Strongly-typed programming language - AbsoluteAstronomy.com

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

and computer programming

Computer programming

Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...

, a type system

Type system

A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...

is said to feature strong typing when it specifies one or more restrictions on how operations involving values of different data type

Data type

In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...

s can be intermixed. The opposite of strong typing is weak typing
Weak typing
In computer science, weak typing is a property attributed to the type systems of some programming languages. It is the opposite of strong typing, and consequently the term weak typing has as many different meanings as strong typing does.One of the more common definitions states that weakly typed...

.

Interpretation

Most generally, "strong typing" implies that the programming language

Programming language

A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

places severe restrictions on the intermixing that is permitted to occur, preventing the compiling

Compiler

A compiler is a computer program that transforms source code written in a programming language into another computer language...

or running of source code

Source code

In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

which uses data in what is considered to be an invalid way. For instance, an addition operation may not be used with an integer and string values; a procedure which operates upon linked list

Linked list

In computer science, a linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a datum and a reference to the next node in the sequence; more complex variants add additional links...

s may not be used upon numbers. However, the nature and strength of these restrictions is highly variable.

Example

	Weak Typing	Strong Typing
Pseudocode Pseudocode In computer science and numerical computation, pseudocode is a compact and informal high-level description of the operating principle of a computer program or other algorithm. It uses the structural conventions of a programming language, but is intended for human reading rather than machine reading...	a = 2 b = "2" concatenate(a, b) # Returns "22" add(a, b) # Returns 4	a = 2 b = "2" concatenate(a, b) # Type Error add(a, b) # Type Error concatenate(str(a), b) # Returns "22" add(a, int(b)) # Returns 4
Languages	Perl Perl Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular... , PHP PHP PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document... , Rexx REXX REXX is an interpreted programming language that was developed at IBM. It is a structured high-level programming language that was designed to be both easy to learn and easy to read... , JavaScript JavaScript JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles.... , BASIC BASIC BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....	Java Java Java is an island of Indonesia. With a population of 135 million , it is the world's most populous island, and one of the most densely populated regions in the world. It is home to 60% of Indonesia's population. The Indonesian capital city, Jakarta, is in west Java... , C C (programming language) C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.... , C++ C++ C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell... , Python Python The Pythonidae, commonly known simply as pythons, from the Greek word python-πυθων, are a family of non-venomous snakes found in Africa, Asia and Australia. Among its members are some of the largest snakes in the world... , C#, Vala Vala (programming language) Vala is a programming language created with the goal of bringing modern language features to C, with no added runtime needs and with little overhead, by targeting the GObject object system. It is being developed by Jürg Billeter and Raffaele Sandrini. The syntax borrows heavily from C#...

Meanings in computer literature

Some of the factors which writers have qualified as "strong typing" include:

Absence of unchecked run-time type errors. This definition comes from Luca Cardelli
Luca Cardelli
Luca Cardelli is an Italian computer scientist who is currently an Assistant Director at Microsoft Research in Cambridge, UK. Cardelli is well-known for his research in type theory and operational semantics. Among other contributions he implemented the first compiler for the functional programming...

's article Typeful Programming
Typeful programming
In computer science, typeful programming is a programming style identified by widespread use of type information handled through mechanical typechecking techniques. The concept was introduced in a paper of the same name by Luca Cardelli in 1991....

. In other writing, the absence of unchecked run-time errors is referred to as safety or type safety; Tony Hoare
C. A. R. Hoare
Sir Charles Antony Richard Hoare , commonly known as Tony Hoare or C. A. R. Hoare, is a British computer scientist best known for the development of Quicksort, one of the world's most widely used sorting algorithms...

's early papers call this property security.
Strong guarantees about the run-time behavior of a program before program execution, whether provided by static analysis, the execution semantics of the language or another mechanism.
Type safety; that is, at compile or run time, the rejection of operations or function calls which attempt to disregard data types. In a more rigorous setting, type safety
Type safety
In computer science, type safety is the extent to which a programming language discourages or prevents type errors. A type error is erroneous or undesirable program behaviour caused by a discrepancy between differing data types...

is proved about a formal language by proving progress and preservation
Type safety
In computer science, type safety is the extent to which a programming language discourages or prevents type errors. A type error is erroneous or undesirable program behaviour caused by a discrepancy between differing data types...

.
The guarantee that a well-defined error or exceptional behavior
Exception handling
Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....

(as opposed to an undefined behavior) occurs as soon as a type-matching failure happens at runtime, or, as a special case of that with even stronger constraints, the guarantee that type-matching failures would never happen at runtime (which would also satisfy the constraint of "no undefined behavior" after type-matching failures, since the latter would never happen anyway).
The mandatory requirement, by a language definition, of compile-time
Compile time
In computer science, compile time refers to either the operations performed by a compiler , programming language requirements that must be met by source code for it to be successfully compiled , or properties of the program that can be reasoned about at compile time.The operations performed at...

checks for type constraint violations. That is, the compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...

ensures that operations only occur on operand types that are valid for the operation. However, that is also the definition of static typing, leading some experts to state: "Static typing is often confused with StrongTyping".
Fixed and invariable typing of data objects. The type of a given data object does not vary over that object's lifetime. For example, class
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...

instances
Object (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...

may not have their class altered.
The absence of ways to evade the type system. Such evasions are possible in languages that allow programmer access to the underlying representation of values, i.e., their bit-pattern.
Omission of implicit type conversion
Type conversion
In computer science, type conversion, typecasting, and coercion are different ways of, implicitly or explicitly, changing an entity of one data type into another. This is done to take advantage of certain features of type hierarchies or type representations...

