import json

def escape(str): return str # TODO: implement

def quote(str): return '"'+escape(str)+'"'

class ASTVisitor(object):

def visit(self, ast):

tag, fields = ast

fn = getattr(self, '_' + tag)

return fn(fields)

def _mapVisit(self, fields, *names):

return [self.visit(fields[name]) for name in names]

def _ident(self, fields): return self.ident(fields['name'])

def _const(self, fields): return self.const(fields['type'], fields['value'])

def _infixop(self, fields):

visited = self._mapVisit(fields, 'lhs', 'rhs')

return self.infixop(fields['opname'], *visited)

def _conditional(self, fields):

visited = self._mapVisit(fields, 'cond', 'conseq', 'altern')

return self.conditional(*visited)

def _call(self, fields):

args = [self.visit(arg) for arg in fields['args']]

return self.call(fields['fname'], args)

class ExprToSql(ASTVisitor): # more like Expr to MySQL

def ident(self, name): return name # TODO: check for SQL Injection

def const(self, type, value):

if type == 'num':

return value

else:

return quote(value)

def infixop(self, opname, lhs, rhs):

if opname in ["+", "-", "*", "/", "%", ">", "<"]:

return lhs + opname + rhs

if opname == "++":

return "CONCAT(%s, %s)" % (lhs, rhs)

raise Exception("Unknown operation %s" % opname)

def conditional(self, cond, conseq, altern):

return "IF(%s, %s, %s)" % (cond, conseq, altern)

def call(self, fname, args):

fn = getattr(self, "fn_%s"%fname)

return fn(args)

def fn_log(self, argsSql):

if len(argsSql) != 1: raise Exception

return "LOG(%s)" % argsSql[0]

def fn_mean(self, argsSql):

if len(argsSql) != 1: raise Exception

return "AVG(%s)" % argsSql[0]

sqlizer = ExprToSql()

def sqlize(str):

return sqlizer.visit(json.loads(str))

print sqlize('["infixop",{"opname":"+","lhs":["const",{"value":"1","type":"num"}],"rhs":["const",{"value":"2","type":"num"}]}]')

print sqlize('["call",{"fname":"mean","args":[["infixop",{"opname":"-","lhs":["call",{"fname":"log","args":[["infixop",{"opname":"*","lhs":["ident",{"name":"mycol"}],"rhs":["const",{"value":"10","type":"num"}]}]]}],"rhs":["const",{"value":"1","type":"num"}]}]]}]')

print sqlize('["infixop",{"opname":"++","lhs":["const",{"value":"some","type":"cat"}],"rhs":["conditional",{"cond":["infixop",{"opname":">","lhs":["infixop",{"opname":"*","lhs":["const",{"value":"6","type":"num"}],"rhs":["const",{"value":"3","type":"num"}]}],"rhs":["const",{"value":"5","type":"num"}]}],"conseq":["const",{"value":" thing","type":"cat"}],"altern":["const",{"value":" stuff","type":"cat"}]}]}]')