Scalar (computing) - Typing

Typing

In statically typed languages such as Java or ML, a variable also has a type, meaning that only certain kinds of values can be stored in it. For example, a variable of type "integer" is prohibited from storing text values.

In dynamically typed languages such as Python, it is values, not variables, which carry type. In Common Lisp, both situations exist simultaneously: a variable is given a type (if undeclared, it is assumed to be T, the universal supertype) which exists at compile time. Values also have types, which can be checked and queried at runtime.

Typing of variables also allows polymorphisms to be resolved at compile time. However, this is different from the polymorphism used in object-oriented function calls (referred to as virtual functions in C++) which resolves the call based on the value type as opposed to the supertypes the variable is allowed to have.

Variables often store simple data, like integers and literal strings, but some programming languages allow a variable to store values of other datatypes as well. Such languages may also enable functions to be parametric polymorphic. These functions operate like variables to represent data of multiple types. For example, a function named length may determine the length of a list. Such a length function may be parametric polymorphic by including a type variable in its type signature, since the amount of elements in the list is independent of the elements' types.