ISWIM (If You See What I Mean) is an abstract computer programming language (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 in 1966.[1]
The operational semantics of ISWIM are defined using Landin's SECD machine and use call-by-value, that is eager evaluation.[2] A goal of ISWIM was to look more like mathematical notation, so Landin abandoned ALGOL'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.[3]
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 types found in modern functional languages.[4] 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.
Implementations and derivatives
No direct implementation of ISWIM was completed but Art Evan's language PAL,[5] and John C. Reynolds' language Gedanken,[6] 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 and with type inference.
Another line of descent from ISWIM is to strip out the imperative features (assignment and the J operator) leaving a purely functional language.[7] It then becomes possible to switch to lazy evaluation. This path led to programming languages SASL, Kent Recursive Calculator (KRC), Hope, Miranda, Haskell, and Clean.
^This article is based on material taken from ISWIM at the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
^Turner, D. A. (2013), Some History of Functional Programming Languages, Lecture Notes in Computer Science, vol. 7829, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 1–20, doi:10.1007/978-3-642-40447-4_1, ISBN978-3-642-40446-7, retrieved 2024-01-28, 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
^Evans, Art (1968). "PAL: a language designed for teaching programming linguistics". Proceedings ACM National Conference. ACM National Conference. Association for Computing Machinery.
^Reynolds, John C. (September 1969). GEDANKEN: a simple typeless language which permits functional data structures and co-routines (Report). Argonne National Laboratory.
