Flix (programming language): Difference between revisions

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'''Flix''' is a [[functional programming|functional]], [[imperative programming|imperative]], and [[logic programming|logic]] [[programming language]] developed at [[Aarhus University]], with funding from the [[Danish Council for Independent Research|Independent Research Fund Denmark]],<ref>{{cite web |title=Forskningsprojekter |url=https://dff.dk/forskningsprojekter?SearchableText=functional+and+declarative+logic+programming&period%3Alist=all&instrument%3Alist=all&filed_method%3Alist=all |website=Danmarks Frie Forskningsfond |language=da}}</ref> and by a community of [[open source]] contributors.<ref>{{cite web |title=Flix Authors |url=https://github.com/flix/flix/blob/master/AUTHORS.md |website=GitHub |date=27 July 2022 |language=en}}</ref> The Flix language supports [[algebraic data types]], [[pattern matching]], [[parametric polymorphism]], [[currying]], [[higher-order function]]s, [[extensible records]],<ref>{{cite journal |last1=Leijen |first1=Daan |title=Extensible records with scoped labels |journal=Trends in Functional Programming}}</ref> [[Communicating sequential processes|channel and process-based concurrency]], and [[tail call elimination]]. Two notable features of Flix are its type and effect system<ref name="oopsla2020a">{{cite journal |last1=Madsen |first1=Magnus |last2=van de Pol |first2=Jaco |title=Polymorphic Types and Effects with Boolean Unification |journal=Proceedings of the ACM on Programming Languages |date=13 November 2020 |volume=4 |issue=OOPSLA |pages=1–29 |doi=10.1145/3428222|s2cid=227044242 |doi-access=free }}</ref> and its support for first-class Datalog constraints.<ref name="oopsla2020b">{{cite journal |last1=Madsen |first1=Magnus |last2=Lhoták |first2=Ondřej |title=Fixpoints for the Masses: Programming with First-class Datalog Constraints |journal=Proceedings of the ACM on Programming Languages |date=13 November 2020 |volume=4 |issue=OOPSLA |pages=125:1–125:28 |doi=10.1145/3428193|s2cid=227107960 |doi-access=free }}</ref>

The Flix type and effect system supports [[Hindley–Milner type system|Hindley-Milner]]-style [[type inference]]. The system separates pure and impure code: if an expression is typed as pure then it cannot produce an effect at run-time. Higher-order functions can enforce that they are given pure (or impure) function arguments. The type and effect system supports [[effect polymorphism]]<ref>{{cite journalbook |last1=Lucassen |first1=J. M. |last2=Gifford |first2=D. K. |title=Polymorphic effect systems |journal=Proceedings of the 15th ACM SIGPLAN-SIGACT Symposiumsymposium on Principles of Programmingprogramming Languageslanguages - POPL '88 |chapter=Polymorphic effect systems |date=1988 |pages=47–57 |doi=10.1145/73560.73564|isbn=0897912527 |s2cid=13015611 |doi-access=free }}</ref><ref>{{cite journal |last1=Leijen |first1=Daan |title=Koka: Programming with Row Polymorphic Effect Types |journal=Electronic Proceedings in Theoretical Computer Science |date=5 June 2014 |volume=153 |pages=100–126 |doi=10.4204/EPTCS.153.8|arxiv=1406.2061 |s2cid=14902937 }}</ref> which means that the effect of a higher-order function may depend on the effect(s) of its argument(s).

