小编Kot*_*lar的帖子

我有以下类型类(省略了不相关的方法):

class Initializable a where
    initialize :: IO a
    initialize = return initializePure

    {# convenience method for implementations, not to be called from outside #}
    initializePure :: a

data Foo = Foo

instance Initializable Foo where
    initializePure = Foo

data Bar = Bar

instance Initializable Bar where
    initialize = initializeBar

有些实现需要IO初始化自己,有些则不需要.

此代码发出警告:

No explicit implementation for
  ‘initializePure’
In the instance declaration for ‘Initializable Bar’

我试图添加这样的MINIMALpragma:

{-# MINIMAL initializePure | initialize #-}

但后来我得到了一个不同的警告:

The MINIMAL pragma does …

以下函数编译和工作:

shares :: Maybe (Int, L.ByteString) -> Maybe Int                                                                                                                                                            
shares a =                                                                                                                                                                                                  
    case a of                                                                                                                                                                                               
        Nothing        -> Nothing                                                                                                                                                                           
        Just (x, y)    -> Just x

但是当以下面的形式重写时:

shares :: Maybe (Int, L.ByteString) -> Maybe Int                                                                                                                                                            
shares Nothing = Nothing                                                                                                                                                                                    
shares Just (x, y) = Just x

我收到错误

Equations for ‘shares’ have different numbers of arguments

我认为它基本上是一样的.

我有以下代码

import Data.List

data Card = One
          | Two
          | Three
          | Four
          | Five
          | Six
          | Seven
          | Eight
          | Nine
          | Ten
          | J
          | Q
          | K
          | A
          deriving (Show, Eq, Ord, Enum)

data Hand = Hand Card Card Card Card Card deriving(Show)

largestCard :: Hand -> Card
largestCard (Hand a b c d e) = One

但是，当我尝试toEnum 3在GHCi中使用时，我得到了Exception: Prelude.Enum.().toEnum: bad argument。如何解决？

最后一个功能只是一个虚拟功能。现在它什么也没做。

在这里的一个帖子中,我找到了一种检查用户是否具有管理权限的方法.但是,当我尝试在此使用布尔逻辑时,它无法工作.

$user = [Security.Principal.WindowsIdentity]::GetCurrent();
(New-Object Security.Principal.WindowsPrincipal $user).IsInRole([Security.Principal.WindowsBuiltinRole]::Administrator)  

if($user = $false){cls;Write-warning "Starting and stopping services can only be done with administrative privileges.`nPlease restart this script from an elevated prompt!";Read-host;exit
}

问题是,虽然从我的计算机运行脚本而没有启动具有管理权限的Powershell,但文本"False"出现了.但是,if语句没有启动.我定义错了吗？

编辑:当我使用$true而不使用$falseif语句时,我会执行并且不从高架提示符运行脚本.

我有

myapp/
    __init__.py
    lib.py
    tests/
        lib_test.py

在lib_test.py我有:

import lib

从myapp运行时:

python tests/lib_test.py

我收到一个错误

ImportError: No module named lib

毫无疑问,这是某种初学者错误.但是我无法弄清楚出了什么问题.