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{{Short description|Defines the inputs and outputs for a function, subroutine or method}}
In [[Computer science|computer science]], a '''type signature''' or '''type annotation''' defines the inputs and outputs for a [[Function (computer programming)|function]], [[Subroutine|subroutine]] or method. A type signature includes the number, types, and order of the [[Parameter (computer programming)|arguments]] required by a function. A type signature is typically used during overload resolution for choosing the correct definition of a function to be called among many overloaded forms.

==Examples==
===C/C++===
In [[C (programming language)|C]] and [[C++]], the type signature is [[Declaration (computer programming)|declared]] by what is commonly known as a [[Function prototype|function prototype]]. In C/C++, a function declaration reflects its use; for example, a [[Function pointer|function pointer]] with the signature {{code|lang=C|(int)(char, double)}} would be called as:

<source lang=C>
char c;
double d;
int retVal = (*fPtr)(c, d);
</source>

===Erlang===
In [[Erlang (programming language)|Erlang]], type signatures may be optionally declared, as:<ref name="erlang.org">{{Cite web |url=https://www.erlang.org/doc/reference_manual/typespec.html#specifications-for-functions |title=Erlang Reference Manual  User's Guide Version 13.1.4 |access-date=2023-04-27 |website=erlang.org |archive-url=https://web.archive.org/web/20230127081737/https://www.erlang.org/doc/reference_manual/typespec.html |archive-date=2023-01-27 |url-status=live |at=7.5  Specifications for Functions |language=en}}</ref>

<source lang=Erlang>
-spec function_name(type1(), type2(), ...) -> out_type().
</source>

For example:
<source lang=Erlang>
-spec is_even(number()) -> boolean().
</source>

===Haskell===
A type signature in [[Haskell (programming language)|Haskell]] generally takes the following form:

<source lang=Haskell>
functionName :: arg1Type -> arg2Type -> ... -> argNType
</source>

Notice that the type of the result can be regarded as everything past the first supplied argument. This is a consequence of [[Currying|currying]], which is made possible by Haskell's support for [[First-class function|first-class function]]s; this function requires two inputs where one argument is supplied and the function is "curried" to produce a function for the argument not supplied. Thus, calling {{Haskell|f x}}, where {{Haskell|f :: a -> b -> c}}, yields a new function {{Haskell|f2 :: b -> c}} that can be called {{Haskell|f2 b}} to produce {{Haskell|c}}.

The actual type specifications can consist of an actual type, such as {{Haskell|Integer}}, or a general [[Type variable|type variable]] that is used in parametric polymorphic functions, such as {{Haskell|a}}, or {{Haskell|b}}, or {{Haskell|anyType}}. So we can write something like:

Since Haskell supports [[Higher-order function|higher-order function]]s, functions can be passed as arguments. This is written as:

This function takes in a function with type signature {{Haskell|a -> a}} and returns data of type {{Haskell|a}} out.

===Java===
In the [[Java virtual machine]], ''internal type signatures'' are used to identify methods and classes at the level of the virtual machine code.

Example: The method {{Java|String String.substring(int, int)}} is represented in [[Java bytecode|bytecode]] as {{Java|Ljava/lang/String.substring(II)Ljava/lang/String;}}.

The signature of the {{code|main}} method looks like this:

<source lang="java">
public static void main(String[] args);
</source>

And in the disassembled bytecode, it takes the form of {{Java|Lsome/package/Main/main:([Ljava/lang/String;)V}}

The method signature for the {{code|main()}} method contains three modifiers:
* {{Java|public}} indicates that the {{Java|main()}} method can be called by any object.
* {{Java|static}} indicates that the {{Java|main()}} method is a class method.
* {{Java|void}} indicates that the {{Java|main()}} method has no return value.

==Signature==
A function signature consists of the function prototype. It specifies the general information about a function like the name, scope and parameters. Many [[Programming language|programming languages]] use [[Name mangling|name mangling]] in order to pass along more semantic information from the [[Compiler|compiler]]s to the linkers. In addition to mangling, there is an excess of information in a function signature (stored internally to most compilers) which is not readily available, but may be accessed.<ref>{{cite web|title=C++ Reference: Programming terms|url=http://www.cs.unm.edu/~storm/C++/ProgrammingTerms/FunctionSignatures.html|access-date=3 December 2013}}</ref>

Understanding the notion of a function signature is an important concept for all computer science studies.
* Modern object orientation techniques make use of interfaces, which are essentially templates made from function signatures.
* [[C++]] uses function overloading with various signatures.

The practice of [[Multiple inheritance|multiple inheritance]] requires consideration of the function signatures to avoid unpredictable results.

Computer science theory, and the concept of polymorphism in particular, make much use of the concept of function signature.

In the [[C (programming language)|C programming language,]] a signature is roughly equivalent to its [[Function prototype|prototype definition]].

