Type signature

Key takeaways

Type signature belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Type signature to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Type signature from memory before moving on to harder problems.

Reference excerpt

In computer science, a type signature or type annotation defines the inputs and outputs of a function, subroutine or method. A type signature includes the number, types, and order of the function's arguments. One important use of a type signature is for function overload resolution, where one particular definition of a function to be called is selected among many overloaded forms.

Examples

C/C++ In C and C++, the type signature is declared by what is commonly known as a function prototype. In C/C++, a function declaration reflects its use; for example, a function pointer with the signature (int)(char, double) would be called as:

Erlang In Erlang, type signatures may be optionally declared, as:

For example:

Haskell A type signature in Haskell generally takes the following form:

Notice that the type of the result can be regarded as everything past the first supplied argument. This is a consequence of currying, which is made possible by Haskell's support for first-class functions; 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 f x, where f :: a -> b -> c, yields a new function f2 :: b -> c that can be called f2 b to produce c. The actual type specifications can consist of an actual type, such as Integer, or a general type variable that is used in parametric polymorphic functions, such as a, or b, or anyType. So we can write something like: functionName :: a -> a -> ... -> a Since Haskell supports higher-order functions, functions can be passed as arguments. This is written as: functionName :: (a -> a) -> a This function takes in a function with type signature a -> a and returns data of type 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 String String.substring(int, int) is represented in bytecode as Ljava/lang/String.substring(II)Ljava/lang/String;. The signature of the main method looks like this:

And in the disassembled bytecode, it takes the form of Lsome/package/Main/main:([Ljava/lang/String;)V The method signature for the main() method contains three modifiers:

public indicates that the main() method can be called by any object. static indicates that the main() method is a class method. void indicates that the 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 languages use name mangling in order to pass along more semantic information from the compilers 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. 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 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, a signature is roughly equivalent to its prototype definition. In the ML family of programming languages, "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, especially 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 parameters. A method signature is the smallest type of a method.

Examples

C In C, the method signature is the method name and the number and type of its parameters, but it is possible to have variadic parameters, but these are not type-safe.

Manipulation of these parameters can be done by using the routines in the standard library header <stdarg.h>.

C++ In C++, in addition to the C-style function return type appearing before, the return type may also follow the parameter list, which is referred to as a trailing return type. The difference is only syntactic; in either case, the resulting signature is identical. If using variadic templates:

Note void doSomething(auto... args); is equivalent to template <typename... Ts> void doSomething(Ts... args);.

C# Similar to the syntax of C, method signatures in C# are composed of a name and the number and type of its parameters, where the last parameter may be an array of values:

Java In 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. For example, the following two methods have different signatures:

The following two methods both have the same signature:

Variadic parameters in Java are represented with ellipses on the final parameter. These behave identically to an array, but in the JVM bytecode the method is marked variadic.

Julia In Julia, function signatures take the following form:

The types in the arguments are used for the 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.

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,

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Type signature

Start with the simplest possible case. Write down what Type signature claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Type signature before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Type signature ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Type signature

In research: Type signature appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Type signature in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Type signature is common in secondary-school and first-year university syllabi. It links to neighbouring topics Subroutines, Type theory, so understanding it makes those chapters shorter.
In everyday life: Look for Type signature outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Type signature in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Type signature means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Type signature out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Type signature in simple terms?

In computer science, a type signature or type annotation defines the inputs and outputs of a function, subroutine or method. A type signature includes the number, types, and order of the function's arguments.

Why does Type signature matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Type signature?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Type signature.