ABSTRACT
From a typing point of view, programming languages can be statically typed, dynamically typed or typeless. Barring very low level languages such as assembly languages, no modern programming language is typeless. In statically typed languages, types are associated with (syntactic) entities in a program at compile time. The language C is an example; variables, expressions and functions have types, and programs are checked for type violations at compile time. Dynamically typed languages support such a notion only at runtime. The Lisp language is an example. A Lisp compiler or interpreter associates no types with the expressions in the supplied program; however, the values created at runtime are associated with types, and at runtime care is exercised to ensure that an operation is performed on a value only if the operation is compatible with the type of the value. Thus, a Lisp expression that adds a number to a string can be flagged at runtime as causing a runtime type error. Many statically typed languages — Simula, C++ and Java, for instance — also allow the inspection of the type of runtime values, and branching based on that type, to inject some of the flexibility of dynamic typing into an otherwise statically typed language.
