The Free On-line Dictionary of Computing (30 December 2018):

ISWIM

    (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)