Design
ISWIM is an imperative 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 J 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. 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 just possible that he had in mind to develop some form of type inference.
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