language (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" (http://www.cs.utah.edu/~wilson/compilers/old/papers/p157-landin.pdf), P.J. Landin, CACM 9(3):157-166, Mar 1966].
(2007-03-20)