Expression-oriented programming language: Difference between revisions

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{{Notability|date=July 2022}}

{{Programming paradigms}}An '''expression-oriented programming language''' is a [[programming language]] where every (or nearly every) construction is an [[expression (programming)|expression]] and thus yields a value. The typical exceptions are [[Macro (computer science)|macro]] definitions, [[preprocessor]] commands, and declarations, which expression-oriented languages often treat as [[statement (programming)|statement]]s rather than expressions. Some expression-oriented languages introduce a [[void return type]] to be yielded by expressions that merely cause [[Side effect (computer science)|side-effects]].

An '''expression-oriented programming language''' is a [[programming language]] in which every (or nearly every) construction is an [[Expression (computer science)|expression]] and thus yields a value.<ref>{{Cite web |title=Glossary - The Rust Programming Language |url=https://web.mit.edu/rust-lang_v1.25/arch/amd64_ubuntu1404/share/doc/rust/html/book/first-edition/glossary.html#expression-oriented-language |access-date=2022-07-06 |website=web.mit.edu}}</ref> The typical exceptions are [[Macro (computer science)|macro]] definitions, [[preprocessor]] commands, and [[Declaration (computer programming)|declarations]], which expression-oriented languages often treat as [[Statement (computer science)|statements]].

[[ALGOL 68]] and [[Lisp (programming language)|Lisp]] are examples of expression-oriented languages. [[Pascal programming language|Pascal]] is not an expression-oriented language. All [[functional programming language]]s are expression-oriented.{{citationneeded|date=September 2020}}

[[Lisp (programming language)|Lisp]]<ref name=":0">{{Cite journal |last1=Syme |first1=Don |date=2020-06-14 |title=The early history of F# |journal=Proceedings of the ACM on Programming Languages |language=en |volume=4 |issue=HOPL |pages=1–58 |doi=10.1145/3386325 |issn=2475-1421 |doi-access=free}}</ref> and [[ALGOL 68]] are expression-oriented languages. [[Pascal (programming language)|Pascal]] is not an expression-oriented language.

==Criticism==

All [[functional programming]] languages are expression-oriented.<ref>{{Cite web |author1=Knoldus |date=2018-02-08 |title=Expression Oriented Programming |url=https://medium.com/knoldus/expression-oriented-programming-d88f621fd3a4 |access-date=2023-08-07 |website=Knoldus – Technical Insights |language=en}}</ref>

Critics<!--[[Assignment_(computer_science)#Assignment_versus_equality|Assignment versus equality]]</ref><ref>[[Relational_operator#Confusion_with_assignment_operators|Confusion with assignment operators]]</ref>-->, including language designers,<ref>[http://java.sun.com/docs/codeconv/html/CodeConventions.doc9.html#547 Java Code Conventions "10.4 Variable Assignments"]</ref> blame expression-orientation for an entire class of [[software bug|programming mistakes]] wherein a programmer introduces an [[Assignment (computer science)|assignment expression]] where they meant to test for [[Equality (relational operator)|equality]]. For example, the designers of [[Ada (programming language)|Ada]] and [[Java (programming language)|Java]] were so worried about this type of mistake, they restricted [[Conditional (programming)|control expressions]] to those that evaluate strictly to the [[boolean data type]].<ref>[https://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-14.9 The Java Language Specification, Java SE 8 Edition "14.9 The if Statement"]</ref><ref>[http://www.adaic.org/whyada/intro4.html Introducing Ada]</ref> The designers of [[Python (programming language)|Python]] had similar worries but took the alternative strategy of implementing assignment as a statement rather than an expression, thus prohibiting assignment from nesting inside of any other statement or expression.<ref>[https://docs.python.org/reference/simple_stmts.html#assignment The Python Language Reference "6.2. Assignment statements"]</ref>

==Criticism and prevention==

{{Synthesis|date=July 2022}}

{{See also|Assignment (computer science)#Assignment versus equality|Relational operator#Confusion with assignment operators}}

Critics, including language designers,<ref>[https://www.oracle.com/java/technologies/javase/codeconventions-programmingpractices.html#547 Java Code Conventions "10.4 Variable Assignments"]</ref>{{Failed verification|date=July 2022}} blame expression-orientation for an entire class of [[software bug|programming mistakes]] wherein a programmer accidentally codes an [[Assignment (computer science)|assignment]] ''expression'', which replaces a variable with an expression rather than testing it for [[Equality (relational operator)|equality]] with that expression.

The designers of [[Ada (programming language)|Ada]] and [[Java (programming language)|Java]] prevent this type of mistake by restricting [[Conditional (computer programming)|control expressions]] to those that evaluate strictly to the [[boolean data type]].<ref>[https://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-14.9 The Java Language Specification, Java SE 8 Edition "14.9 The if Statement"]</ref><ref>[https://www.adaic.org/learn/materials/intro/part4/ Introducing Ada]</ref>

The designers of [[Python (programming language)|Python]] implemented assignment as a statement rather than an expression, thus prohibiting assignment from nesting inside any other statement or expression.<ref>[https://docs.python.org/3/reference/simple_stmts.html#assignment The Python Language Reference "6.2. Assignment statements"]</ref> (Until version 3.8 added 'assignment expressions', with a different syntax.<ref>[https://www.python.org/dev/peps/pep-0572/ "PEP 572: Assignment Expressions"]</ref>)

In some expression-oriented languages, expressions that merely cause [[side effect (computer science)|side effects]] return [[void type]]s.

==Examples==

{{More citations needed|date=July 2022}}

{{Expand section|date=February 2018}}

*[[ALGOL 68]]

*[[LispBLISS]]
*[[Icon (programming language)|LispIcon]]

*[[Lisp (programming language)|Lisp]]<ref name=":0" />

*[[ML (programming language)|ML]]

*[[Perl]]

*[[REBOLRebol]]

*[[Ruby (programming language)|Ruby]]

*[[HaskellElixir (programming language)|HaskellElixir]]

*[[RustErlang (programming language)|RustErlang]]

*[[Haskell]]

*[[Rust (programming language)|Rust]]<ref>{{Cite web |title=Functions - The Rust Programming Language |url=https://web.mit.edu/rust-lang_v1.25/arch/amd64_ubuntu1404/share/doc/rust/html/book/first-edition/functions.html#expressions-vs-statements |access-date=2022-07-06 |website=web.mit.edu}}</ref>

*[[Scala (programming language)|Scala]]

*[[Smalltalk]]

<!-- *[[PHP]]<ref>[http://php.net/manual/en/language.expressions.php PHP Manual "Expressions"]</ref> -- See talks. PHP manual saying something does not really mean that it is true =( -->

*[[Kotlin (programming language)|Kotlin]]

*[[OCaml]]<ref>{{Cite web |title=COS 326: Functional Programming |url=https://www.cs.princeton.edu/~dpw/courses/cos326-12/notes/type-check.php |access-date=2022-07-06 |website=www.cs.princeton.edu}}</ref>

<!-- *[[PHP]]<ref>[httphttps://www.php.net/manual/en/language.expressions.php PHP Manual "Expressions"]</ref> -- See talks. PHP manual saying something does not really mean that it is true =( -->

==See also==

*[[Command-queryCommand–query separation]]

*[[functionalFunctional programming]]

== Notes ==

{{Expand section|date=February 2018}}

<references group="nb" />

==References==

{{Reflist}}

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[[Category:Programming language classification]]