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from parser import Expr
class Function:
def __init__(self, callable_, *arities):
self.callable = callable_
if len(arities) == 0:
self.arities = None
else:
self.arities = arities
def call(self, expr):
if self.arities is not None and len(expr.args) not in self.arities:
#if self.arity >= 0 and len(args) != self.arity:
fmt = f"[{self.arities[0]}"
for arity in self.arities[1:]:
fmt += f", {arity}"
fmt += "]"
raise Exception(f"expected {fmt} arguments, received {len(expr.args)}")
return self.callable(expr)
class Environment:
def __init__(self, parent=None):
self.parent = parent
self.environment = {}
def register(self, key, value):
self.environment[key] = value
def contains(self, key):
if key in self.environment:
return True
elif self.parent is not None:
return self.parent.contains(key)
else:
return False
def get(self, key):
if not self.contains(key):
raise Exception(f"undefined symbol: '{key}")
if key in self.environment:
return self.environment[key]
else:
return self.parent.get(key)
def interpret(exprs):
ret = None
for expr in exprs:
ret = evaluate(expr)
return ret
def evaluate(expr):
if isinstance(expr, Expr.Literal):
return expr.value
elif isinstance(expr, Expr.Symbol):
# defined symbols will evaluate to their expression,
# undefined will return their own name
# TODO this is bad
if not GLOBALS.contains(expr.name):
raise Exception(f"no such symbol: {expr}")
return interpretEnv(expr, GLOBALS.get(expr.name))
name = expr.symbol.name
if name == "def":
return interpretDef(expr, GLOBALS)
elif GLOBALS.contains(name):
return GLOBALS.get(name).call(expr)
else:
raise Exception(f"unable to evaluate: {expr}")
GLOBALS = Environment()
def interpretOr(expr):
# or returns true for the first expression that returns true
if len(expr.args) < 2:
raise Exception("'or' has at least two operands")
for arg in expr.args:
ev = evaluate(arg)
if ev not in (True, False):
raise Exception("'or' needs boolean arguments")
if ev == True:
return True
return False
GLOBALS.register("or", Function(interpretOr))
def interpretAnd(expr):
# and returns false for the first expression that returns false
if len(expr.args) < 2:
raise Exception("'and' has at least two operands")
for arg in expr.args:
ev = evaluate(arg)
if ev not in (True, False):
raise Exception("'and' needs boolean arguments")
if ev == False:
return False
return True
GLOBALS.register("and", Function(interpretAnd))
def interpretEq(expr):
# equal
first = evaluate(expr.args[0])
second = evaluate(expr.args[1])
return first == second
GLOBALS.register("eq?", Function(interpretEq, 2))
def interpretComparison(expr):
left = evaluate(expr.args[0])
if type(left) not in (int, float):
raise Exception("'left' must be a number")
right = evaluate(expr.args[1])
if type(right) not in (int, float):
raise Exception("'right' must be a number")
if expr.symbol.name == ">":
return left > right
elif expr.symbol.name == ">=":
return left >= right
elif expr.symbol.name == "<":
return left < right
elif expr.symbol.name == "<=":
return left <= right
GLOBALS.register(">", Function(interpretComparison, 2))
GLOBALS.register(">=", Function(interpretComparison, 2))
GLOBALS.register("<", Function(interpretComparison, 2))
GLOBALS.register("<=", Function(interpretComparison, 2))
def interpretTerm(expr):
if len(expr.args) < 1:
raise Exception("term has at least one operand")
res = None
for arg in expr.args:
ev = evaluate(arg)
if type(ev) not in (int, float):
raise Exception("term must be a number")
if res is None:
res = ev
elif expr.symbol.name == "+":
res += ev
elif expr.symbol.name == "-":
res -= ev
return res
GLOBALS.register("+", Function(interpretTerm))
GLOBALS.register("-", Function(interpretTerm))
def interpretFactor(expr):
if expr.symbol.name == "/":
num = evaluate(expr.args[0])
if type(num) not in (int, float):
raise Exception("numerator must be a number")
denom = evaluate(expr.args[1])
if type(denom) not in (int, float):
raise Exception("denominator must be a number")
return num / denom # TODO floats and ints
else:
if len(expr.args) < 2:
raise Exception("'*' requires at least two operands")
first = evaluate(expr.args[0])
if type(first) not in (int, float):
raise Exception("'*' operand must be a number")
res = first
for arg in expr.args[1:]:
tmp = evaluate(arg)
if type(tmp) not in (int, float):
raise Exception("'*' operand must be a number")
res = res * tmp
return res
GLOBALS.register("*", Function(interpretFactor))
GLOBALS.register("/", Function(interpretFactor, 2))
def interpretNot(expr):
res = evaluate(expr.args[0])
if res not in (True, False):
raise Exception("'not' only works on booleans")
return not res
GLOBALS.register("not", Function(interpretNot, 1))
def interpretIf(expr):
# if cond t-branch [f-branch]
cond = evaluate(expr.args[0])
if cond not in (True, False):
raise Exception("'if' condition must be boolean")
if cond:
return evaluate(expr.args[1])
elif len(expr.args) == 3:
return evaluate(expr.args[2])
return None # this shouldn't be reached
GLOBALS.register("if", Function(interpretIf, 2, 3))
def interpretPrint(expr):
ev = evaluate(expr.args[0])
if not isinstance(ev, str):
raise Exception("can only 'print' strings")
print(ev)
return None # print returns nothing
GLOBALS.register("print", Function(interpretPrint, 1))
def interpretDef(expr, env):
if not isinstance(expr.args[0], Expr.Symbol):
raise Exception("'def' requires a string literal as a name")
name = expr.args[0].name # NOTE: we are not evaluating the name!!
if not isinstance(name, str):
raise Exception("'def' requires a string literal as a name")
env.register(name, expr.args[1])
return None
GLOBALS.register("def", Function(interpretDef, 2))
def interpretEnv(expr, env_expr):
return evaluate(env_expr) # TODO more than this
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