ISWIM
 | This article includes a list of general references, but it lacks sufficient corresponding inline citations. (December 2011) |
| Paradigm | Imperative, functional |
|---|---|
| Designed by | Peter Landin |
| First appeared | 1966 |
| Influenced by | |
| ALGOL 60, Lisp | |
| Influenced | |
| SASL, Miranda, ML, Haskell, Clean, Lucid | |
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]
Although not implemented, it has proved very influential in the development of programming languages, especially functional programming languages such as SASL, Miranda, ML, Haskell and their successors, and dataflow programming languages like Lucid.
Design
ISWIM is an
The operational semantics of ISWIM are defined using Landin's
begin ... end blocks and replaced them with the off-side rule and scoping based on indentationA 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 types found in modern functional languages.[3] 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 attempted but Art Evan's language PAL,[4] and John C. Reynolds' language Gedanken,[5] 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.
References
- S2CID 13409665.
- ^ Plotkin, Gordon (1975). Call-by-Name, Call-by Value and the Lambda Calculus (PDF) (Report).
- ISBN 978-3-642-40446-7, 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.
- S2CID 14379260.