Haskell 安装

1.Ubuntu 安装过程中出现了一些问题：

　　1）W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: 　　　　http://packages.ros.org/ros/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654

　　     解决方案：apt-key adv --keyserver keyserver.ubuntu.com --recv-keys F42ED6FBAB17C654

　　2）E: The repository 'http://ppa.launchpad.net/nm-l2tp/network-manager-l2tp/ubuntu bionic Release' does not have a Release file.

　　　  N: Updating from such a repository can't be done securely, and is therefore disabled by default.

　　　  解决方法：sudo apt-add-repository -r ppa:nm-l2tp/network-manager-l2tp

               sudo apt update -q
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2. CentOS 7 安装过程