ISWIM

ISWIM
编程范型	指令式, 函数式
設計者	Peter J. Landin
发行时间	1966年，60年前
作用域	词法
受影响于
	ALGOL 60, Lisp
影響語言
	PAL, SASL, ML, NPL, Lucid, Haskell

ISWIM（“If you See What I Mean”的首字母缩写），是的一种抽象的计算机编程语言或编程语言家族，它由Peter J. Landin设计，并描述在他1966年于ACM通讯发表的文章《The Next 700 Programming Languages》之中^[1]。

尽管没有实现，它被证明为在编程语言开发中非常有影响力的语言，特别是对于函数式编程语言，比如SASL、ML、NPL、Haskell和它们的后继者，还有数据流程编程语言如Lucid。

设计

ISWIM是具有函数式核心的指令式编程语言，它构成自加了语法糖的叫做“应用表达式”（AE）的lambda演算，并增加了可变变量和赋值，还有强力控制机制即对应J算子（英语：J operator）的程序点算子pp^[2]。ISWIM的操作语义，使用Landin的SECD抽象机来定义，并且使用了传值调用，因而是及早求值的^[3]。

ISWIM基于了lambda演算，从而具有高阶函数和词法作用域变量，其应用表达式为了“同时”或“平行”定义而扩展了在λ和.之间的标识符列表，为递归定义而采用了Y不动点算子。ISWIM的目标之一，就是要看起来更像数学表示，所以在定义结构之时，放弃了ALGOL的语句括号begin……end，转而采用了越位规则和基于缩进的作用域。

ISWIM程序基于了一个单一表达式，它由where或let子句、条件表达式和函数定义所限定。ISWIM在表示法上的特色，是与CPL一起，最早使用了where子句，尤其是let子句适合表达ALGOL 60的块结构而为后继者语言所承袭：

AE

ISWIM

{λf.f(b+2c)+f(2b-c)}
[λx.x(x+a)]

{λ(f,b,c).f(b+2c)+f(2b-c)}
[λx.x(x+a),
  u/(u+1),
  v/(v+1)]

{λg.g({λf.f}[λx.ax²+bx+c],
  u/(u+1),
  v/(v+1))}
[λ(f,p,q).f(p+2q,2p-q)]

{λf.f(6)}[Yλf.λn.(n=0)→1 else→nf(n-1)]

{λx.xy(x+y)}
[{λy.a²+a√y}
  [a²+b²]]

{λ(a,b,f).
  {λ(g,h).
    {λk.X}
    [λ(u,v).λx.K]}
  [Yλ(g,h).
    (λx.G,λ(y,z).H)]}
[A,B,λ(x,y).F]

f(b+2c)+f(2b-c)
where f(x) = x(x+a)

f(b+2c)+f(2b-c)
where f(x) = x(x+a)
  and b = u/(u+1)
  and c = v/(v+1)

g(f where f(x) = ax²+bx+c,
  u/(u+1),
  v/(v+1))
where g(f,p,q) = f(p+2q,2p-q)

f(6) where rec f(n) = (n=0)→1; nf(n-1)

xy(x+y)
where x = a²+a√y
  where y = a²+b²

X
where k(u,v)(x) = K
where rec g(x) = G
  and h(y,z) = H
where a = A
  and b = B
  and f(x,y) = F

let f(x) = x(x+a)
f(b+2c)+f(2b-c)

let f(x) = x(x+a)
  and b = u/(u+1)
  and c = v/(v+1)
f(b+2c)+f(2b-c)

let g(f,p,q) = f(p+2q,2p-q)
g((let f(x) = ax²+bx+c; f),
  u/(u+1),
  v/(v+1))

let rec f(n) = ((n=0)→1; nf(n-1)); f(6)

let x =
  let y = a²+b²; a²+a√y
xy(x+y)

let a = A
  and b = B
  and f(x,y) = F
let rec g(x) = G
  and h(y,z) = H
let k(u,v)(x) = K
X

在ISWIM论文中首次出现了用来定义结构的代数数据类型定义，这是通过自然语言的either……or……和……and……描述完成的，但明确的表示出了“积（英语：Product type）之和”^[4]：

ISWIM

Standard ML

an  $\mathrm {aexpression}$  ( $\mathrm {aexp}$ ) is
  either  $simple$ , and has
    a  $body$ , which is an  $\mathrm {identifier}$ 
  or a  $combination$ , in which case it has
    a  $rator$ , which is an  $\mathrm {aexp}$ ,
    and a  $rand$ , which is an  $\mathrm {aexp}$ ,
  or a  $conditional$ , ……
  or  $one{\text{-}}armed$ , ……
  or a  $listing$ , ……
  or  $beet$ , …… .

datatype aexp
  = SIMPLE of body 
  | COMBINATION of rator * rand
  | CONDITIONAL ……
  | ONE_ARMED ……
  | LISTING ……
  | BEET ……
and body = BODY of identifier
and rator = RATOR of aexp
and rand = RAND of aexp

实现和派生

没有进行过直接实现ISWIM的尝试，但Arthur Evans的PAL^[5]，和John C. Reynolds（英语：John C. Reynolds）的Gedanken^[6]，获取了Landin的多数概念，包括强力的控制转移操作，这两者都是动态类型语言。Robin Milner的ML，可以被认为等价于没有J算子，而有类型推论的ISWIM。

从ISWIM衍生出的另一条路线，是去掉指令式特征即赋值和J算子，而只留下纯函数式语言^[7]，接着就有可能切换到惰性求值。这条路线导致了SASL、Hope、KRC、Miranda、Haskell、Clean。

