Editing Nested function

{{Short description|A named function defined within a function}}

In [[computer programming]], a '''nested function''' (or '''nested procedure''' or '''subroutine''') is a [[identifier|named]] [[subroutine|function]] that is defined within another, enclosing, block and is [[lexically scoped]] within the enclosing block {{endash}} meaning it is only callable by name within the body of the enclosing block and can use [[identifiers]] declared in outer [[Block (programming)|blocks]], including outer functions. The enclosing block is typically, but not always, another function.
<!-- Is there a clear but more concise wording? -->

[[Programming language]] support for nested functions varies. With respect to [[structured programming]] languages, it is supported in some outdated languages such as [[ALGOL]], [[Simula 67]] and [[Pascal (programming language)|Pascal]] and in the commonly used [[JavaScript]]. It is commonly supported in [[dynamic language|dynamic]] and [[functional language|functional]] languages.
However, it is not supported in some commonly used languages including standard [[C language|C]] and [[C++]].

Other programming technologies provide similar benefit. For example, a [[Lambda function (computer programming)|lambda function]] also allows for a function to be defined inside of a function (as well as elsewhere) and allows for similar data hiding and encapsulation. Notably, a lambda function has no name (is anonymous) and therefore cannot be called by name and has no visibility aspect.

== Attributes ==

The [[Scope (computer science)|scope]] of a nested function is the block that contains it {{endash}} be it a function block or block within a function body. It is not visible (cannot be called by name) outside its containing block.

A nested function can use [[identifiers]] (i.e. the name of functions, variables, types, classes) declared in any enclosing block, except when they are masked by inner declarations with the same names.

A nested function can be declared within a nested function, recursively, to form a deeply nested structure. 
A deeply nested function can access identifiers declared in all of its enclosing blocks, including enclosing functions.

Nested functions may in certain situations lead to the creation of a [[Closure (computer programming)|closure]]. If it is possible for the nested function to [[Escape analysis|escape]] the enclosing function, for example if functions are [[first class object]]s and a nested function is passed to another function or returned from the enclosing function, then a closure is created and calls to this function can access the environment of the original function. The frame of the immediately enclosing function must continue to be alive until the last referencing closure dies and [[non-local variable|non-local]] [[automatic variable]]s referenced in closures can therefore not be [[stack allocation|stack allocated]] in languages that allow the closure to persist beyond the lifetime of the enclosing block. This is known as the [[funarg problem]] and is a key reason why nested functions was not implemented in some simpler languages as it significantly complicates code generation and analysis, especially when functions are nested to various levels, sharing different parts of their environment.

== Value ==

The nested function technology allows a [[programmer]] to write [[source code]] that includes beneficial attributes such as [[information hiding]], [[encapsulation (computer programming)|encapsulation]] and [[Decomposition (computer science)|decomposition]]. The programmer can divide a task into subtasks which are only meaningful within the context of the task such that the subtask functions are hidden from callers that are not designed to use them.

Block scoping allows functions to share the state of enclosing blocks (including enclosing functions) without passing [[Parameter (computer programming)|parameters]] or using [[global variable]]s.{{sfn|Bright|2004}}

== Uses ==

=== Helper ===

A nested function typically acts as a [[wrapper function|helper function]] or a [[recursion (computer science)|recursive function]].

=== Control flow ===

Nested functions can be used for unstructured [[control flow]], by using the return statement for general unstructured control flow. This can be used for finer-grained control than is possible with other built-in features of the language – for example, it can allow early termination of a for loop if <code>break</code> is not available, or early termination of a nested [[for loop]] if a multi-level <code>break</code> or exceptions are not available.

=== Higher-order functions ===

{{Main|Higher-order function}}

In some languages, it is possible to create a nested function that accesses a set of parameters from the outer function, that is a [[Closure (computer programming)|closure]], and have that function be the outer function's return value. Thus it is possible to return a function that is set to fulfill a certain task with little or no further parameters given to it, which can increase performance quite significantly.<ref name="Higher-Order Functions and Lambdas – Kotlin Programming Language">[http://kotlinlang.org/docs/reference/inline-functions.html Higher-Order Functions and Lambdas - Kotlin Programming Language]</ref>

== Examples ==

=== Simple example ===

A simple example in Pascal:
<syntaxhighlight lang=pascal>
function E(x: real): real;
    function F(y: real): real;
    begin
        F := x + y
    end;
begin
    E := F(3) + F(4)
end;
</syntaxhighlight>
The function <code>F</code> is nested within <code>E</code>. Note that <code>E</code>'s parameter <code>x</code> is also visible in <code>F</code> (as <code>F</code> is a part of <code>E</code>) while both <code>x</code> and <code>y</code> are invisible outside <code>E</code> and <code>F</code> respectively.