The term "signature" may carry other meanings in computer science:
* [[File signature]]s can identify or verify the content of a file.
* Database signatures can identify or verify the schema or a version of a database.
* In the [[ML (programming language)|ML]] family of [[Programming language|programming language]]s, "signature" is used as a keyword referring to a construct of the module system that plays the role of an interface.

==Method signature==

In [[Computer programming|computer programming]], especially [[Object-oriented programming|object-oriented programming]], a method is commonly identified by its unique '''method signature''', which usually includes the method name and the number, types, and order of its [[Parameter (computer programming)|parameters]].<ref>{{cite web
| access-date = 2011-05-31
| author = Paul Leahy
| website= About.com Guide
| title = Method Signature
| quote = A method signature is part of the method declaration. It is the combination of the method name and the parameter list.
| url = http://java.about.com/od/m/g/methodsignature.htm}}</ref> A method signature is the smallest type of a method.

===Examples===
====C/C++====
In C/C++, the method signature is the method name and the number and type of its parameters, but it is possible to have a last parameter that consists of an array of values:

<source lang="c">
int printf(const char*, ... );
</source>

Manipulation of these parameters can be done by using the routines in the standard library header {{C-lang|<stdarg.h>}}.

In C++, the return type can also follow the parameter list, which is referred to as a [[Trailing return type|trailing return type]]. The difference is only syntactic; in either case, the resulting signature is identical:
<!-- The missing comma is deliberate -->
<source lang="cpp">
auto printf(const char*... ) -> int;
</source>

====C#====
Similar to the syntax of C, method signatures in [[C Sharp (programming language)|C#]] are composed of a name and the number and type of its parameters, where the last parameter may be an array of values:<ref>
{{cite web
| access-date = 2011-08-03
| date       = 2002-03-25
| first      = Hanspeter
| last       = Mössenböck
| page       = 52
| publisher  = Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik
| title      = Advanced C#: Variable Number of Parameters
| url        = http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/Part1.pdf}}
</ref>

<source lang="csharp">
void Add(out int sum, params int[] value);
[...]
Add(out sum, 3, 5, 7, 11, -1);  // sum == 25
</source>

====Java====
In [[Java (programming language)|Java]], a method signature is composed of a name and the number, type, and order of its parameters. Return types and thrown exceptions are not considered to be a part of the method signature, nor are the names of parameters; they are ignored by the compiler for checking method uniqueness.

The method signatures help distinguish overloaded methods (methods with the same name) in a class. Return types are not included in overloading. Only method signatures should be used to distinguish overloaded methods.<ref>{{Cite web|title=Chapter 4. The class File Format|url=https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.3.4|access-date=2021-10-17|website=docs.oracle.com}}</ref>

For example, the following two methods have different signatures:

<source lang="java">
void doSomething(String[] x); // doSomething(String[])
void doSomething(String x);   // doSomething(String)
</source>

The following two methods both have the same signature:

<source lang="java">
int doSomething(int x);                   // doSomething(int)
void doSomething(int y) throws Exception; // doSomething(int)
</source>

====Julia====
In [[Julia (programming language)|Julia]], function signatures take the following form:

<source lang="julia">
commission(sale::Int, rate::Float64)::Float64
</source>

The types in the arguments are used for the [[Multiple dispatch|multiple dispatch]]. The return type is validated when the function returns a value, and a runtime exception is raised if the type of the value does not agree with the specified type.

Abstract types are allowed and are encouraged for implementing general behavior that is common to all subtypes. The above function can therefore be rewritten as follows. In this case, the function can accept any Integer and Real subtypes accordingly.

<source lang="julia">
commission(sale::Integer, rate::Real)::Real
</source>

Types are completely optional in function arguments. When unspecified, it is equivalent to using the type Any, which is the super-type of all types. It is idiomatic to specify argument types but not return type.

====Objective-C====
In the [[Objective-C]] programming language, method signatures for an object are declared in the interface header file. For example,

<source lang="objc">
- (id)initWithInt:(int)value;
</source>

defines a method {{ObjC|initWithInt}} that returns a general object (an {{ObjC|id}}) and takes one integer argument. Objective-C only requires a type in a signature to be explicit when the type is not {{ObjC|id}}; this signature is equivalent:

<source lang="objc">
- initWithInt:(int)value;
</source>

====Rust====
In [[Rust (programming language)|Rust]], function signatures take the following form:

<source lang="rust">
fn commission(sale: u32, rate: f64) -> f64;
</source>

== See also ==

* {{annotated link|Argument of a function}}
* {{annotated link|Philosophy:Signature (logic)}}

==References==
{{reflist}}

{{DEFAULTSORT:Type Signature}}
<!--Categories-->
[[Category:Type theory]]
[[Category:Subroutines]]

{{Sourceattribution|Type signature}}