引用

^ Landin, P. J. The Next 700 Programming Languages (PDF). Communications of the ACM (Association for Computing Machinery). March 1966, 9 (3): 157–165 [2021-08-28]. S2CID 13409665. doi:10.1145/365230.365257. （原始内容 (PDF)存档于2022-03-23）.
^ Landin, P.J. A Generalization of Jumps and Labels (PDF). 1965.
Thielecke, H. An Introduction to Landin's "A Generalization of Jumps and Labels". Higher-Order and Symbolic Computation. December 1998, 11 (2): 117–123. S2CID 1562780. doi:10.1023/A:1010060315625.
^ Gordon Plotkin（英语：Gordon Plotkin）. Call-by-Name, Call-by Value and the Lambda Calculus (PDF) (报告). 1975 [2020-04-26]. （原始内容 (PDF)存档于2020-02-01）.
^ Turner, D. A., Some History of Functional Programming Languages (PDF), Lecture Notes in Computer Science 7829, Berlin, Heidelberg: Springer Berlin Heidelberg: 1–20, 2013 [2024-01-28], ISBN 978-3-642-40446-7, doi:10.1007/978-3-642-40447-4_1, The ISWIM paper also has the first appearance of algebraic type definitions used to define structures. This is done in words, but the sum-of-products idea is clearly there.
^ Arthur Evans. PAL: a language designed for teaching programming linguistics. Proceedings ACM National Conference. ACM National Conference. Association for Computing Machinery. 1968.
A. Evans. PAL -- A Reference Manual and a Primer (PDF) (报告). Department of Electrical Engineering, Massachusetts Institute of Technology. February 1968 [2021-09-09]. （原始内容 (PDF)存档于2022-03-06）.
A. Evans. Appendix 2.1. The Complete Syntax for PAL (PDF) (报告). February 1968 [2021-09-10]. （原始内容 (PDF)存档于2022-03-06）.
J. M. Wozencraft, A. Evans. Notes on Programming Linguistics (PDF). M.I.T. Department of Electrical Engineering. 1971 [2021-09-11]. （原始内容 (PDF)存档于2022-03-06）.
^ John C. Reynolds（英语：John C. Reynolds）. GEDANKEN: a simple typeless language which permits functional data structures and co-routines (PDF) (报告). Argonne National Laboratory. September 1969 [2021-09-09]. （原始内容 (PDF)存档于2021-09-09）.
John C. Reynolds（英语：John C. Reynolds）. Definitional interpreters for higher-order programming languages. ACM '72: Proceedings of the ACM annual conference - Volume. August 1972 [2022-11-19]. doi:10.1145/800194.805852. （原始内容存档于2022-11-27）.
^ Ivanović, Mirjana; Budimac, Zoran. A definition of an ISWIM-like language via Scheme. ACM SIGPLAN Notices. April 1993, 28 (4): 29–38. S2CID 14379260. doi:10.1145/152739.152743.

本條目部分或全部内容出自以GFDL授權發佈的《自由線上電腦詞典》（FOLDOC）。

[1] Landin, P. J. The Next 700 Programming Languages (PDF). Communications of the ACM (Association for Computing Machinery). March 1966, 9 (3): 157–165 [2021-08-28]. S2CID 13409665. doi:10.1145/365230.365257. （原始内容 (PDF)存档于2022-03-23）.

[2] Landin, P.J. A Generalization of Jumps and Labels (PDF). 1965.
Thielecke, H. An Introduction to Landin's "A Generalization of Jumps and Labels". Higher-Order and Symbolic Computation. December 1998, 11 (2): 117–123. S2CID 1562780. doi:10.1023/A:1010060315625.

[3] Gordon Plotkin（英语：Gordon Plotkin）. Call-by-Name, Call-by Value and the Lambda Calculus (PDF) (报告). 1975 [2020-04-26]. （原始内容 (PDF)存档于2020-02-01）.

[4] Turner, D. A., Some History of Functional Programming Languages (PDF), Lecture Notes in Computer Science 7829, Berlin, Heidelberg: Springer Berlin Heidelberg: 1–20, 2013 [2024-01-28], ISBN 978-3-642-40446-7, doi:10.1007/978-3-642-40447-4_1, The ISWIM paper also has the first appearance of algebraic type definitions used to define structures. This is done in words, but the sum-of-products idea is clearly there.

[5] Arthur Evans. PAL: a language designed for teaching programming linguistics. Proceedings ACM National Conference. ACM National Conference. Association for Computing Machinery. 1968.
A. Evans. PAL -- A Reference Manual and a Primer (PDF) (报告). Department of Electrical Engineering, Massachusetts Institute of Technology. February 1968 [2021-09-09]. （原始内容 (PDF)存档于2022-03-06）.
A. Evans. Appendix 2.1. The Complete Syntax for PAL (PDF) (报告). February 1968 [2021-09-10]. （原始内容 (PDF)存档于2022-03-06）.
J. M. Wozencraft, A. Evans. Notes on Programming Linguistics (PDF). M.I.T. Department of Electrical Engineering. 1971 [2021-09-11]. （原始内容 (PDF)存档于2022-03-06）.

[6] John C. Reynolds（英语：John C. Reynolds）. GEDANKEN: a simple typeless language which permits functional data structures and co-routines (PDF) (报告). Argonne National Laboratory. September 1969 [2021-09-09]. （原始内容 (PDF)存档于2021-09-09）.
John C. Reynolds（英语：John C. Reynolds）. Definitional interpreters for higher-order programming languages. ACM '72: Proceedings of the ACM annual conference - Volume. August 1972 [2022-11-19]. doi:10.1145/800194.805852. （原始内容存档于2022-11-27）.

[7] Ivanović, Mirjana; Budimac, Zoran. A definition of an ISWIM-like language via Scheme. ACM SIGPLAN Notices. April 1993, 28 (4): 29–38. S2CID 14379260. doi:10.1145/152739.152743.

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