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jsinterp.py
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jsinterp.py
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import optimizer
class JSReturn(Exception):
"""Phrasing "return" as an exception allows us to break out of
any nested statement evaluation and return to the caller
immediately."""
def __init__(self, retval):
self.retval = retval
# Could also be done with
# raise Exception(retval)
# exception Exception as retval:
def env_lookup(vname,env):
if vname in env[1]:
return (env[1])[vname]
elif env[0] == None:
return None
else:
return env_lookup(vname,env[0])
def env_update(vname,value,env):
if vname in env[1]:
(env[1])[vname] = value
elif not (env[0] == None):
env_update(vname,value,env[0])
def global_env_update(vname,value,env):
if vname in env[1] or (env[0] == None):
(env[1])[vname] = value
elif not (env[0] == None):
global_env_update(vname,value,env[0])
def eval_elt(elt,env):
if elt[0] == 'function':
fname = elt[1]
fparams = elt[2]
fbody = elt[3]
fvalue = ("function",fparams,fbody,env)
(env[1])[fname] = fvalue
elif elt[0] == 'stmt':
eval_stmt(elt[1],env)
else:
print "ERROR: eval_elt: unknown element " + elt
def eval_stmts(stmts,env):
for stmt in stmts:
eval_stmt(stmt,env)
def eval_stmt(stmt,env):
stype = stmt[0]
if stype == "if-then":
cexp = stmt[1]
then_branch = stmt[2]
if eval_exp(cexp,env):
eval_stmts(then_branch,env)
elif stype == "while":
cexp = stmt[1]
while_body = stmt[2]
while eval_exp(cexp,env):
eval_stmts(while_body,env)
elif stype == "if-then-else":
cexp = stmt[1]
then_branch = stmt[2]
else_branch = stmt[3]
if eval_exp(cexp,env):
eval_stmts(then_branch,env)
else:
eval_stmts(else_branch,env)
elif stype == "var":
vname = stmt[1]
rhs = stmt[2]
(env[1])[vname] = eval_exp(rhs,env)
# env_update(vname, eval_exp(rhs,env), env)
elif stype == "assign":
vname = stmt[1]
rhs = stmt[2]
global_env_update(vname, eval_exp(rhs,env), env)
#if vname in env:
# env_update(vname, eval_exp(rhs,env), env)
#else:
# JavaScript allows you to make a global
# variable by assigning to it.
# print "ERROR: unknown variable assigned " + vname
elif stype == "return":
retval = eval_exp(stmt[1],env)
raise JSReturn(retval)
elif stype == "exp":
eval_exp(stmt[1],env)
else:
print "ERROR: unknown statement type ",
print stype
def eval_exp(exp,env):
etype = exp[0]
if etype == "identifier":
vname = exp[1]
value = env_lookup(vname,env)
if value == None:
print "ERROR: unbound variable " + vname
else:
return value
elif etype == "number":
return float(exp[1])
elif etype == "string":
return exp[1]
elif etype == "true":
return True
elif etype == "false":
return False
elif etype == "not":
return not(eval_exp(exp[1],env))
elif etype == "function":
fparams = exp[1]
fbody = exp[2]
return ("function",fparams,fbody,env)
elif etype == "binop":
a = eval_exp(exp[1],env)
op = exp[2]
b = eval_exp(exp[3],env)
if op == "+":
return a+b
elif op == "-":
return a-b
elif op == "/":
return a/b
elif op == "*":
return a*b
elif op == "%":
return a%b
elif op == "==":
return a==b
elif op == "<=":
return a<=b
elif op == "<":
return a<b
elif op == ">=":
return a>=b
elif op == ">":
return a>b
elif op == "&&":
return a and b
elif op == "||":
return a or b
else:
print "ERROR: unknown binary operator ",
print op
exit(1)
elif etype == "call":
fname = exp[1]
args = exp[2]
fvalue = env_lookup(fname,env)
if fname == "write":
argval = eval_exp(args[0],env)
output_sofar = env_lookup("javascript output",env)
env_update("javascript output",output_sofar + str(argval),env)
elif fvalue[0] == "function":
fparams = fvalue[1]
fbody = fvalue[2]
fenv = fvalue[3]
if len(fparams) <> len(args):
print "ERROR: wrong number arguments to " + fname
else:
# make a new environment frame
newenv = (fenv,{ })
for i in range(len(args)):
argval = eval_exp(args[i],env)
(newenv[1])[fparams[i]] = argval
# evaluate the body in the new frame
try:
eval_stmts(fbody,newenv)
return None
except JSReturn as r:
return r.retval
else:
print "ERROR: call to non-function " + fname
# Complicated!
else:
print "ERROR: unknown expression type ",
print etype
return None
def interpret(ast):
ast = optimizer.optimize(ast)
global_env = (None, {"javascript output" : ""})
for elt in ast:
eval_elt(elt,global_env)
return (global_env[1])["javascript output"]