First-class Function - Language Support

Language Support

Functional programming languages, such as Scheme, ML, Haskell, F#, and Scala all have first-class functions. When Lisp, one of the earliest functional languages, was designed not all aspects of first-class function were then properly understood, resulting in functions being dynamically scoped. The later Common Lisp dialect, does have lexically scoped first-class functions.

Many scripting languages, including Perl, Python, PHP, Lua, Tcl/Tk, JavaScript, Ruby, and Io, have first-class functions.

For imperative languages, a distinction has to be made between Algol and its descendants such as Pascal, the traditional C family, and the modern garbage-collected variants. The Algol family has allowed nested functions and higher-order taking function as arguments, but not higher-order functions that return functions as results (except Algol 68, which allows this). The reason for this was that it was not known how to deal with non-local variables if a nested-function was returned as a result (and Algol 68 produces runtime errors in such cases).

The C family allowed both passing functions as arguments and returning them as results, but avoided any problems by not supporting nested functions. (The gcc compiler allows them as an extension.) As the usefulness of returning functions primarily lies in the ability to return nested functions that have captured non-local variables, instead of top-level functions, these language are generally not considered to have first-class functions.

Modern imperative languages often support garbage-collection making the implementation of first-class functions feasible. First-class function have often only been supported in later revisions of the language, including C# 2.0 and Apple's Blocks extension to C, C++ and Objective C. C++11 has added support for anonymous functions and closures to the language, but the non-garbage collected nature of the language and capture semantics of non-local variables still make it problematic for functions to be returned as results.

Language		Higher-order functions		Non-local variables			Partial application	Notes
Language		Arguments	Results	Nested functions	Anonymous functions	Closures	Partial application	Notes
Algol family	ALGOL 60	Yes	No	Yes	No	No	No	Have function types.
	ALGOL 68	Yes	Yes	Yes	Yes	No	No
	Pascal	Yes	No	Yes	No	No	No
	Oberon	Yes	Non-nested only	Yes	No	No	No
C family	C	Yes	Yes	No	No	No	No	Has function pointers.
	C++	Yes	Yes	No	C++11	C++11	No	Has function pointers, function objects. (Also, see below.)
	C#	Yes	Yes	No	2.0 / 3.0	2.0	No	Has delegates (2.0) and lambda expressions (3.0).
	Objective-C	Yes	Yes	No	2.0 + Blocks	2.0 + Blocks	No	Has function pointers.
	Java	Partial	Partial	No	No	8	No	Has anonymous inner classes.
Functional languages	Lisp	Syntax	Syntax	Yes	Yes	Common Lisp	No	(see below)
	Scheme	Yes	Yes	Yes	Yes	Yes	SRFI 26
	Clojure	Yes	Yes	Yes	Yes	Yes	Yes
	ML	Yes	Yes	Yes	Yes	Yes	Yes
	Haskell	Yes	Yes	Yes	Yes	Yes	Yes
	Scala	Yes	Yes	Yes	Yes	Yes	Yes
Scripting languages	JavaScript	Yes	Yes	Yes	Yes	Yes	ECMAScript 5	Partial application possible with user-land code on ES3
	PHP	Yes	Yes	Unscoped	5.3	5.3	No	(see below)
	Perl	Yes	Yes	6	Yes	Yes	6
	Python	Yes	Yes	Yes	Partial	Yes	2.5	(see below)
	Ruby	Syntax	Syntax	Unscoped	Yes	Yes	1.9	(see below)
Other languages	Mathematica	Yes	Yes	Yes	Yes	Yes	No
	Smalltalk	Yes	Yes	Yes	Yes	Yes	Partial	Partial application possible through library.
	Fortran	Yes	Yes	Yes	No	No	No

C++: C++11 closures can capture non-local variables by copy or by reference, but without extending their lifetime.
Java: Java 8 closures can only capture immutable ("effectively final") non-local variables.
Lisp: Lexically scoped Lisp variants support closures, dynamically scoped variants do not.; In Common Lisp, the identifier of a function in the function namespace cannot be used as a first-class value. It must be converted into a value using the (function ) or #' operators. Conversely, to apply a function encapsulated in such a way as a value, one must use the (funcall ) operator instead of calling it directly.
PHP: Nested functions are created at global scope once the outer function is executed.
Python: Python's anonymous functions can only have an expression as the body; Explicit partial application with functools.partial since version 2.5, and operator.methodcaller since version 2.6.
Ruby: The identifier of a regular "function" in Ruby (which is really a method) cannot be used as a value or passed. It must first be retrieved into a Method or Proc object to be used as first-class data. The syntax for calling such a function object differs from calling regular methods.; Nested method definitions do not actually nest the scope.; Explicit currying with Proc#curry.

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