The evolution of programming languages is the stepwise
introduction of abstractions hiding the underlying computer
hardware and the details of program execution.  Assembly languages
introduce mnemonic instructions and symbolic labels for hiding
machine codes and addresses.  Fortran introduces arrays and
expressions in standard mathematical notation for hiding
registers.  Algol-like languages introduce structured statements
for hiding gotos and jump labels.  Object-oriented languages
introduce visibility levels and encapsulation for hiding the
representation of data and the management of memory.  Along these
lines, declarative languages, the most prominent representatives
of which are functional and logic languages, hide the order of
evaluation by removing assignment and other control statements.  A
declarative program is a set of logical statements describing
properties of the application domain.  The execution of a
declarative program is the computation of the value(s) of an
expression with respect to these properties.  Thus, the
programming effort shifts from encoding the steps for computing a
result to structuring the application data and the relationships
between the application components.