Editing Reference (computer science) (section)

===Functional languages===
In [[Standard ML]], [[OCaml]], and many other functional languages, most values are persistent: they cannot be modified by assignment. Assignable "reference cells" provide [[mutable|mutable variables]], data that can be modified. Such reference cells can hold any value, and so are given the [[polymorphism (computer science)|polymorphic]] type <code>α ref</code>, where <code>α</code> is to be replaced with the type of value pointed to. These mutable references can be pointed to different objects over their lifetime. For example, this permits building of circular data structures. The reference cell is functionally equivalent to a mutable array of length 1.

To preserve safety and efficient implementations, references cannot be [[Type conversion|type-cast]] in ML, nor can pointer arithmetic be performed. In the functional paradigm, many structures that would be represented using pointers in a language like C are represented using other facilities, such as the powerful [[algebraic datatype]] mechanism. The programmer is then able to enjoy certain properties (such as the guarantee of  immutability) while programming, even though the compiler often uses machine pointers "under the hood".