ISWIM (If You See What I Mean) is an abstract computer

programming language A programming language is a system of notation for writing computer programs. Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...

(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 both ...

and first described in his article "The Next 700 Programming Languages", published in the ''

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

'' 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 declarat ...

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 pioneered several programming language ...

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.

Design

ISWIM is an

imperative programming In computer science, imperative programming is a programming paradigm of software that uses Statement (computer science), statements that change a program's state (computer science), state. In much the same way that the imperative mood in natural ...

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 In mathematical logic, the lambda calculus (also written as ''λ''-calculus) is a formal system for expressing computability, computation based on function Abstraction (computer science), abstraction and function application, application using var ...

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 itself ...

s 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: '' Landin's contribution'') virtual machine and abstract machine intended as a target for compilers of functional programming languages. The letters stand for stack, environment, control, dump, respe ...

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 ...

. 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 The off-side rule describes syntax of a computer programming language that defines the bounds of a code block via indentation. The term was coined by Peter Landin, possibly as a pun on the offside law in association football. An off-side ...

and scoping based on

indentation __FORCETOC__ In the written form of many languages, indentation describes empty space ( white space) used before or around text to signify an important aspect of the text such as: * Beginning of a paragraph * Hierarchy subordinate concept * Qu ...

. 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 data type, i.e., a data type formed by combining other types. Two common classes of algebraic types are product ty ...

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, sometimes called type reconstruction, 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 bran ...

Implementations and derivatives

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

PAL Phase Alternating Line (PAL) is a color encoding system for analog 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) was a British computer scientist, and a Turing Award winner.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 In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an Expression (computer science), expression until its value is needed (non-strict evaluation) and which avoids repeated eva ...

. 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 i ...

(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 own life, or the world at large. As a verb, Merriam-Webster defines ''hope'' as "to expect with confid ...

, 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