, that is, conversions that are inserted by the compiler on the programmer's behalf. For these authors, a programming language is strongly typed if type conversions are allowed only when an explicit notation, often called a cast, is used to indicate the desire of converting one type to another.
Disallowing any kind of type conversion. Values of one type cannot be converted to another type, explicitly or implicitly.
A complex, fine-grained type system
Type system
A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...

with compound types.
Brian Kernighan
Brian Kernighan
Brian Wilson Kernighan is a Canadian computer scientist who worked at Bell Labs alongside Unix creators Ken Thompson and Dennis Ritchie and contributed to the development of Unix. He is also coauthor of the AWK and AMPL programming languages. The 'K' of K&R C and the 'K' in AWK both stand for...

: "[..]each object in a program has a well-defined type which implicitly defines the legal values of and operations on the object. The language guarantees that it will prohibit illegal values and operations, by some mixture of compile- and run-time checking."

Variation across programming languages

Note that some of these definitions are contradictory, others are merely orthogonal, and still others are special cases (with additional constraints) of other, more "liberal" (less strong) definitions. Because of the wide divergence among these definitions, it is possible to defend claims about most programming languages that they are either strongly or weakly typed. For instance:

Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

, C#, Pascal
Pascal (programming language)
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...

, Ada
Ada (programming language)
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...

and C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

require all variables
Variable (programming)
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...

to have a defined type and support the use of explicit casts of arithmetic values to other arithmetic types. Java, C#, Ada and Pascal are often said to be more strongly typed than C, a claim that is probably based on the fact that C supports more kinds of implicit conversions, and C also allows pointer values to be explicitly cast while Java and Pascal do not. Java itself may be considered more strongly typed than Pascal as manners of evading the static type system in Java are controlled by the Java Virtual Machine's
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...

dynamic type system. C# is similar to Java in that respect, though it allows disabling dynamic type checking by explicitly putting code segments in an "unsafe context". Pascal's type system has been described as "too strong", because the size of an array or string is part of its type, making some programming tasks very difficult.
The object-oriented programming languages Smalltalk
Smalltalk
Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...

, Ruby
Ruby (programming language)
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...

, Python
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...

, and Self are all "strongly typed" in the sense that typing errors are prevented at runtime and they do little implicit type conversion
Type conversion
In computer science, type conversion, typecasting, and coercion are different ways of, implicitly or explicitly, changing an entity of one data type into another. This is done to take advantage of certain features of type hierarchies or type representations...

, but these languages make no use of static type checking: the compiler does not check or enforce type constraint rules. The term duck typing
Duck typing
In computer programming with object-oriented programming languages, duck typing is a style of dynamic typing in which an object's current set of methods and properties determines the valid semantics, rather than its inheritance from a particular class or implementation of a specific interface...

is now used to describe the dynamic typing paradigm used by the languages in this group.
The Lisp family of languages are all "strongly typed" in the sense that typing errors are prevented at runtime. Some Lisp dialects like Common Lisp
Common Lisp
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...

do support various forms of type declarations and some compilers (CMUCL
CMUCL
CMUCL is a free Common Lisp implementation, originally developed at Carnegie Mellon University.CMUCL runs on most Unix-like platforms, including Linux and BSD; there is an experimental Windows port as well. Steel Bank Common Lisp is derived from CMUCL...

and related) use these declarations together with type inference to enable various optimizations and also limited forms of compile time type checks.
Standard ML
Standard ML
Standard ML is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of theorem provers.SML is a modern descendant of the ML...

, Objective Caml
Objective Caml
OCaml , originally known as Objective Caml, is the main implementation of the Caml programming language, created by Xavier Leroy, Jérôme Vouillon, Damien Doligez, Didier Rémy and others in 1996...

and Haskell
Haskell (programming language)
Haskell is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. In Haskell, "a function is a first-class citizen" of the programming language. As a functional programming language, the...

have purely static type systems, in which the compiler automatically infers a precise type for all values. These languages (along with most functional
Functional programming
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...

languages) are considered to have stronger type systems than Java, as they permit no implicit type conversions. While OCaml's libraries allow one form of evasion (Object magic), this feature remains unused in most applications.
Visual Basic
Visual Basic
Visual Basic is the third-generation event-driven programming language and integrated development environment from Microsoft for its COM programming model...

is a hybrid language. In addition to including statically typed variables, it includes a "Variant" data type that can store data of any type. Its implicit casts are fairly liberal where, for example, one can sum string variants and pass the result into an integer literal.
Assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

and Forth have been said to be untyped. There is no type checking; it is up to the programmer to ensure that data given to functions is of the appropriate type. Any type conversion required is explicit.

For this reason, writers who wish to write unambiguously about type systems often eschew the term "strong typing" in favor of specific expressions such as "type safety".

Interpretation

Example

Meanings in computer literature

Variation across programming languages

See also