Dus*_*etz 10 python monads haskell clojure monad-transformers
我正在尝试编写seq -m和error-m来对可能返回错误的事情进行列表理解.我的输出有意想不到的类型,但除了它实际上似乎是明智的.我在下面展开了我的代码,但这里也是一个工作要点.
这是我的monadic业务逻辑
def get_loan(name):
m_qualified_amounts = (
bind(get_banks(name), lambda bank:
bind(get_accounts(bank, name), lambda account:
bind(get_balance(bank, account), lambda balance:
bind(get_qualified_amount(balance), lambda qualified_amount:
unit(qualified_amount))))))
return m_qualified_amounts
names = ["Irek", "John", "Alex", "Fred"]
for name, loans in zip(names, map(get_loan, names)):
print "%s: %s" % (name, loans)
产量
Irek: [None, 'Insufficient funds for loan, current balance is 35000', None, 'Insufficient funds for loan, current balance is 70000', None, 'Unable to get balance due to technical issue for Wells Fargo: 3']
John: [None, 'Insufficient funds for loan, current balance is 140000']
Alex: [[245000], None, [280000], None]
Fred: (None, 'No bank associated with name Fred')
我希望看到元组列表 - 列表是列表推导的结果,最终列表中的每个项目应该是error-monad(value, error元组)中的值.它就像是被删除了太多级别的嵌套seq_bind.
这是我对monad的定义,如果它不正确,它非常接近,因为两个monad都是孤立地工作,而不是合并.
def success(val): return val, None
def error(why): return None, why
def get_value(m_val): return m_val[0]
def get_error(m_val): return m_val[1]
# error monad
def error_unit(x): return success(x)
def error_bind(mval, mf):
assert isinstance(mval, tuple)
error = get_error(mval)
if error: return mval
else: return mf(get_value(mval))
def flatten(listOfLists):
"Flatten one level of nesting"
return [x for sublist in listOfLists for x in sublist]
# sequence monad
def seq_unit(x): return [x]
def seq_bind(mval, mf):
assert isinstance(mval, list)
return flatten(map(mf, mval))
# combined monad !!
def unit(x): return error_unit(seq_unit(x))
def bind(m_error_val, mf):
return error_bind(m_error_val, lambda m_seq_val: seq_bind(m_seq_val, mf))
monadic API
def get_banks(name):
if name == "Irek": return success(["Bank of America", "Wells Fargo"])
elif name == "John": return success(["PNC Bank"])
elif name == "Alex": return success(["TD Bank"])
else: return error("No bank associated with name %s" % name)
def get_accounts(bank, name):
if name == "Irek" and bank == "Bank of America": return success([1, 2])
elif name == "Irek" and bank == "Wells Fargo": return success([3])
elif name == "John" and bank == "PNC Bank": return success([4])
elif name == "John" and bank == "Wells Fargo": return success([5, 6])
elif name == "Alex" and bank == "TD Bank": return success([7, 8])
else: return error("No account associated with (%s, %s)" % (bank, name))
def get_balance(bank, account):
if bank == "Wells Fargo":
return error("Unable to get balance due to technical issue for %s: %s" % (bank, account))
else:
return success([account * 35000]) #right around 200,000 depending on acct number
def get_qualified_amount(balance):
if balance > 200000:
return success([balance])
else:
return error("Insufficient funds for loan, current balance is %s" % balance)
还在寻找改进代码的方法.标记为haskell和clojure,因为这在这些语言中是惯用的,python社区对此不感兴趣.
我不是Python专家,但这个定义:
def bind(mval, mf):
return error_bind(mval, lambda mval: seq_bind(mval, mf))
......让我非常怀疑.据推测,mf应该返回东西是包裹在两个error和seq单子类型,与error-ness最; 但是,你要传递给它seq_bind,它需要一个返回seq-ness最外层的函数.
你可能想看看的来源ErrorT和LogicT单子变压器在Haskell以了解其可能会做正确的想法.(LogicT与你期望的相比,你可能会发现令人惊讶的复杂 - 这是因为天真ListT 实际上并不是monad变换器!)
