val, None), where val is the resulting value. >>> twos = Pair(SchemeInt(2), Pair(SchemeInt(2), nil)) >>> plus = PrimitiveProcedure(scheme_add, False) >>> plus.apply(twos, None) (scnum(4), None) """ "*** YOUR CODE HERE ***" class LambdaProcedure(Procedure): """A procedure defined by a lambda expression or the complex define form.""" def __init__(self, formals, body, env = None): """A procedure whose formal parameter list is FORMALS (a Scheme list), whose body is the single Scheme expression BODY, and whose parent environment is the Frame ENV. A lambda expression containing multiple expressions, such as (lambda (x) (display x) (+ x 1)) can be handled by using (begin (display x) (+ x 1)) as the body.""" self.formals = formals self.body = body self.env = env def _symbol(self): return 'lambda' def __str__(self): # UPDATED 4/16/2014 @ 13:20 return "({0} {1} {2})".format(self._symbol(), str(self.formals), str(self.body)) def __repr__(self): args = (self.formals, self.body, self.env) return "{0}Procedure({1}, {2}, {3})".format(self._symbol().capitalize(), *(repr(a) for a in args)) def __eq__(self, other): return type(other) is type(self) and \ self.formals == other.formals and \ self.body == other.body and \ self.env == other.env def apply(self, args, env): if proper_tail_recursion: # Implemented in Question 22. "*** YOUR CODE HERE ***" else: "*** YOUR CODE HERE ***" class MuProcedure(LambdaProcedure): """A procedure defined by a mu expression, which has dynamic scope. _________________ < Scheme is cool! > ----------------- \ ^__^ \ (oo)\_______ (__)\ )\/\ ||----w | || || """ def _symbol(self): return 'mu' def apply(self, args, env): if proper_tail_recursion: # Implemented in Question 22. "*** YOUR CODE HERE ***" else: "*** YOUR CODE HERE ***" # Call-by-name (nu) extension. class NuProcedure(LambdaProcedure): """A procedure whose parameters are to be passed by name.""" def _symbol(self): return 'nu' "*** YOUR CODE HERE ***" class Thunk(LambdaProcedure): """A by-name value that is to be called as a parameterless function when its value is fetched to be used.""" "*** YOUR CODE HERE ***" ################# # Special forms # ################# # All of the 'do_..._form' methods return a value and an environment, # as for the 'apply' method on Procedures. That is, they either return # (V, None), indicating that the value of the special form is V, or they # return (Expr, Env), indicating that the value of the special form is what # you would get by evaluating Expr in the environment Env. def do_lambda_form(vals, env, function_type=LambdaProcedure): """Evaluate a lambda form with formals VALS[0] and body VALS.second in environment ENV, creating a procedure of type FUNCTION_TYPE (a subtype of Procedure).""" check_form(vals, 2) formals = vals[0] check_formals(formals) "*** YOUR CODE HERE ***" def do_mu_form(vals, env): """Evaluate a mu (dynamically scoped lambda) form with formals VALS[0] and body VALS.second in environment ENV.""" return do_lambda_form(vals, env, function_type=MuProcedure) def do_nu_form(vals, env): """Evaluate a mu (call-by-name scoped lambda) form with formals VALS[0] and body VALS.second in environment ENV.""" return do_lambda_form(vals, env, function_type=NuProcedure) def do_define_form(vals, env): """Evaluate a define form with parameters VALS in environment ENV.""" check_form(vals, 2) target = vals[0] if scheme_symbolp(target): check_form(vals, 2, 2) "*** YOUR CODE HERE ***" elif scheme_pairp(target): "*** YOUR CODE HERE ***" else: raise SchemeError("bad argument to define") def do_quote_form(vals, env): """Evaluate a quote form with parameters VALS. ENV is ignored.""" check_form(vals, 1, 1) "*** YOUR CODE HERE ***" def do_let_form(vals, env): """Evaluate a let form with parameters VALS in environment ENV.""" check_form(vals, 2) bindings = vals[0] exprs = vals.second if not scheme_listp(bindings): raise SchemeError("bad bindings list in let form") # Add a frame containing bindings names, values = nil, nil "*** YOUR CODE HERE ***" new_env = env.make_call_frame(names, values) # Evaluate all but the last expression after bindings, and return the last last = len(exprs)-1 for i in range(0, last): scheme_eval(exprs[i], new_env) return exprs[last], new_env ######################### # Logical Special Forms # ######################### def do_if_form(vals, env): """Evaluate if form with parameters VALS in environment ENV.""" check_form(vals, 2, 3) "*** YOUR CODE HERE ***" def do_and_form(vals, env): """Evaluate short-circuited and with parameters VALS in environment ENV.""" "*** YOUR CODE HERE ***" def quote(value): """Return a Scheme expression quoting the Scheme VALUE. >>> s = quote('hello') >>> print(s) (quote hello) >>> scheme_eval(s, Frame(None)) # "hello" is undefined in this frame. intern('hello') """ return Pair("quote", Pair(value, nil)) def do_or_form(vals, env): """Evaluate short-circuited or with parameters VALS in environment ENV.""" "*** YOUR CODE HERE ***" def do_cond_form(vals, env): """Evaluate cond form with parameters VALS in environment ENV.""" num_clauses = len(vals) for i, clause in enumerate(vals): check_form(clause, 1) if clause.first is else_sym: if i < num_clauses-1: raise SchemeError("else must be last") test = scheme_true if clause.second is nil: raise SchemeError("badly formed else clause") else: test = scheme_eval(clause.first, env) if test: "*** YOUR CODE HERE ***" return okay, None def do_begin_form(vals, env): """Evaluate begin form with parameters VALS in environment ENV.""" check_form(vals, 0) if scheme_nullp(vals): return okay, None "*** YOUR CODE HERE ***" # Collected symbols with significance to the interpreter and_sym = intern("and") begin_sym = intern("begin") cond_sym = intern("cond") define_macro_sym = intern("define-macro") define_sym = intern("define") else_sym = intern("else") if_sym = intern("if") lambda_sym = intern("lambda") let_sym = intern("let") mu_sym = intern("mu") nu_sym = intern("nu") or_sym = intern("or") quasiquote_sym = intern("quasiquote") quote_sym = intern("quote") set_bang_sym = intern("set!") unquote_splicing_sym = intern("unquote-splicing") unquote_sym = intern("unquote") # Collected special forms SPECIAL_FORMS = { and_sym: do_and_form, begin_sym: do_begin_form, cond_sym: do_cond_form, define_sym: do_define_form, if_sym: do_if_form, lambda_sym: do_lambda_form, let_sym: do_let_form, mu_sym: do_mu_form, nu_sym: do_nu_form, or_sym: do_or_form, quote_sym: do_quote_form, } # Utility methods for checking the structure of Scheme programs def check_form(expr, min, max = None): """Check EXPR (default SELF.expr) is a proper list whose length is at least MIN and no more than MAX (default: no maximum). Raises a SchemeError if this is not the case.""" if not scheme_listp(expr): raise SchemeError("badly formed expression: " + str(expr)) length = len(expr) if length < min: raise SchemeError("too few operands in form") elif max is not None and length > max: raise SchemeError("too many operands in form") def check_formals(formals): """Check that FORMALS is a valid parameter list, a Scheme list of symbols in which each symbol is distinct. Raise a SchemeError if the list of formals is not a well-formed list of symbols or if any symbol is repeated. >>> check_formals(read_line("(a b c)")) """ "*** YOUR CODE HERE ***" ################ # Input/Output # ################ def read_eval_print_loop(next_line, env, quiet=False, startup=False, interactive=False, load_files=()): """Read and evaluate input until an end of file or keyboard interrupt.""" if startup: for filename in load_files: scheme_load(scstr(filename), True, env) while True: try: src = next_line() while src.more_on_line: expression = scheme_read(src) result = scheme_eval(expression, env) if not quiet and result is not None: scheme_print(result) except (SchemeError, SyntaxError, ValueError, RuntimeError) as err: if (isinstance(err, RuntimeError) and 'maximum recursion depth exceeded' not in err.args[0]): raise print("Error:", err) except KeyboardInterrupt: # <Control>-C if not startup: raise print("\nKeyboardInterrupt") if not interactive: return except EOFError: # <Control>-D, etc. return def scheme_load(*args): """Load a Scheme source file. ARGS should be of the form (SYM, ENV) or (SYM, QUIET, ENV). The file named SYM is loaded in environment ENV, with verbosity determined by QUIET (default true).""" if not (2 <= len(args) <= 3): vals = args[:-1] raise SchemeError("wrong number of arguments to load: {0}".format(vals)) sym = args[0] quiet = args[1] if len(args) > 2 else True env = args[-1] if (scheme_stringp(sym)): sym = intern(str(sym)) check_type(sym, scheme_symbolp, 0, "load") with scheme_open(str(sym)) as infile: lines = infile.readlines() args = (lines, None) if quiet else (lines,) def next_line(): return buffer_lines(*args) read_eval_print_loop(next_line, env.global_frame(), quiet=quiet) return okay def scheme_open(filename): """If either FILENAME or FILENAME.scm is the name of a valid file, return a Python file opened to it. Otherwise, raise an error.""" try: return open(filename) except IOError as exc: if filename.endswith('.scm'): raise SchemeError(str(exc)) try: return open(filename + '.scm') except IOError as exc: raise SchemeError(str(exc)) def create_global_frame(): """Initialize and return a single-frame environment with built-in names.""" env = Frame(None) env.define("eval", PrimitiveProcedure(scheme_eval, True)) env.define("apply", PrimitiveProcedure(scheme_apply, True)) env.define("load", PrimitiveProcedure(scheme_load, True)) for names, fn in get_primitive_bindings(): for name in names: proc = PrimitiveProcedure(fn) env.define(name, proc) return env @main def run(*argv): next_line = buffer_input interactive = True load_files = () if argv: try: filename = argv[0] if filename == '-load': load_files = argv[1:] else: input_file = open(argv[0]) lines = input_file.readlines() def next_line(): return buffer_lines(lines) interactive = False except IOError as err: print(err) sys.exit(1) read_eval_print_loop(next_line, create_global_frame(), startup=True, interactive=interactive, load_files=load_files) tscheme_exitonclick()