In computer programming, a nested function (or nested procedure or subroutine) is a function which is defined within another function, the enclosing function. Due to simple recursive scope rules, a nested function is itself invisible outside of its immediately enclosing function, but can see (access) all local objects (data, functions, types, etc.) of its immediately enclosing function as well as of any function(s) which, in turn, encloses that function. The nesting is theoretically possible to unlimited depth, although only a few levels are normally used in practical programs.

Nested functions are used in many approaches to structured programming, including early ones, such as ALGOL, Simula 67 and Pascal, and also in many modern dynamic languages and functional languages. However, they are traditionally not supported in the (originally simple) C-family of languages.

Effects

Nested functions assume function scope or block scope. The scope of a nested function is inside the enclosing function, i.e. inside one of the constituent blocks of that function, which means that it is invisible outside that block and also outside the enclosing function. A nested function can access other local functions, variables, constants, types, classes, etc. that are in the same scope, or in any enclosing scope, without explicit parameter passing, which greatly simplifies passing data into and out of the nested function. This is typically allowed for both reading and writing.

Nested functions may in certain situations (and languages) lead to the creation of a closure. If it is possible for the nested function to escape the enclosing function, for example if functions are first class objects 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 automatic variables referenced in closures can therefore not be stack allocated. 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.

Examples

An example using Pascal syntax (with ALGOL, Modula 2, Oberon, Ada, etc. similar):

function E(x: real): real;
    function F(y: real): real;
    begin
        F := x + y
    end;
begin
    E := F(3) + F(4)
end;

The function F is nested within E. Note that E's parameter x is also visible in F (as F is a part of E) while both x and y are invisible outside E and F respectively.

Similarly, in Standard ML:

fun e (x : real) =
  let
    fun f y = x+y
  in
    f 3 + f 4
  end;

One way to write the same example in Haskell syntax:

e :: Float -> Float
e x = f 3 + f 4 where f y = x + y

The same example in GNU C syntax^[1] (C extended with nested functions):

float E(float x)
{
    float F(float y)
    {
        return x + y;
    }
    return F(3) + F(4);
}

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);
}

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);
	}
}

Purpose

Lexically nested function definitions are a form of information hiding and are useful for dividing procedural tasks into subtasks which are only meaningful locally. This avoids cluttering other parts of the program with functions and variables that are unrelated to those parts.

They are typically used as helper functions or as recursive functions inside another function (as in the quicksort example above). This has the structural benefit of organizing the code, avoids polluting the scope, and also allows functions to share state easily.^[3] As nested function can access local variables of the enclosing function, sharing of state is possible without passing parameters to the nested function or use a global variable, simplifying code.

In languages with nested functions, functions may normally also contain local constants, and types (in addition to local variables, parameters, and functions), encapsulated and hidden in the same nested manner, at any level of depth. This may further enhance the code structuring possibilities.

Alternatives

The main alternative to nested functions in languages that lack support for them is to place all relevant functions and variables in a separate module (file) and expose only the top-level wrapper function publicly. In C this will generally be done by using static functions for encapsulation and static variables for communication.^[5] This achieves encapsulation and sharing of state, though not the logical organization given by lexical nesting of functions, and comes at the cost of having a separate file. It is also not possible in more than a single level.

Another alternative is to share state between the functions through function parameters, most often passing references as arguments to avoid the cost of copying. In C this is generally implemented by a pointer to a structure containing the context.^[5] This significantly increases the complexity of the function calls.^[3]

In PHP and other languages the anonymous function is the only alternative: the nested function is declared not as usual function, but by reference, as a local variable. To use local variables in the anonymous function, use closure.

Languages

Well known languages supporting lexically nested functions include:

• ALGOL-based languages such as ALGOL 68, Simula, Pascal, Modula-2, Modula-3, Oberon, Seed7 and Ada
• Modern versions of Lisp (with lexical scope) such as Scheme, and Common Lisp
• ECMAScript (JavaScript and ActionScript)
• Dart^[6]
• Kotlin (local functions^[7])
• Rust
• Scala (nested functions^[8])
• Various degrees of support in scripting languages such as Ruby, Python, Lua, PHP and Perl
• GCC supports nested functions in C, as a language extension.^[9]
• C#, starting with C# 7.0
• The D language, a C-related language with nested functions.
• Fortran, starting with Fortran-90, supports a single level of nested (CONTAINed) subroutines and functions.
• MATLAB (full support)
• Wolfram Language

Implementation

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. 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.^[5][12] 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.

See also

• Call stack
• Closure (computer science)
• Function composition (computer science)
• Inner class
• Nesting (computing)

References

External links

• comp.lang.c FAQ: Nested Functions
• "6.4 Nested procedure and functions". FreePascal documentation.