Flix supports [[Datalog]] programs as [[First-class citizen|first-class]] values. A Datalog program value, i.e. a collection of Datalog facts and rules, can be passed to and returned from functions, stored in data structures, and composed with other Datalog program values. The [[Minimal model program|minimal model]] of a Datalog program value can be computed and is itself a Datalog program value. In this way, Flix can be viewed as a [[metaprogramming|meta programming]] language for Datalog. Flix supports [[Stratification (mathematics)#In mathematical logic|stratified negation]] and the Flix compiler ensures stratification at compile-time.<ref name="Programming Flix - Fixpoints">{{cite web |title=Programming Flix - Fixpoints |url=https://doc.flix.dev/fixpoints/ |website=flix.dev}}</ref> Flix also supports an enriched form of Datalog constraints where predicates are given [[Lattice (order)|lattice]] semantics.<ref>{{cite journal |last1=Madsen |first1=Magnus |last2=Yee |first2=Ming-Ho |last3=Lhoták |first3=Ondřej |title=From Datalog to flix: a declarative language for fixed points on lattices |journal=ACM SIGPLAN Notices |date=August 2016 |volume=51 |issue=6 |pages=194–208 |doi=10.1145/2980983.2908096}}</ref><ref>{{cite journalbook |last1=Madsen |first1=Magnus |last2=Lhoták |first2=Ondřej |title=Safe and sound program analysis with Flix |journal=Proceedings of the 27th ACM SIGSOFT International Symposium on Software Testing and Analysis -|chapter=Safe ISSTAand 2018sound program analysis with Flix |date=2018 |pages=38–48 |doi=10.1145/3213846.3213847|isbn=9781450356992 |s2cid=49427988 }}</ref><ref>{{cite journal |last1=Keidel |first1=Sven |last2=Erdweg |first2=Sebastian |title=Sound and reusable components for abstract interpretation |journal=Proceedings of the ACM on Programming Languages |date=10 October 2019 |volume=3 |issue=OOPSLA |pages=1–28 |doi=10.1145/3360602|s2cid=203631644 |doi-access=free }}</ref><ref>{{cite book |last1=Gong |first1=Qing |title=Extending Parallel Datalog with Lattice |publisher=Pennsylvania State University}}</ref>

== Overview ==

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Monomorphization avoids [[Value type and reference type|boxing]] of primitive values at the cost of longer compilation times and larger executable binaries. Flix has some support for interoperability with programs written in [[Java programming language|Java]].<ref>{{cite web |title=Programming Flix - Interoperability |url=https://doc.flix.dev/interoperability/ |website=flix.dev}}</ref>

Flix supports [[tail call elimination]] which ensures that function calls in tail position never consume stack space and hence cannot cause the call stack to overflow.<ref>{{cite journalbook |last1=Madsen |first1=Magnus |last2=Zarifi |first2=Ramin |last3=Lhoták |first3=Ondřej |title=Proceedings of the 27th International Conference on Compiler Construction |chapter=Tail call elimination and data representation for functional languages on the Java virtual machine |journal=Proceedings of the 27th International Conference on Compiler Construction - CC 2018 |date=2018 |pages=139–150 |doi=10.1145/3178372.3179499|isbn=9781450356442 |s2cid=3432962 }}</ref> Since the [[Java bytecode instruction listings|JVM instruction set]] lacks explicit support for tail calls, such calls are emulated using a form of reusable stack frames.<ref>{{cite journal |last1=Tauber |first1=Tomáš |last2=Bi |first2=Xuan |last3=Shi |first3=Zhiyuan |last4=Zhang |first4=Weixin |last5=Li |first5=Huang |last6=Zhang |first6=Zhenrui |last7=Oliveira |first7=Bruno C. D. S. |title=Memory-Efficient Tail Calls in the JVM with Imperative Functional Objects |journal=Programming Languages and Systems |series=Lecture Notes in Computer Science |date=2015 |volume=9458 |pages=11–28 |doi=10.1007/978-3-319-26529-2_2|isbn=978-3-319-26528-5 }}</ref> Support for tail call elimination is important since all iteration in Flix is expressed through [[recursion]].

The Flix compiler disallows most forms of unused or redundant code, including: unused local variables, unused functions, unused formal parameters, unused type parameters, and unused type declarations, such unused constructs are reported as compiler errors.<ref>{{cite web |title=Redundancies as Compile-Time Errors |url=https://flix.dev/blog/redundancies-as-compile-time-errors/ |website=flix.dev}}</ref> [[Variable shadowing]] is also disallowed. The stated rationale is that unused or redundant code is often correlated with erroneous code<ref>{{cite journal |last1=Engler |first1=D. |title=Using redundancies to find errors |journal=IEEE Transactions on Software Engineering |date=October 2003 |volume=29 |issue=10 |pages=915–928 |doi=10.1109/TSE.2003.1237172}}</ref>