In
computer programming
Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...
, one of the many ways that
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
s are colloquially classified is whether the language's
type system
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
makes it strongly typed or weakly typed (loosely typed). However, there is no precise technical definition of what the terms mean and different authors disagree about the implied meaning of the terms and the relative rankings of the "strength" of the type systems of mainstream programming languages. For this reason, writers who wish to write unambiguously about type systems often eschew the terms "strong typing" and "weak typing" in favor of specific expressions such as "
type safety".
Generally, a strongly typed language has stricter typing rules at
compile time
In computer science, compile time (or compile-time) describes the time window during which a computer program is compiled.
The term is used as an adjective to describe concepts related to the context of program compilation, as opposed to concept ...
, which implies that errors and exceptions are more likely to happen during compilation. Most of these rules affect variable assignment, function return values, procedure arguments and function calling.
Dynamically typed
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
languages (where
type checking happens at
run time) can also be strongly typed. Note that in dynamically typed languages, values, rather than variables, have types.
A weakly typed language has looser typing rules and may produce unpredictable or even erroneous results or may perform implicit type conversion at runtime. Advocates of dynamically typed (generally "weakly typed") languages find such concerns to be overblown and believe that static typing actually introduces an exponentially larger set of problems and inefficiencies. A different but related concept is
latent typing
In computer programming, latent typing refers to a type system where types are associated with values and not variables. An example latently typed language is Scheme. This typically requires run-time type checking and so is commonly used synonym ...
.
History
In 1974,
Liskov and S. Zilles defined a strongly-typed language as one in which "whenever an object is passed from a calling function to a called function, its type must be compatible with the type declared in the called function."
In 1977, K. Jackson wrote, "In a strongly typed language each data area will have a distinct type and each process will state its communication requirements in terms of these types."
Definitions of "strong" or "weak"
A number of different language design decisions have been referred to as evidence of "strong" or "weak" typing. Many of these are more accurately understood as the presence or absence of
type safety,
memory safety
Memory safety is the state of being protected from various software bugs and Vulnerability (computing), security vulnerabilities when dealing with random-access memory, memory access, such as buffer overflows and dangling pointers. For example, J ...
,
static type-checking, or
dynamic type-checking.
"Strong typing" generally refers to use of programming language types in order to both capture invariants of the code, and ensure its correctness, and definitely exclude certain classes of programming errors. Thus there are many "strong typing" disciplines used to achieve these goals.
Implicit type conversions and "type punning"
Some programming languages make it easy to use a value of one type as if it were a value of another type. This is sometimes described as "weak typing".
For example, Aahz Maruch observes that "
Coercion
Coercion () is compelling a party to act in an involuntary manner by the use of threats, including threats to use force against a party. It involves a set of forceful actions which violate the free will of an individual in order to induce a desi ...
occurs when you have a
statically typed
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
language and you use the syntactic features of the language to force the usage of one type as if it were a different type (consider the common use of void* in C). Coercion is usually a symptom of weak typing. Conversion, on the other hand, creates a brand-new object of the appropriate type."
As another example,
GCC describes this as
type-punning and warns that it will break strict
aliasing. Thiago Macieira discusses several problems that can arise when type-punning causes the
compiler
In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
to make inappropriate
optimizations.
There are many examples of languages that allow
implicit type conversion
In computer science, type conversion, type casting, type coercion, and type juggling are different ways of changing an expression from one data type to another. An example would be the conversion of an integer value into a floating point valu ...
s, but in a type-safe manner. For example, both C++ and C# allow programs to define operators to convert a value from one type to another with well-defined semantics. When a C++ compiler encounters such a conversion, it treats the operation just like a function call. In contrast, converting a value to the C type is an unsafe operation that is invisible to the compiler.
Pointers
Some programming languages expose
pointers as if they were numeric values, and allow users to perform arithmetic on them. These languages are sometimes referred to as "weakly typed", since pointer arithmetic can be used to bypass the language's type system.
Untagged unions
Some programming languages support
untagged unions, which allow a value of one type to be viewed as if it were a value of another type.
Static type-checking
In
Luca Cardelli's article ''Typeful Programming'', a "strong type system" is described as one in which there is no possibility of an unchecked runtime type error. In other writing, the absence of unchecked run-time errors is referred to as ''safety'' or ''type safety'';
Tony Hoare
Sir Charles Antony Richard Hoare (Tony Hoare or C. A. R. Hoare) (born 11 January 1934) is a British computer scientist who has made foundational contributions to programming languages, algorithms, operating systems, formal verification, and c ...
's early papers call this property ''security''.
Dynamic type-checking
Some programming languages do not have static type-checking. In many such languages, it is easy to write programs that would be rejected by most static type-checkers. For example, a variable might store either a number or the Boolean value "false".
Variation across programming languages
Note that some of these definitions are contradictory, others are merely conceptually independent, 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 (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
,
Pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
,
Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, Tur ...
and
C require all
variables to have a declared type, and support the use of explicit casts of arithmetic values to other arithmetic types. Java, C#, Ada and Pascal are sometimes 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 methods of evading the static type system in Java are controlled by the
Java virtual machine
A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes ...
's type system. C# and VB.NET are similar to Java in that respect, though they allow disabling of 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.
*
Smalltalk
Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by Alan Ka ...
,
Ruby
A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sa ...
,
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
, and
Self
The self is an individual as the object of that individual’s own reflective consciousness. Since the ''self'' is a reference by a subject to the same subject, this reference is necessarily subjective. The sense of having a self—or ''selfhood ...
are all "strongly typed" in the sense that typing errors are prevented at runtime and they do little implicit
type conversion, but these languages make no use of static type checking: the compiler does not check or enforce type constraint rules. The term
duck typing is now used to describe the
dynamic typing paradigm used by the languages in this group.
* The
Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lisping ...
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 (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
or
Clojure do support various forms of type declarations and some compilers (
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 f ...
and related) use these declarations together with
type inference
Type inference refers to the automatic detection of the type of an expression in a formal language. These include programming languages and mathematical type systems, but also natural languages in some branches of computer science and linguistics ...
to enable various optimizations and also limited forms of compile time type checks.
*
Standard ML
Standard ML (SML) 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 the ...
,
F#,
OCaml
OCaml ( , formerly Objective Caml) is a general-purpose programming language, general-purpose, multi-paradigm programming language which extends the Caml dialect of ML (programming language), ML with object-oriented programming, object-oriented ...
,
Haskell,
Go and
Rust
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH ...
are statically type-checked, but the compiler automatically infers a precise type for most values.
*
Assembly language
In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
and
Forth
Forth or FORTH may refer to:
Arts and entertainment
* ''forth'' magazine, an Internet magazine
* ''Forth'' (album), by The Verve, 2008
* ''Forth'', a 2011 album by Proto-Kaw
* Radio Forth, a group of independent local radio stations in Scotla ...
can be characterized as ''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.
See also
*
Comparison of programming languages
*
Data type
In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most progra ...
includes a more thorough discussion of typing issues
*
Design by contract (strong typing as implicit contract form)
*
Latent typing
In computer programming, latent typing refers to a type system where types are associated with values and not variables. An example latently typed language is Scheme. This typically requires run-time type checking and so is commonly used synonym ...
*
Memory safety
Memory safety is the state of being protected from various software bugs and Vulnerability (computing), security vulnerabilities when dealing with random-access memory, memory access, such as buffer overflows and dangling pointers. For example, J ...
*
Type safety
*
Type system
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
References
{{DEFAULTSORT:Strong and Weak Typing
Type systems