Type system

dynamicstatictype checkingdynamically typedstatically typeddynamic typingstatic typingtypedtype systemsstatically-typed
In programming languages, a type system is a set of rules that assigns a property called type to the various constructs of a computer program, such as variables, expressions, functions or modules.wikipedia
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C (programming language)

CC programming languageC language
An example of a simple type system is that of the C language.
C (, as in the general-purpose, procedural computer programming language supporting structured programming, lexical variable scope, and recursion, while a static type system prevents unintended operations.

Extended static checking

optional tools
Type systems are often specified as part of programming languages, and built into the interpreters and compilers for them; although the type system of a language can be extended by optional tools that perform added kinds of checks using the language's original type syntax and grammar.
ESC can be thought of as an extended form of type checking.

Type theory

theory of typestypestype
Type theory is the study of type systems.
Type theory is closely related to (and in some cases overlaps with) type systems, which are a programming language feature used to reduce bugs.

Data type

typedatatypetypes
In programming languages, a type system is a set of rules that assigns a property called type to the various constructs of a computer program, such as variables, expressions, functions or modules. An implementation of a type system could in theory associate identifications called data type (a type of a value), class (a type of an object), and kind (a type of a type, or metatype).
A data type also represents a constraint placed upon the interpretation of data in a type system, describing representation, interpretation and structure of values or objects stored in computer memory.

Haskell (programming language)

HaskellHaskell programming languageHackage
Haskell is also strongly typed but its types are automatically inferred so that explicit conversions are often (but not always) unnecessary. However, a language can be statically typed without requiring type declarations (examples include Haskell, Scala, OCaml, F#, and to a lesser extent C# and C++), so explicit type declaration is not a necessary requirement for static typing in all languages.
Haskell is a statically typed, purely functional programming language with type inference and lazy evaluation.

Variable (computer science)

variablevariablesscalar
In programming languages, a type system is a set of rules that assigns a property called type to the various constructs of a computer program, such as variables, expressions, functions or modules.
Depending on the type system of a programming language, variables may only be able to store a specified datatype (e.g. integer or string).

Programming language

programming languageslanguagedialect
Type systems are often specified as part of programming languages, and built into the interpreters and compilers for them; although the type system of a language can be extended by optional tools that perform added kinds of checks using the language's original type syntax and grammar. In programming languages, a type system is a set of rules that assigns a property called type to the various constructs of a computer program, such as variables, expressions, functions or modules.
Many important restrictions of this type, like checking that identifiers are used in the appropriate context (e.g. not adding an integer to a function name), or that subroutine calls have the appropriate number and type of arguments, can be enforced by defining them as rules in a logic called a type system.

Dependent type

dependent typesdependently typeddependent
A programming language compiler can also implement a dependent type or an effect system, which enables even more program specifications to be verified by a type checker. Advocates of dependent typing, implemented in languages such as Dependent ML and Epigram, have suggested that almost all bugs can be considered type errors, if the types used in a program are properly declared by the programmer or correctly inferred by the compiler.
It is an overlapping feature of type theory and type systems.

Software bug

bugsbugsoftware bugs
The main purpose of a type system is to reduce possibilities for bugs in computer programs by defining interfaces between different parts of a computer program, and then checking that the parts have been connected in a consistent way.
Programming languages include features to help prevent bugs, such as static type systems, restricted namespaces and modular programming.

Formal methods

formal methodformalformal analysis
To prove the absence of these more-general-than-types defects, other kinds of formal methods, collectively known as program analyses, are in common use.
Formal methods are best described as the application of a fairly broad variety of theoretical computer science fundamentals, in particular logic calculi, formal languages, automata theory, discrete event dynamic system and program semantics, but also type systems and algebraic data types to problems in software and hardware specification and verification.

Compiler

compiledcompilerscompile
A compiler may also use the static type of a value to optimize the storage it needs and the choice of algorithms for operations on the value.

Class (computer programming)

classclassesPartial class
An implementation of a type system could in theory associate identifications called data type (a type of a value), class (a type of an object), and kind (a type of a type, or metatype).
The various object-oriented programming languages enforce member accessibility and visibility to various degrees, and depending on the language's type system and compilation policies, enforced at either compile-time or run-time.

Covariance and contravariance (computer science)

Variancecontravariancecovariance and contravariance
In some languages subtypes may also possess covariant or contravariant return types and argument types respectively.
Many programming language type systems support subtyping.

Runtime (program lifecycle phase)

run timerun-timeruntime
This checking can happen statically (at compile time), dynamically (at run time), or as a combination of static and dynamic checking.
Type checking, register allocation, code generation, and code optimization are typically done at compile time, but may be done at runtime depending on the particular language and compiler.

Scala (programming language)

ScalaScala programming languageScala.js
However, a language can be statically typed without requiring type declarations (examples include Haskell, Scala, OCaml, F#, and to a lesser extent C# and C++), so explicit type declaration is not a necessary requirement for static typing in all languages.
Scala is a general-purpose programming language providing support for functional programming and a strong static type system.

Subtyping

subtypesubtype polymorphismsupertype
A program associates each value with at least one specific type, but it also can occur that one value is associated with many subtypes.
The type system of a programming language essentially defines its own subtyping relation, which may well be trivial should the language support no (or very little) conversion mechanisms.

Clojure

Rich HickeyClojureScriptClojure programming language
Certain languages, for example Clojure, Common Lisp, or Cython are dynamically type-checked by default, but allow programs to opt into static type checking by providing optional annotations.
While its type system is entirely dynamic, recent efforts have also sought the implementation of gradual typing.

Epigram (programming language)

Epigram
Advocates of dependent typing, implemented in languages such as Dependent ML and Epigram, have suggested that almost all bugs can be considered type errors, if the types used in a program are properly declared by the programmer or correctly inferred by the compiler.
Epigram's type system is strong enough to express program specifications.

Effect system

A programming language compiler can also implement a dependent type or an effect system, which enables even more program specifications to be verified by a type checker.
An effect system is typically an extension of a type system.

OCaml

MetaOCamlOCaml Package ManagerOPAM
However, a language can be statically typed without requiring type declarations (examples include Haskell, Scala, OCaml, F#, and to a lesser extent C# and C++), so explicit type declaration is not a necessary requirement for static typing in all languages.
ML-derived languages are best known for their static type systems and type-inferring compilers.

JavaScript

Server-side JavaScriptclient-side JavaScriptJS
JavaScript has curly-bracket syntax, dynamic typing, prototype-based object-orientation, and first-class functions.

Racket (programming language)

RacketDrRacketPLT
The programming environment DrRacket, a pedagogic environment based on Lisp, and a precursor of the language Racket is also soft-typed.
In the years that followed, a team including Flatt, Robby Findler, Shriram Krishnamurthi, Cormac Flanagan, and many others produced DrScheme, a programming environment for novice Scheme programmers and a research environment for soft typing.

Python (programming language)

PythonPython programming languagePython 2
For example, C++ templates are typically more cumbersome to write than the equivalent Ruby or Python code since C++ has stronger rules regarding type definitions (for both functions and variables).
Python uses dynamic typing and a combination of reference counting and a cycle-detecting garbage collector for memory management.

Type safety

type-safetype safesafe
It is challenging to find a sufficiently expressive type system that satisfies all programming practices in a type safe manner.
In object oriented languages type safety is usually intrinsic in the fact that a type system is in place.

Duck typing

duckduck typedassuming that method exists
Dynamic typing typically allows duck typing (which enables easier code reuse).
Certain, mostly statically typed languages such as C# 4+ and Boo have features