ISWIM is an
imperative programming language with a functional core, consisting of a
syntactic sugaring of
lambda calculus to which are added mutable variables and assignment and a powerful control mechanism: the
program point operator. Being based on lambda calculus, ISWIM has
higher-order functions and
lexically scoped variables. The operational semantics of ISWIM are defined using Landin's
SECD machine and use call-by-value, that is
eager evaluation. 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. 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. 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 ==