Computing Dictionary

ISWIM definition

(If You See What I Mean) An influential but unimplemented computer programming language described in the article by Peter J. Landin cited below. Landin attempted to capture all known programming language concepts, including assignment and control operators such as goto and coroutines, within a single lambda calculus based framework.
ISWIM is an imperative language with a functional core, consisting of sugared lambda calculus plus mutable variables and assignment. A powerful control mechanism, Landin's J operator, enables capture of the current continuation (the call/cc operator of Scheme is a simplified version). Being based on lambda calculus ISWIM had higher order functions and lexically scoped variables.
The operational semantics of ISWIM are defined using Landin's SECD machine and use call-by-value (eager evaluation). To make ISWIM look more like mathematical notation, Landin replaced ALGOL's semicolons and begin end blocks with the off-side rule and scoping based on indentation.
An ISWIM program is a single expression qualified by "where" clauses (auxiliary definitions including equations among variables), conditional expressions and function definitions. With CPL, ISWIM was one of the first programming languages to use "where" clauses.
New data types could be defined as a (possibly recursive) sum of products like the algebraic data types found in modern functional languages. ISWIM variables were probably dynamically typed but Landin may have planned some form of type inference.
Concepts from ISWIM appear in Art Evan's PAL and John Reynold's Gedanken, Milner's ML and purely functional languages with lazy evaluation like SASL, Miranda and Haskell.
["The Next 700 Programming Languages" (, P.J. Landin, CACM 9(3):157-166, Mar 1966].
The Free On-line Dictionary of Computing, © Denis Howe 2010
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