Similarly, in [[Standard ML]]:

<syntaxhighlight lang=ocaml>
fun e (x : real) =
  let
    fun f y = x+y
  in
    f 3 + f 4
  end;
</syntaxhighlight>

In [[Haskell (programming language)|Haskell]]:

<syntaxhighlight lang=haskell>
e :: Float -> Float
e x = f 3 + f 4 where f y = x + y
</syntaxhighlight>

In [[PL/I]]:

{{pre|
e: procedure(x) returns(float);
  declare x float;
  f: procedure(y) returns(float);
    declare y float;
    return x + y
    end;
  return f(3.0) + f(4.0);
  end;
}}

In [[Python (programming language)|Python]]:

<syntaxhighlight lang=python>
def e(x: float) -> float:
    def f(y: float) -> float:
        return x + y
    return f(3.0) + f(4.0)
</syntaxhighlight>

In [[GNU Compiler Collection|GNU C]]<ref>{{cite book |last1=Rothwell |first1=Trevis J. |title=The GNU C Reference Manual |date=2011 |publisher=Free Software Foundation, Inc |page=63}}</ref> {{endash}} which extends standard C with nested functions:

<syntaxhighlight lang=c>
float E(float x)
{
    float F(float y)
    {
        return x + y;
    }
    return F(3) + F(4);
}
</syntaxhighlight>

=== Quicksort ===

The following is an implementation of [[quicksort]]:<ref>
{{usurped|1=[https://archive.today/20130703055646/http://www.dreamincode.net/forums/topic/262883-nesting-functions-why/page__p__1530693&%23entry1530693 Re: Nesting functions- Why?]}}, {{usurped|1=[https://web.archive.org/web/20100405113726/http://www.dreamincode.net/forums/user/52176-baavgai/ baavgai]}}, 14 January 2012</ref><!-- code tweaked for readability -->
<syntaxhighlight lang=C>
void sort(int *items, int size) {
    void quickSort(int first, int last) {
        void swap(int p, int q) {
            int tmp = items[p];
            items[p] = items[q];
            items[q] = tmp;
        }
        
        int partition() {
            int pivot = items[first], index = first;
            swap(index, last);
            for (int i = first; i < last; i++)
                if (items[i] < pivot)
                    swap(index++, i);
            swap(index, last);
            return index;
        }

        if (first < last) {
            int pivotIndex = partition();
            quickSort(first, pivotIndex - 1);
            quickSort(pivotIndex + 1, last);
        }
    }
    quickSort(0, size - 1);
}
</syntaxhighlight>

The following is an implementation of the [[Quicksort#Hoare partition scheme|Hoare partition based quicksort]] using [[C++11#Lambda functions and expressions|C++11]] [[Anonymous function#C.2B.2B .28since C.2B.2B11.29|lambda expression syntax]] which is an alternative technology that also allows hiding a function inside a function:

<syntaxhighlight lang=c++>
template<typename RandomAccessIterator>
auto Sort(RandomAccessIterator Begin, RandomAccessIterator End)->void {
	auto Partition = [&]() {
		//Hoare partition scheme
		auto &Pivot = *Begin;
		auto ForwardCursor = Begin;
		auto BackwardCursor = End - 1;
		auto PartitionPositionFound = false;
		auto LocatePartitionPosition = [&]() {
			while (*ForwardCursor < Pivot)
				++ForwardCursor;
			while (Pivot < *BackwardCursor)
				--BackwardCursor;
			if (ForwardCursor >= BackwardCursor)
				PartitionPositionFound = true;
			else
				Swap(*ForwardCursor, *BackwardCursor);
		};
		//Trivial helper function
		auto MoveOnAndTryAgain = [&]() {
			++ForwardCursor;
			--BackwardCursor;
		};
		//Brief outline of the actual partition process
		while (true) {
			LocatePartitionPosition();
			if (PartitionPositionFound)
				return BackwardCursor + 1;
			else
				MoveOnAndTryAgain();
		}
	};
	//Brief outline of the quicksort algorithm
	if (Begin < End - 1) {
		auto PartitionPosition = Partition();
		Sort(Begin, PartitionPosition);
		Sort(PartitionPosition, End);
	}
}
</syntaxhighlight>

== Languages ==

Notable languages supporting nested functions include:

*[[ALGOL]]-based languages such as [[ALGOL 68]], [[Simula]], [[Pascal (programming language)|Pascal]], [[Modula-2]], [[Modula-3]], [[Oberon (programming language)|Oberon]], [[PL/I]], [[Seed7]] and [[Ada (programming language)|Ada]]
*Modern versions of [[Lisp (programming language)|Lisp]] (with lexical scope) such as [[Scheme (programming language)|Scheme]], and [[Common Lisp]]
*[[ECMAScript]] ([[JavaScript]] and [[ActionScript]])
*[[Dart (programming language)|Dart]]<ref>{{Cite web|url=https://dart.dev/guides/language/language-tour#lexical-scope|title=A tour of the Dart language}}</ref>
*[[Kotlin (programming language)|Kotlin]] (local functions<ref>{{Cite web|url=https://kotlinlang.org/docs/functions.html#local-functions|title = Functions &#124; Kotlin}}</ref>)
*[[Rust (programming language)|Rust]]
*[[Scala (programming language)|Scala]] (nested functions<ref>{{Cite web|url=https://docs.scala-lang.org/tour/nested-functions.html|title=Nested Methods}}</ref>)
*Various degrees of support in scripting languages such as [[Ruby (programming language)|Ruby]], [[Python (programming language)|Python]], [[Lua (programming language)|Lua]], [[PHP]] and [[Perl]]
*[[GNU Compiler Collection|GCC]] supports nested functions in C, as a language extension.<ref>{{cite web|url=https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html|title=Nested Functions – Using the GNU Compiler Collection (GCC)|accessdate=2007-01-06|publisher=GNU Project}}</ref>
*[[C Sharp (programming language)|C#]], starting with C# 7.0
*The [[D (programming language)|D]] language, a C-related language with nested functions.
*[[Fortran]], starting with [[Fortran#Fortran 90|Fortran-90]], supports ''a single level'' of nested (''CONTAINed'') subroutines and functions.
*[[MATLAB]] (full support)
*[[Wolfram Language]]
*[[Go (programming language)| Golang]] (Function closures<ref>{{Cite web|url=https://go.dev/tour/moretypes/25|title=A tour of Go}}</ref>)

=== Functional languages ===

In most [[functional programming]] languages, such as Scheme, nested functions are a [[Programming idiom|common way]] of implementing [[algorithm]]s with loops in them. A simple ([[tail recursion|tail]]) [[recursion|recursive]] inner function is created, which behaves as the algorithm's main loop, while the outer function performs startup actions that only need to be done once. In more complex cases, a number of mutually recursive functions may be created as inner functions.

== Alternatives ==

Various alternative techniques can be used to achieve similar programming results as via nested functions.

=== Modularity ===

A common alternative is to leverage a language's modularity technology. Some functions are exposed for use outside of the module and some are only visible within the module.

In C, this can be implemented by declaring functions and variables as ''static'' to hide them from code outside the file.<ref name=cfaq>"[http://c-faq.com/misc/nestfcns.html Question 20.24: Why doesn't C have nested functions?], comp.lang.c FAQ</ref> This allows for data hiding, encapsulation and decomposition, but at a different level of [[granularity]] than with nested functions. This modularity does not support more than one level of nesting.

In [[object-oriented languages]], a class typically provides a scope in which functions and state can be hidden from consumers of the class but accessible within the class. Some languages allow classes to be nested.

=== Parameters ===

To implement data hiding, functions can pass around shared data as parameters, but this increases the complexity of function calls.{{sfn|Bright|2004}}

In C, this is generally implemented by passing a pointer to a structure containing the shared data.<ref name=cfaq />

=== Lambda ===

In [[PHP]] and other languages, the [[anonymous function|lambda]] is an alternative. A function is defined in a code statement rather than declared with the usual function syntax. It has no name but is callable via a [[function reference]]. Such functions can be defined inside of a function as well as in other scopes. To use local variables in the anonymous function, use [[Closure (computer science)|closure]].

=== Alternatives by language ===

The following languages provide features that are similar to nested functions:

*[[C++]] {{endash}} classes allow for similar data hiding and encapsulation; defining a class within a class provides similar structure (see [[Function object#In C and C.2B.2B|Function object in C++]])

*[[C++11]] and later {{endash}} via lambda expressions (see quicksort example above)<ref>{{Cite web|url=http://www.rosettacode.org/wiki/Nested_function#C.2B.2B|title = Nested function - Rosetta Code}}</ref>

*[[Eiffel (programming language)|Eiffel]] {{endash}} explicitly disallows nesting of routines to keep the language simple; does allow the convention of using a special variable, '''Result''', to denote the result of a (value-returning) function

*[[C Sharp (programming language)|C#]] and [[Visual Basic .NET|Visual Basic]] {{endash}} via lambda expressions

*[[Java (programming language)|Java]] {{endash}} since Java 8, via [[Anonymous function#Java|lambda expressions]]<ref>{{Cite web|url=http://www.rosettacode.org/wiki/Nested_function#Java|title = Nested function - Rosetta Code}}</ref><!--How can a lambda expression "simulate" nested scopes, in any number of levels, containing any language element (functions, variables, constants, types) ?-->, and in older versions, via an [[anonymous class]] containing a single method

