The Free On-line Dictionary of Computing (30 December 2018):
(FP) A program in a functional language consists
of a set of (possibly recursive) function definitions and
an expression whose value is output as the program's result.
Functional languages are one kind of declarative language.
They are mostly based on the typed lambda-calculus with
constants. There are no side-effects to expression
evaluation so an expression, e.g. a function applied to
certain arguments, will always evaluate to the same value (if
its evaluation terminates). Furthermore, an expression can
always be replaced by its value without changing the overall
result (referential transparency).
The order of evaluation of subexpressions is determined by the
language's evaluation strategy. In a strict
(call-by-value) language this will specify that arguments
are evaluated before applying a function whereas in a
non-strict (call-by-name) language arguments are passed
Programs written in a functional language are generally
compact and elegant, but have tended, until recently, to run
slowly and require a lot of memory.
Examples of purely functional languages are Clean, FP,
Haskell, Hope, Joy, LML, Miranda, and SML. Many
other languages such as Lisp have a subset which is purely
functional but also contain non-functional constructs.
See also lazy evaluation, reduction.
or the same in dvi-format
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