我目前正在学习Haskell，在FP方面我是一个绝对的初学者。
现在我正在尝试使用列表推导的不同内容。
listComprehension = [(a,b,c) | a <- xs, b <- xs, c <- ys, even c, c == a+b] ++
[(a,b,c) | a <- xs, b <- xs, c <- ys, even c, c == a-b]
where xs = [1..4]; ys = [(-100)..100]
为什么不说：
listComprehension = [(a,b,c) | a <- xs, b <- xs, c <- ys, even c, c == a+b || c == a-b]
where xs = [1..4]; ys = [(-100)..100]
我希望元素的排序方式与我的第一个例子完全相同。 我想要所有元素，其中c = a + b，然后是所有元素，其中c = a-b。
请注意，在我的第二个代码中，订单不是我想要的。我似乎无法弄清楚我是如何在列表理解中命令的，特别是当我有元组时...
谢谢！
答案 0 :(得分：6)
在第一个生成器中选择函数Welcome to the Google Cloud SDK!
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Traceback (most recent call last):
File "/Users/<user>/Downloads/google-cloud-sdk/bin/bootstrapping/install.py", line 218, in <module>
main()
File "/Users/<user>/Downloads/google-cloud-sdk/bin/bootstrapping/install.py", line 196, in main
Install(pargs.override_components, pargs.additional_components)
File "/Users/<user>/Downloads/google-cloud-sdk/bin/bootstrapping/install.py", line 141, in Install
_CLI.Execute(['--quiet', 'components', 'list'])
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/calliope/cli.py", line 870, in Execute
self._HandleAllErrors(exc, command_path_string, specified_arg_names)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/calliope/cli.py", line 908, in _HandleAllErrors
exceptions.HandleError(exc, command_path_string, self.__known_error_handler)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/calliope/exceptions.py", line 526, in HandleError
core_exceptions.reraise(exc)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/exceptions.py", line 111, in reraise
six.reraise(type(exc_value), exc_value, tb)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/calliope/cli.py", line 844, in Execute
resources = calliope_command.Run(cli=self, args=args)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/calliope/backend.py", line 756, in Run
resources = command_instance.Run(args)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/surface/components/list.py", line 98, in Run
only_local_state=args.only_local_state)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/update_manager.py", line 674, in List
to_print, current_version, latest_version = self._GetPrintListWithDiff()
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/update_manager.py", line 704, in _GetPrintListWithDiff
_, diff = self._GetStateAndDiff(command_path='components.list')
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/update_manager.py", line 600, in _GetStateAndDiff
command_path=command_path)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/update_manager.py", line 583, in _GetLatestSnapshot
*effective_url.split(','), command_path=command_path)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/snapshots.py", line 178, in FromURLs
for url in urls]
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/snapshots.py", line 199, in _DictFromURL
response = installers.ComponentInstaller.MakeRequest(url, command_path)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/installers.py", line 293, in MakeRequest
return ComponentInstaller._RawRequest(req, timeout=timeout)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/updater/installers.py", line 337, in _RawRequest
should_retry_if=RetryIf, sleep_ms=500)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/util/retry.py", line 178, in RetryOnException
exceptions.reraise(exc_info[1], tb=exc_info[2])
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/exceptions.py", line 111, in reraise
six.reraise(type(exc_value), exc_value, tb)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/util/retry.py", line 159, in TryFunc
return func(*args, **kwargs), None
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/url_opener.py", line 78, in urlopen
return opener.open(req, data, timeout)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 429, in open
response = self._open(req, data)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 447, in _open
'_open', req)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 407, in _call_chain
result = func(*args)
File "/Users/<user>/Downloads/google-cloud-sdk/lib/googlecloudsdk/core/url_opener.py", line 63, in https_open
return self.do_open(build, req)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 1195, in do_open
h.request(req.get_method(), req.get_selector(), req.data, headers)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 1042, in request
self._send_request(method, url, body, headers)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 1082, in _send_request
self.endheaders(body)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 1038, in endheaders
self._send_output(message_body)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 882, in _send_output
self.send(msg)
File "/usr/local/Cellar/python@2/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 844, in send
self.connect()
File "/Users/<user>/Downloads/google-cloud-sdk/lib/third_party/httplib2/python2/httplib2/__init__.py", line 1120, in connect
raise SSLHandshakeError(e)
httplib2.python2.httplib2.SSLHandshakeError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)
。
(+),(-)顺便说一句，这是一种效率低下的方法。我们不应该尝试每个listComprehension =
[(a,b,c) | f <- [(+),(-)], a <- xs, b <- xs, c <- ys, even c, c == f a b ]
where xs = [1..4]; ys = [(-100)..100]
，而应该c <- ys，然后检查let c = f a b（和均匀度）。我们以这种方式使代码快200倍。
答案 1 :(得分：0)
之前我已经在列表推导中使用了替代函数（[even，odd]）。我并不总是对他们最有效率感到满意。我看到的缺点是他们会通过源列表进行两次或多次传递。为此，我选择了不同的方法，但是，对于较小的数据集，效率并不是很重要。
这就是我写的
lc xs ys = [[(a,b,c)|(a,b,c)<-ls,c==a+b] ++ [(a,b,c)|(a,b,c)<-ls,c==a-b]]
where ls = [(a,b,c)| a<-xs, b<-xs, c<-ys, even c]
这比原来的功能更快，比其他答案更快。更引人注目的是它使用的内存比其中任何一个都少。
从长列表中选择值是另一回事，而从生成的值中设置它们是另一回事。
lc xs = [(a,b,c)|a<-xs,b<-xs,c<-[(a-b),(a+b)],even c]
partition (\(a,b,c) -> c == a+b) lc [1..4]