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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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====Delayed stacking {{anchor|Leaf procedure}}{{anchor|Leaf function}}====
One disadvantage of the call stack mechanism is the increased cost of a procedure call and its matching return.{{ clarify | date = November 2015 | reason = increased relative to what?}} The extra cost includes incrementing and decrementing the stack pointer (and, in some architectures, checking for [[stack overflow]]), and accessing the local variables and parameters by frame-relative addresses, instead of absolute addresses. The cost may be realized in increased execution time, or increased processor complexity, or both.
This overhead is most obvious and objectionable in ''leaf procedures'' or ''[[leaf
<!---If the procedure or function itself uses stack handling commands, outside of the prologue and epilogue, e.g. to store intermediate calculation values, the programmer needs to keep track of the number of 'push' and 'pop' instructions so as not to corrupt the original return address.-->
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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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