A strict programming language is a

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 ...

which employs a strict

programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...

, allowing only

strict function In computer science and computer programming, a function f is said to be strict if, when applied to a non-terminating expression, it also fails to terminate. A strict function in the denotational semantics of programming languages is a function '' ...

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

lazy evaluation In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an expression until its value is needed (non-strict evaluation) and which also avoids repeated evaluations (sharing). The b ...

Examples

Nearly all

s in common use today are strict. Examples include C#,

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Perl Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...

(all versions, i.e. through version 5 and version 7),

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 ...

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, and ML. Some strict programming languages include features that mimic laziness. Raku, formerly known as Perl 6, has lazy lists. Python has generator functions.

Julia Julia is usually a feminine given name. It is a Latinate feminine form of the name Julio and Julius. (For further details on etymology, see the Wiktionary entry "Julius".) The given name ''Julia'' had been in use throughout Late Antiquity (e.g. ...

provides a macro system to build non-strict functions, as does

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. Examples for non-strict languages are

Haskell Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lang ...

, Miranda, and Clean.

Explanation

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

data constructor In computer programming, especially functional programming and type theory, an algebraic data type (ADT) is a kind of composite type, i.e., a type formed by combining other types. Two common classes of algebraic types are product types (i.e., t ...

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

prime number A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...

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 In the context of Combinatorial game theory, which typically studies sequential games with perfect information, a game tree is a graph representing all possible game states within such a game. Such games include well-known ones such as chess, check ...

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. 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 to be useful. * All

hardware architecture In engineering, hardware architecture refers to the identification of a system's physical components and their interrelationships. This description, often called a hardware design model, allows hardware designers to understand how their compon ...

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. *

Space complexity The space complexity of an algorithm or a computer program is the amount of memory space required to solve an instance of the computational problem as a function of characteristics of the input. It is the memory required by an algorithm until it ex ...

of non-strict programs is difficult to understand and predict. Strict programming languages are often associated with

eager evaluation In a programming language, an evaluation strategy is a set of rules for evaluating expressions. The term is often used to refer to the more specific notion of a ''parameter-passing strategy'' that defines the kind of value that is passed to the f ...

, and non-strict languages with

, but other evaluation strategies 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 In computer programming, a thunk is a subroutine used to inject a calculation into another subroutine. Thunks are primarily used to delay a calculation until its result is needed, or to insert operations at the beginning or end of the other subro ...

Citations

References

* * {{Refend Programming paradigms Evaluation strategy