A strict programming language is one in which only strict function

Strict function

A strict function in the denotational semantics of programming languages is a function f where f\left = \perp. The entity \perp, called bottom, denotes an expression which does not return a normal value, either because it loops endlessly or because it aborts due to an error such as division by...

s (functions whose parameters must be evaluated completely before they may be called) may be defined by the user. A non-strict programming language allows the user to define non-strict functions, and hence may allow lazy evaluation

Lazy evaluation

In programming language theory, lazy evaluation or call-by-need is an evaluation strategy which delays the evaluation of an expression until the value of this is actually required and which also avoids repeated evaluations...

.

Examples

Nearly all 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....

s in common use today are strict. Examples include 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...

, C#, 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...

, 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...

 (up through version 5), 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...

, 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...

, 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...

, Scheme, and ML. The best known non-strict languages are 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...

, Miranda, and Clean.

Explanation

In most non-strict languages the non-strictness extends to data constructor

Algebraic data type

In computer programming, particularly functional programming and type theory, an algebraic data type is a datatype each of whose values is data from other datatypes wrapped in one of the constructors of the datatype. Any wrapped datum is an argument to the constructor...

s. This allows conceptually infinite data structures (such as the list of all prime number

Prime number

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. A natural number greater than 1 that is not a prime number is called a composite number. For example 5 is prime, as only 1 and 5 divide it, whereas 6 is composite, since it has the divisors 2...

s) to be manipulated in the same way as ordinary finite data structures. It also allows for the use of very large but finite data structures such as the complete game tree

Game tree

In game theory, a game tree is a directed graph whose nodes are positions in a game and whose edges are moves. The complete game tree for a game is the game tree starting at the initial position and containing all possible moves from each position; the complete tree is the same tree as that...

 of chess

Chess

Chess is a two-player board game played on a chessboard, a square-checkered board with 64 squares arranged in an eight-by-eight grid. It is one of the world's most popular games, played by millions of people worldwide at home, in clubs, online, by correspondence, and in tournaments.Each player...

.

Non-strictness has several disadvantages which have prevented widespread adoption:

• Because of the uncertainty regarding if and when expressions will be evaluated, non-strict languages generally must be purely functional
  Purely functional
  Purely functional is a term in computing used to describe algorithms, data structures or programming languages that exclude destructive modifications...
  
   to be useful.
• All hardware architecture
  Computer architecture
  In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
  
  s in common use are optimized for strict languages, so the best compilers for non-strict languages produce slower code than the best compilers for strict languages, with the notable exception of the Glasgow Haskell Compiler
  Glasgow Haskell Compiler
  The Glorious Glasgow Haskell Compilation System, more commonly known as the Glasgow Haskell Compiler or GHC, is an open source native code compiler for the functional programming language Haskell. The lead developers are Simon Peyton Jones and Simon Marlow...
  
   which outperforms many strict language compilers.
• Space complexity of non-strict programs is difficult to understand and predict.

Strict programming languages are often associated with eager evaluation

Eager evaluation

In computer programming, eager evaluation or greedy evaluation is the evaluation strategy in most traditional programming languages. In eager evaluation an expression is evaluated as soon as it gets bound to a variable. The term is typically used to contrast lazy evaluation, where expressions are...

, and non-strict languages with lazy evaluation

Lazy evaluation

In programming language theory, lazy evaluation or call-by-need is an evaluation strategy which delays the evaluation of an expression until the value of this is actually required and which also avoids repeated evaluations...

, but other evaluation strategies

Evaluation strategy

In computer science, an evaluation strategy is a set of rules for evaluating expressions in a programming language. Emphasis is typically placed on functions or operators: an evaluation strategy defines when and in what order the arguments to a function are evaluated, when they are substituted...

 are possible in each case. The terms "eager programming language" and "lazy programming language" are often used as synonyms for "strict programming language" and "non-strict programming language" respectively.

In many strict languages, some advantages of non-strict functions can be obtained through the use of macros or thunk

Thunk

Thunk may refer to:* Thunk , a piece of code to perform a delayed computation * Thunk : a feature of some virtual function table implementations...

s.