在Haskell中,使用Monad Transformers这样堆叠monad组合.撇开Daniel Wagner的观点,ListT暂时不是monad.你有两个类型的monad:
List a 看起来像 [x,y,z](Error e) a看起来x, None或None, err如果将one转换为monad变换器并将它们组合起来,有两种方法:
(ErrorT e) List a 看起来像 [ (x,None), (y,None), (None, err) ]ListT (ErrorT e) a看起来像[x,y,z], None或None, [x,y,z]你想要一个对的列表,所以我希望你想要第一个表格.但是你的简单测试并不同意这一点.您unit不会返回(1.)中的对列表,而是返回一对列表和None,即(2.).
所以你要么倒退,要么你有一个更复杂的monad.我会尝试修改你的要点看起来像(1.).
我认为这段代码可能会做你想要的:
def flatten(listOfLists):
"Flatten one level of nesting"
assert isinstance(listOfLists, list)
if len(listOfLists) > 0:
assert isinstance(listOfLists[0], list)
return [x for sublist in listOfLists for x in sublist]
# sequence monad
def seq_unit(x): return [x]
def seq_bind(mval, mf): return flatten(map(mf, mval))
# Decompose ErrorT e m a
def get_value(m_val): return m_val[0]
def get_error(m_val): return m_val[1]
# hard coded "(ErrorT e) List a" instance of throwError, note that seq_unit is hardcoded
def error_throwError(err): return (None, err)
def errorT_list_throwError(err): return seq_unit(error_throwError(err))
# "(ErrorT e) List a" monad
def error_unit(x): return (x,None)
def errorT_list_unit(x): return seq_unit(error_unit(x))
def error_bind(mval, mf):
assert isinstance(mval, tuple)
error = get_error(mval)
if error:
return error_throwError(error)
else:
return mf(get_value(mval))
# Cannot have multi-line lambda
def errorT_list_bind_helper(mval, mf):
assert isinstance(mval, tuple)
error = get_error(mval)
if error:
return errorT_list_throwError(error)
else:
return mf(get_value(mval))
def errorT_list_bind(mval, mf): return seq_bind(mval, lambda v: errorT_list_bind_helper(v, mf))
# combined monad !! (ErrorT e) List a
unit = errorT_list_unit
bind = errorT_list_bind
throwError = errorT_list_throwError
# hard coded "lift :: List a -> (ErrorT e) List a"
def lift(mval):
assert isinstance(mval, list)
# return [ (val,None) for val in mval ]
# return [ errorT_list_unit(val) for val in mval ]
return seq_bind(mval, lambda v : unit(v))
def get_banks(name):
if name == "Irek": return lift(["Bank of America", "Wells Fargo"])
elif name == "John": return unit("PNC Bank")
elif name == "Alex": return unit("TD Bank")
else: return throwError("No bank associated with name %s" % name)
def get_accounts(bank, name):
if name == "Irek" and bank == "Bank of America": return lift([1, 2])
elif name == "Irek" and bank == "Wells Fargo": return unit(3)
elif name == "John" and bank == "PNC Bank": return unit(4)
elif name == "John" and bank == "Wells Fargo": return lift([5, 6])
elif name == "Alex" and bank == "TD Bank": return lift([7, 8])
else: return throwError("No account associated with (%s, %s)" % (bank, name))
def get_balance(bank, account):
if bank == "Wells Fargo":
return throwError("Unable to get balance due to technical issue for %s: %s" % (bank, account))
else:
return unit(account * 35000) #right around 200,000 depending on acct number
def get_qualified_amount(balance):
if balance > 200000:
return unit(balance)
else:
return throwError("Insufficient funds for loan, current balance is %s" % balance)
# monadic business logic
def get_loan(name):
m_qualified_amounts = (
bind(get_banks(name), lambda bank:
bind(get_accounts(bank, name), lambda account:
bind(get_balance(bank, account), lambda balance:
bind(get_qualified_amount(balance), lambda qualified_amount:
unit(qualified_amount))))))
assert isinstance(m_qualified_amounts, list)
assert isinstance(m_qualified_amounts[0], tuple)
return m_qualified_amounts
names = ["Irek", "John", "Alex", "Fred"]
for name, loans in zip(names, map(get_loan, names)):
print "%s: %s" % (name, loans)
输出是
Irek: [(None, 'Insufficient funds for loan, current balance is 35000'), (None, 'Insufficient funds for loan, current balance is 70000'), (None, 'Unable to get balance due to technical issue for Wells Fargo: 3')]
John: [(None, 'Insufficient funds for loan, current balance is 140000')]
Alex: [(245000, None), (280000, None)]
Fred: [(None, 'No bank associated with name Fred')]