CLU

维基百科,自由的百科全书
跳到导航 跳到搜索
CLU
编程范型多范型: 面向对象, 过程式
設計者芭芭拉·利斯科夫和她的学生
實作者麻省理工学院
发行时间1975年,​47年前​(1975
穩定版本
Native CLU 1.5 (SPARC, VAX) / 1989年5月26日,​32年前​(1989-05-26[1]

Portable CLU / 2009年11月6日,​12年前​(2009-11-06[2]

型態系統强类型
網站www.pmg.lcs.mit.edu/CLU.html
主要實作產品
Native CLU[1], Portable CLU[2], clu2c[3]
啟發語言
ALGOL 60, Lisp[4], Simula[5]
影響語言
Ada, Argus, C++, Lua, Python[6], Ruby, Sather英语Sather, Swift[7]

CLU是一门编程语言,由芭芭拉·利斯科夫和她的学生在1974年到1975年于麻省理工学院(MIT)创造。虽然它没有被广泛使用,但它引入了抽象数据类型[8],和许多现在广泛使用的特性,而被视为面向对象编程发展的重要一步。

主要贡献还包括:传共享调用迭代器、多值返回(并行赋值形式)、参数化类型可变类型。值得注意的是它使用了具有构造器方法,但没有继承

聚簇[编辑]

CLU的语法基于了ALGOL,这是多数新语言设计的起点。关键增补是“聚簇”(cluster)概念,它是CLU的类型扩展系统和语言名字的根源(CLUster)[9]。聚簇一般对应于面向对象语言中“类”的概念。例如,下面是CLU用来实现复数的语法:

   complex_number = cluster is add, subtract, multiply, ...
       rep = record [ real_part: real, imag_part: real ]
       add = proc ... end add;
       subtract = proc ... end subtract;
       multiply = proc ... end multiply;
       ...
   end complex_number;

聚簇是一个模块,它封装了除了那些在is子句中显式命名的成员之外的所有成员。这些成员对应于现在面向对象语言中一个类的公开成员。聚簇还定义了可以在聚簇之外引用名字的一个类型(在这个案例中是complex_number),但是它的表示类型(这里的rep)对于外部客户是隐藏的。

聚簇名字是全局的,不提供名字空间机制来组织聚簇,也不允许它们在其他聚簇内部被“局部”创建。

CLU不进行隐式类型转换。在聚簇中,显式类型转换updown抽象类型和表示之间进行变更。有一个全体类型any,和一个过程force[]来检查一个对象是否是一个特定类型。对象可以是可变的或不可变的,后者是基础类型,比如整数、布尔值、字符和字符串[9]

其他特征[编辑]

CLU类型系统的另一个关键特征是迭代器,它一个接一个的、顺序的从一个搜集返回对象[9]。迭代器提供了一致的应用编程接口(API),而不管所用于的是什么数据。因此给complex_number的一个搜集的迭代器,同给integer的数组的迭代器,可以互换的使用。CLU迭代器的显著特征是它们被实现为协程,每个值都是通过yield语句提供给调用者的。像CLU中这样的迭代器,现在是很多现代语言比如C#RubyPython的常见特征,然而它们近来经常被称为生成器。下面是迭代器的例子:

   % 产生从1到n的奇数
   odds = iter(n:int) yields int
       i:int
       i = 1
       while i < n do
           yield i
           i := i + 2
       end
   end odds  
     
   for i:int in odds(13) do
       print int$unparse(i) || "\n"
   end

CLU还包括了异常处理,它参考了在其他语言中的各种尝试;异常使用signal引发,并通过except处理。不同于具有异常处理的多数其他语言,异常不会被隐式的沿着调用链重新发起。不同之处还有,在CLU中异常被当作是正常执行流程的一部份,并作为“正常”而有效的一种类型安全的方式,用来退出循环或从函数返回;它允许用except when直接指定在其他条件适用时的返回值。既未捕获也未显式的重新发起的异常,被立即转换成特殊失败异常,这典型的会终止程序。

CLU经常被引证为具有类型安全的可变类型的第一个语言,在这里叫作oneof,早于ML语言拥有的叫做代数数据类型英语Algebraic data type标签联合

CLU中最后一个显著特征是并行赋值(多赋值),这里多于一个变量可以出现在赋值算符的左侧。例如,书写x,y := y,x将交换xy的值。以相同的方式,函数可以返回多个值,比如x,y,z := f(t)。并行赋值(但未包括多返回值),在CLU之前已经出现在CPL(1963年)之中,叫作“同时赋值”[10],然而确是CLU使之流行,并被引证为对后来语言中出现的并行赋值有直接的影响。

在CLU程序中所有对象都存活在堆中,而内存管理是自动化的。

CLU支持参数化类型的用户定义数据抽象。它是提供类型安全限定的参数化类型的第一个语言,它使用where子句结构,来表达在实际类型实际参数上的约束。

影响[编辑]

CLU和AdaC++模板的主要启发者[11]

CLU的异常处理机制影响了后来的语言如C++Java[12]

Sather英语SatherPythonC#所包含的迭代器,最早出现在CLU中。

PerlLua采用的多赋值和从函数调用返回多个值来自CLU[13]

PythonRuby从它引入了传共享调用yield语句[14]和多赋值[15]

参考资料[编辑]

