A note on higher-order functions versus logical variables

A commonly cited advantage of Prolog and similar logic languages is that data structures may contain logical (i.e. free) variables. These variables may be regarded as hooks into the data structure, to places where unification may be used to insert new information into the data structure. We show that the same technique can be used in a functional programming language. A data structure is represented as a function. The formal parameter of the function is used to provide direct access to the part of a data structure where new information is to be inserted.