== Implementation ==

{{see also|Man or boy test}}

Implementation of nested functions can be more involved than it may appear, as a reference to a nested function that references non-local variables creates a [[Closure (computer science)|closure]]. For this reason nested functions are not supported in some languages such as C, C++ or Java as this makes compilers more difficult to implement.<ref name=cfaq /><ref>[https://stackoverflow.com/a/1348456/2025416 answer] by Dave Vandervies, Aug 28 '09 at 17:45, to "[https://stackoverflow.com/questions/1348095/why-are-nested-functions-not-supported-by-the-c-standard Why are nested functions not supported by the C standard?]"</ref> However, some compilers do support them, as a compiler specific extension. A well known example of this is the [[GNU C]] implementation of C which shares code with compilers for languages such as Pascal, Ada and Modula.

=== Access of non-local objects ===

There are several ways to implement nested procedures in a lexically scoped language, but the classic way is as follows:

:Any [[non-local object]], X, is reached via access-links in the [[call stack|activation frames]] on the machine stack. The caller, C, assists the called procedure, P, by pushing a ''direct'' link to the ''latest'' activation of P's immediate lexical encapsulation, (P), prior to the call itself. P may then quickly find the right activation for a certain X by following a ''fixed number'' (P.depth – X.depth) of links (normally a small number).

:The caller creates this direct link by (itself) following C.depth – P.depth + 1 older links, leading up to the latest activation of (P), and then ''temporarily'' bridging over these with a direct link to that activation; the link later disappears together with P, whereby the older links beneath it may come into use again.

:Note that P is visible for, and may therefore be called by, C if (P) = C / (C) / ((C)) / etc.

This original method is faster than it may seem, but it is nevertheless often optimized in practical modern compilers (using [[Call stack#Display|''displays'']] or similar techniques).

Another way to implement nested functions that is used by some compilers is to convert ("lift") nested functions into non-nested functions (where extra, hidden, parameters replace the access links) using a process known as [[lambda lifting]] during an intermediate stage in the compilation.

=== Functions as values ===

In order for local functions with [[lexically scoped]] [[non-local variable|nonlocals]] to be passed as results, the language runtime code must also implicitly pass the environment (data) that the function sees inside its encapsulating function, so that it is reachable also when the current activation of the enclosing function no longer exists.<ref>Such a combination of function code and its environment is sometimes called a [[Closure (computer programming)|closure]].</ref> This means that the environment must be stored in another memory area than (the subsequently reclaimed parts of) a chronologically based execution stack, which, in turn, implies some sort of freely [[dynamic memory allocation]]. Many older Algol based languages (or dialects thereof) does therefore not allow local functions that access nonlocals to be passed as return values, or do they not allow functions as return values at all, although passing of such functions as arguments may still be possible.

=== No-execute stacks ===

[[GNU Compiler Collection|GCC]]'s implementation of nested functions causes a loss of [[NX bit|no-execute]] [[Call stack|stacks]] (NX stacks). This implementation calls nested functions through a [[jump instruction]] placed on the machine stack at runtime. This requires the stack to be executable. No-execute stacks and nested functions are therefore mutually exclusive in GCC. If a nested function is used in the development of a program, then the NX stack is silently lost, unless GCC is called with the <code>&#8209;Wtrampoline</code> option to alert of the condition. Software engineered using [[Software Development Security|Secure Development Lifecycle]] often do not allow the use of nested functions in this particular compiler due to the loss of NX stacks.<ref>{{cite web
 | url = http://www.owasp.org/index.php/C-Based_Toolchain_Hardening#GCC.2FBinutils
 | title = C-Based Toolchain Hardening
 | last = Walton
 | first = Jeffrey
 | publisher = The Open Web Application Security Project (OWASP)
 | access-date = 28 February 2017
}}</ref>

== See also ==

*[[Call stack]]
*[[Closure (computer science)]]
*[[Function composition (computer science)]]
*[[Inner class]]
*[[Nesting (computing)]]

== References ==

{{reflist}}
{{refbegin}}
* {{cite web |url=http://www.drdobbs.com/nested-functions/184401792 |title=Nested Functions |first=Walter |last=Bright |date=1 May 2004 |work=[[Dr. Dobb's]] }}
{{refend}}

== External links ==
* [http://c-faq.com/misc/nestfcns.html comp.lang.c FAQ: Nested Functions]
* [http://www.freepascal.org/docs-html/prog/progse23.html "6.4 Nested procedure and functions"]. FreePascal documentation.

[[Category:Source code]]
[[Category:Subroutines]]