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 Peter John Landin (5 June 1930 – 3 June 2009) was a British computer scientist. He was one of the first to realise that the lambda calculus could be used to model a programming language, an insight that is essential to the development of bo ...

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 with ...

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 Function application, applying and Function composition (computer science), composing Function (computer science), functions. It is a declar ...

languages such as SASL, Miranda, ML,

Haskell Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lan ...

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 s ...

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 usi ...

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 c ...

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 ...

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 functions and

lexically scoped In computer programming, the scope of a name binding (an association of a name to an entity, such as a variable) is the part of a program where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts ...

variables. The operational semantics of ISWIM are defined using Landin's

SECD machine The SECD machine is a highly influential (''see: '') virtual machine and abstract machine intended as a target for functional programming language compilers. The letters stand for Stack, Environment, Control, Dump—the internal registers of the mac ...

and use call-by-value, that is

eager evaluation In a programming language, an evaluation strategy is a set of rules for evaluating expressions. The term is often used to refer to the more specific notion of a ''parameter-passing strategy'' that defines the kind of value that is passed to the f ...

. 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 A computer programming language is said to adhere to the off-side rule of syntax if blocks in that language are expressed by their indentation. The term was coined by Peter Landin, possibly as a pun on the offside rule in association football. ...

and scoping based on

indentation __FORCETOC__ In the written form of many languages, an indentation or indent is an empty space at the beginning of a line to signal the start of a new paragraph. Many computer languages have adopted this technique to designate "paragraphs" or o ...

. 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 linguistics ...

Implementations and derivatives

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

PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...

'', 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 Arthur John Robin Gorell Milner (13 January 1934 – 20 March 2010), known as Robin Milner or A. J. R. G. Milner, was a British computer scientist, and a Turing Award winner.

'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 Function composition, post-composes a Lambda (programming), lambda expression with the continuation to the current lambda-context. The resulting “function” is first ...

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 In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an expression until its value is needed (non-strict evaluation) and which also avoids repeated evaluations (sharing). The b ...

. 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,

, and Clean.

References

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