ISWIM (

acronym An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in ''NATO'' (''North Atlantic Treaty Organization''), but sometimes use syllables, as ...

for If you See What I Mean) is an abstract computer

programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...

(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 ''Communications of the ACM'' is the monthly journal of the Association for Computing Machinery (ACM). It was established in 1958, with Saul Rosen as its first managing editor. It is sent to all ACM members. Articles are intended for readers wi ...

in 1966. Although not implemented, it has proved very influential in the development of programming languages, especially

functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions tha ...

languages such as SASL, Miranda, ML, Haskell and their successors, and

dataflow programming In computer programming, dataflow programming is a programming paradigm that models a program as a directed graph of the data flowing between operations, thus implementing dataflow principles and architecture. Dataflow programming languages share ...

languages like

Lucid LUCID (Langton Ultimate Cosmic ray Intensity Detector) is a cosmic ray detector built by Surrey Satellite Technology Ltd and designed at Simon Langton Grammar School for Boys, in Canterbury, England. Its main purpose is to monitor cosmic rays ...

Design

ISWIM is an

imperative programming In computer science, imperative programming is a programming paradigm of software that uses statements that change a program's state. In much the same way that the imperative mood in natural languages expresses commands, an imperative program ...

language with a functional core, consisting of a

syntactic sugar In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...

ing of

lambda calculus Lambda calculus (also written as ''λ''-calculus) is a formal system in mathematical logic for expressing computation based on function abstraction and application using variable binding and substitution. It is a universal model of computation th ...

to which are added mutable variables and assignment and a powerful control mechanism: the ''program point'' operator. Being based on lambda calculus, ISWIM has

higher-order function In mathematics and computer science, a higher-order function (HOF) is a function that does at least one of the following: * takes one or more functions as arguments (i.e. a procedural parameter, which is a parameter of a procedure that is itse ...

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. A goal of ISWIM was to look more like mathematical notation, so Landin abandoned

ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...

's semicolons between statements and begin ... end blocks and replaced them with the off-side rule and scoping based on indentation. A notationally distinctive feature of ISWIM is its use of ''where'' 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, ISWIM was one of the first programming languages to use ''where'' clauses. 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 In computer programming, especially functional programming and type theory, an algebraic data type (ADT) is a kind of composite type, i.e., a type formed by combining other types. Two common classes of algebraic types are product types (i.e., ...

s found in modern functional languages. 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

; but it is also possible that he intended to develop some form of

type inference Type inference refers to the automatic detection of the type of an expression in a formal language. These include programming languages and mathematical type systems, but also natural languages in some branches of computer science and linguistic ...

Implementations and derivatives

No direct implementation of ISWIM was attempted but Art Evan's language '' PAL'', and

John C. Reynolds John Charles Reynolds (June 1, 1935 – April 28, 2013) was an American computer scientist. Education and affiliations John Reynolds studied at Purdue University and then earned a Doctor of Philosophy (Ph.D.) in theoretical physics from Harvard U ...

' language ''Gedanken'', captured most of Landin's concepts, including powerful transfer-of-control operations. Both of these were typed dynamically. Robin Milner's ML may be considered equivalent to ISWIM without the

J operator In computer science, Peter Landin's J operator is a programming construct that post-composes a lambda expression with the continuation to the current lambda-context. The resulting “function” is first-class and can be passed on to subsequent ...

and with

. Another line of descent from ISWIM is to strip out the imperative features (assignment and the J operator) leaving a purely functional language. It then becomes possible to switch to lazy evaluation. This path led to programming languages SASL,

Kent Recursive Calculator KRC (Kent Recursive Calculator) is a lazy functional language developed by David Turner from November 1979 to October 1981 based on SASL, with pattern matching, guards and ZF expressions (now more usually called list comprehensions). Two imp ...

(KRC),

Hope Hope is an optimistic state of mind that is based on an expectation of positive outcomes with respect to events and circumstances in one's life or the world at large. As a verb, its definitions include: "expect with confidence" and "to cherish ...

, Miranda, Haskell, and Clean.

References

{{Reflist Programming languages created in 1966 Academic programming languages Experimental programming languages Functional languages History of computing in the United Kingdom