Editing ISWIM (section)

== 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]].