日期：2016年1月13日

在RHEL系统上使用“subscription-manager”注册和激活“subscription”

在RHEL系统中注册和使用subscription是两个过程：

NOTE: With Red Hat Subscription-Manager, registration and utilization of a subscription is actually a two-part process. First register a system, then apply a subscription.

可以使用下面命令一次完成两个过程：

# subscription-manager register --username <username> --password <password> --auto-attach

在我的RHEL 7.2系统上执行上述命令：

# subscription-manager register --username=xxxx --password=xxxx --auto-attach
Registering to: subscription.rhn.redhat.com:443/subscription
The system has been registered with ID: 333486bb-xxxxxx

Installed Product Current Status:
Product Name: Red Hat Enterprise Linux Server
Status:       Subscribed

然后检查状态：

# subscription-manager list

+-------------------------------------------+
    Installed Product Status
+-------------------------------------------+
Product Name:   Red Hat Enterprise Linux Server
Product ID:     69
Version:        7.2
Arch:           x86_64
Status:         Subscribed
Status Details:
Starts:         06/29/2015
Ends:           06/28/2016

接下来就可以使用“yum install”，“yum update”等命令安装和更新软件了，非常方便。

参考资料：
How to Register and Enable Red Hat Subscription, Repositories and Updates for RHEL 7.0 Server；
How to register and subscribe a system to the Red Hat Customer Portal using Red Hat Subscription-Manager；
RHEL : Register Subscription。

 

Haskell笔记 （10）—— Lazy evaluation

Haskell中对表达式的计算使用的是Lazy evaluation（也称之为nonstrict evaluation）：即只有表达式的值真正需要时，才会被计算。Haskell中把用来跟踪还没有计算的表达式的记录称之为trunk。

参考自stackoverflow：

Mostly because it can be more efficient — values don’t need to be computed if they’re not going to be used. For example, I may pass three values into a function, but depending on the sequence of conditional expressions, only a subset may actually be used. In a language like C, all three values would be computed anyway; but in Haskell, only the necessary values are computed.

It also allows for cool stuff like infinite lists. I can’t have an infinite list in a language like C, but in Haskell, that’s no problem. Infinite lists are used fairly often in certain areas of mathematics, so it can be useful to have the ability to manipulate them.

Lazy evaluation的优点：高效（延迟执行代码），支持像infinite lists这样的cool stuff。