Editing ISWIM

{{Short description|Programming language}}
{{Redirect|I See What You Mean|the sculpture in Denver, Colorado|I See What You Mean (Argent)}}
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{{Infobox programming language
| name = ISWIM
| logo = 
| paradigm = [[Imperative programming|Imperative]], [[Functional programming|functional]]
| year = {{Start date and age|1966}}
| designer = [[Peter Landin]] 
| developer = 
| latest_release_version = 
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| typing = 
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| dialects = 
| influenced by = [[ALGOL 60]], [[Lisp (programming language)|Lisp]]
| influenced = [[SASL (programming language)|SASL]], [[Miranda (programming language)|Miranda]], [[ML (programming language)|ML]], [[Haskell]], [[Clean (programming language)|Clean]], [[Lucid (programming language)|Lucid]]
| operating system =
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}}
'''ISWIM''' ('''If You See What I Mean''') is an abstract computer [[programming language]] (or a family of languages) devised by [[Peter Landin]] and first described in his article "The Next 700 Programming Languages", published in the ''[[Communications of the ACM]]'' in 1966.<ref>{{Cite journal |last=Landin |first=P. J. |date=March 1966 |title=The Next 700 Programming Languages |url=https://www.cs.cmu.edu/~crary/819-f09/Landin66.pdf |journal=Communications of the ACM |volume=9 |issue=3 |pages=157–165 |publisher=[[Association for Computing Machinery]]|doi=10.1145/365230.365257 |s2cid=13409665 }}</ref>

Although not implemented, it has proved very influential in the development of programming languages, especially [[functional programming]] languages such as [[SASL (programming language)|SASL]], [[Miranda (programming language)|Miranda]], [[ML (programming language)|ML]], [[Haskell]] and their successors, and [[dataflow programming]] languages like [[Lucid (programming language)|Lucid]].

== Design ==
ISWIM is an [[imperative programming]] language with a functional core, consisting of a [[syntactic sugar]]ing of [[lambda calculus]] to which are added mutable variables and assignment and a powerful control mechanism: the [[J operator|''program point'' operator]]. Being based on lambda calculus, ISWIM has [[higher-order function]]s and [[lexically scoped]] variables.

The operational semantics of ISWIM are defined using Landin's [[SECD machine]] and use call-by-value, that is [[eager evaluation]].<ref>{{Cite report |last=Plotkin |first=Gordon |author-link=Gordon Plotkin |date=1975 |title=Call-by-Name, Call-by Value and the Lambda Calculus |url=http://homepages.inf.ed.ac.uk/gdp/publications/cbn_cbv_lambda.pdf}}</ref> A goal of ISWIM was to look more like mathematical notation, so Landin abandoned [[ALGOL]]'s semicolons between statements and <code>begin</code> ... <code>end</code> blocks and replaced them with the [[off-side rule]] and scoping based on [[Indentation style|indentation]].

A notationally distinctive feature of ISWIM is its use of ''<code>where</code>'' clauses. An ISWIM program is a single expression qualified by ''where'' clauses (auxiliary definitions including equations among variables), conditional expressions and function definitions. Along with [[CPL (programming language)|CPL]], ISWIM was one of the first programming languages to use ''where'' clauses.<ref>{{FOLDOC|ISWIM}}</ref>

A notable semantic feature was the ability to define new data types, as a (possibly recursive) sum of products. This was done using a somewhat verbose natural language style description, but apart from notation amounts exactly to the [[algebraic data type]]s found in modern functional languages.<ref>{{Citation |last=Turner |first=D. A. |title=Some History of Functional Programming Languages |date=2013 |series=Lecture Notes in Computer Science |volume=7829 |pages=1–20 |url=http://dx.doi.org/10.1007/978-3-642-40447-4_1 |access-date=2024-01-28 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/978-3-642-40447-4_1 |isbn=978-3-642-40446-7 |quote=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|url-access=subscription }}</ref> ISWIM variables did not have explicit type declarations and it seems likely (although not explicitly stated in the 1966 paper) that Landin intended the language to be dynamically typed, like LISP and unlike [[ALGOL]]; but it is also possible that he intended to develop some form of [[type inference]].

== Implementations and derivatives ==
No direct implementation of ISWIM was completed but Art Evan's language ''[[PAL (programming language)|PAL]]'',<ref>{{Cite conference |last=Evans |first=Art |date=1968 |title=PAL: a language designed for teaching programming linguistics |book-title=Proceedings ACM National Conference |conference=ACM National Conference |publisher=[[Association for Computing Machinery]]}}</ref> and [[John C. Reynolds]]' language ''Gedanken'',<ref>{{Cite report |last=Reynolds |first=John C. |author-link=John C. Reynolds |date=September 1969 |title=GEDANKEN: a simple typeless language which permits functional data structures and co-routines |publisher=Argonne National Laboratory}}</ref> captured most of Landin's concepts, including powerful transfer-of-control operations. Both of these were [[Type system|typed dynamically]]. [[Robin Milner]]'s [[ML (programming language)|ML]] may be considered equivalent to I<small>SWIM</small> without the [[J operator]] and with [[type inference]].

Another line of descent from ISWIM is to strip out the imperative features (assignment and the J operator) leaving a purely functional language.<ref>{{Cite journal |last1=Ivanović |first1=Mirjana |last2=Budimac |first2=Zoran |date=April 1993 |title=A definition of an ISWIM-like language via Scheme |journal=ACM SIGPLAN Notices |volume=28 |issue=4|pages=29–38 |doi=10.1145/152739.152743 |s2cid=14379260 |doi-access=free }}</ref> It then becomes possible to switch to [[lazy evaluation]]. This path led to programming languages [[SASL (programming language)|SASL]], [[Kent Recursive Calculator]] (KRC), [[Hope (programming language)|Hope]], [[Miranda (programming language)|Miranda]], [[Haskell (programming language)|Haskell]], and [[Clean (programming language)|Clean]].

== References ==
{{Reflist}}

[[Category:Programming languages created in 1966]]
[[Category:Academic programming languages]]
[[Category:Experimental programming languages]]
[[Category:Functional languages]]
[[Category:History of computing in the United Kingdom]]