  1. ^ 1.0 1.1 Curtis, Dorothy. CLU home page. Programming Methodology Group, Computer Science and Artificial Intelligence Laboratory. Massachusetts Institute of Technology. 2009-11-06 [2016-05-26]. (原始内容存档于2016-06-02). 
  2. ^ 2.0 2.1 Curtis, Dorothy. Index of /pub/pclu. Programming Methodology Group, Computer Science and Artificial Intelligence Laboratory. Massachusetts Institute of Technology. 2009-11-06 [2016-05-26]. 
  3. ^ Ushijima, Tetsu. clu2c. clu2c. woodsheep.jp. [2016-05-26]. (原始内容存档于2016-03-04). 
  4. ^ Barbara Liskov. A history of CLU (PDF). 1992 [2022-04-27]. (原始内容 (PDF)存档于2021-11-05). CLU looks like an Algol-like language, but its semantics is like that of Lisp: CLU objects reside in an object universe (or heap), and a variable just identifies (or refers to) an object. We decided early on to have objects in the heap, although we had numerous discussions about the cost of garbage collection. This decision greatly simplified the data abstraction mechanism ……. A language that allocates objects only on the stack is not sufficiently expressive; the heap is needed for objects whose sizes must change and for objects whose lifetime exceeds that of the procedure that creates them. …… Therefore, the choice is: just heap, or both. ……
    One unusual aspect of CLU is that our procedures have no free (global) variables ……. The view of procedures in CLU is similar to that in Lisp: CLU procedures are not nested (except that there can be local procedures within a cluster) but instead are defined at the "top" level, and can be called from any other module. In Lisp such procedures can have free variables that are scoped dynamically, a well-known source of confusion.
     
  5. ^ Barbara Liskov. A history of CLU (PDF). 1992 [2022-04-27]. (原始内容 (PDF)存档于2021-11-05). Programming languages that existed when the concept of data abstraction arose did not support abstract data types, but some languages contained constructs that were precursors of this notion. …… The mechanism that matched the best was the class mechanism of Simula 67. A Simula class groups a set of procedures with some variables. A class can be instantiated to provide an object containing its own copies of the variables; the class contains code that initializes these variables at instantiation time. However, Simula classes did not enforce encapsulation ……, and Simula was lacking several other features needed to support data abstraction, ……. 
  6. ^ Lundh, Fredrik. Call By Object. effbot.org. [21 November 2017]. (原始内容存档于2019-11-23). replace "CLU" with "Python", "record" with "instance", and "procedure" with "function or method", and you get a pretty accurate description of Python's object model. 
  7. ^ Lattner, Chris. Chris Lattner's Homepage. Chris Lattner. 2014-06-03 [2014-06-03]. (原始内容存档于2018-12-25). The Swift language is the product of tireless effort from a team of language experts, documentation gurus, compiler optimization ninjas, and an incredibly important internal dogfooding group who provided feedback to help refine and battle-test ideas. Of course, it also greatly benefited from the experiences hard-won by many other languages in the field, drawing ideas from Objective-C, Rust, Haskell, Ruby, Python, C#, CLU, and far too many others to list. 
  8. ^ Liskov, Barbara; Zilles, Stephen. Programming with abstract data types. Proceedings of the ACM SIGPLAN symposium on Very high level languages. 1974: 50–59. doi:10.1145/800233.807045. 
  9. ^ 9.0 9.1 9.2 Liskov, B.; Snyder, A.; Atkinson, R.; Schaffert, C. Abstraction mechanisms in CLU (PDF). Communications of the ACM. August 1977, 20 (8): 564–576 [2021-03-09]. doi:10.1145/359763.359789. (原始内容 (PDF)存档于2021-08-09). 
  10. ^ Barron, D. W.; Buxton, J. N.; Hartley, D. F.; Nixon, E.; Strachey, C. The main features of CPL. Computer Journal. 1963, 6 (2): 134–143 [2021-03-09]. doi:10.1093/comjnl/6.2.134. (原始内容存档于2012-07-07). 
  11. ^ Stroustrup, Bjarne. The C++ Programming Language (Third Edition and Special Edition). Bjarne Stroustrup's homepage. 2004-09-08 [2020-09-21]. (原始内容存档于2019-05-14). 
  12. ^ Bruce Eckel's MindView, Inc: Does Java need Checked Exceptions?. Mindview.net. [2011-12-15]. (原始内容存档于2002-04-05). 
  13. ^ Ierusalimschy, R.; De Figueiredo, L. H.; Celes, W. The evolution of Lua. Proceedings of the third ACM SIGPLAN conference on History of programming languages – HOPL III (PDF). 2007: 2–1–2–26 [2020-09-21]. ISBN 978-1-59593-766-7. doi:10.1145/1238844.1238846. (原始内容存档 (PDF)于2020-08-17). 
  14. ^ Ruby's Roots and Matz's Leadership. Appfolio Engineering. 2019-11-08 [2019-11-15]. (原始内容存档于2019-11-14). Matz feels that blocks are the greatest invention of Ruby (I agree.) He got the idea from a 1970s language called CLU from MIT, which called them 'iterators'... 
  15. ^ Functional Programming HOWTO — Python 3.8.3 documentation. docs.python.org. [2020-05-25]. (原始内容存档于2012-10-24). 

外部链接[编辑]