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Subroutines were implemented in [[Konrad Zuse]]'s [[Z4 (computer)|Z4]] in 1945.
In 1945, [[Alan
In January 1947 John Mauchly presented general notes at 'A Symposium of Large Scale Digital Calculating Machinery'
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== Features ==
{{uncited section|date=August 2025}}
In general, a callable unit is a list of instructions that, starting at the first instruction, executes sequentially except as directed via its internal logic. It can be invoked (called) many times during the [[Execution (computing)|execution]] of a program. Execution continues at the next instruction after the call instruction when it [[Return statement|returns]] control.
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In many contexts, a callable may have [[side-effect (computer science)|side effect]] behavior such as modifying passed or global data, reading from or writing to a [[peripheral device]], accessing a [[computer file|file]], halting the program or the machine, or temporarily pausing program execution.
Side effects are considered
In strictly [[functional programming]] languages such as [[Haskell (programming language)|Haskell]], a function can have no [[Side effect (computer science)|side effects]], which means it cannot change the state of the program. Functions always return the same result for the same input. Such languages typically only support functions that return a value, since there is no value in a function that has neither return value nor side effect.
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=== Nested call {{endash}} recursion ===
If supported by the language, a callable may call itself, causing its execution to suspend while another ''nested'' execution of the same callable executes. [[Recursion (computer science)|Recursion]] is a useful means to simplify some complex algorithms and break down complex problems. Recursive languages provide a new copy of local variables on each call. If the programmer desires the recursive callable to use the same variables instead of using locals, they typically declare them in a shared context such static or global.
Languages going back to [[ALGOL]], [[PL/I]] and [[C (programming language)|C]] and modern languages, almost invariably use a call stack, usually supported by the instruction sets to provide an activation record for each call. That way, a nested call can modify its local variables without affecting any of the suspended calls variables.
Recursion allows direct implementation of functionality defined by [[mathematical induction]] and recursive [[divide and conquer algorithm]]s. Here is an example of a recursive function in C/C++ to find [[Fibonacci
<syntaxhighlight lang="c">
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Some languages, e.g., [[Ada (programming language)|Ada]], [[Pascal (programming language)|Pascal]], [[PL/I]], [[Python (programming language)|Python]], support declaring and defining a function inside, e.g., a function body, such that the name of the inner is only visible within the body of the outer.
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.
=== Reentrancy ===
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* [[Functional programming]]
* [[Fused operation]]
* [[Generator (computer programming)]]
* [[Intrinsic function]]
* [[Lambda function (computer programming)]], a function that is not bound to an identifier
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