Editing Functional programming (section)

=== Simulating state ===
There are tasks (for example, maintaining a bank account balance) that often seem most naturally implemented with state. Pure functional programming performs these tasks, and I/O tasks such as accepting user input and printing to the screen, in a different way.

The pure functional programming language [[Haskell]] implements them using [[Monad (functional programming)|monads]], derived from [[category theory]].<ref>Michael Barr, Charles Well - Category theory for computer science.</ref> Monads offer a way to abstract certain types of computational patterns, including (but not limited to) modeling of computations with mutable state (and other side effects such as I/O) in an imperative manner without losing purity. While existing monads may be easy to apply in a program, given appropriate templates and examples, many students find them difficult to understand conceptually, e.g., when asked to define new monads (which is sometimes needed for certain types of libraries).<ref>{{cite web |last=Newbern |first=J. |title=All About Monads: A comprehensive guide to the theory and practice of monadic programming in Haskell |url=http://monads.haskell.cz/html/index.html/html/ |access-date=2008-02-14}}</ref>

Functional languages also simulate states by passing around immutable states. This can be done by making a function accept the state as one of its parameters, and return a new state together with the result, leaving the old state unchanged.<ref>{{Cite web|url=https://fsharpforfunandprofit.com/posts/13-ways-of-looking-at-a-turtle/#2-basic-fp---a-module-of-functions-with-immutable-state|title=Thirteen ways of looking at a turtle |website=fF# for fun and profit|language=en|access-date=2018-11-13}}</ref>
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Impure functional languages usually include a more direct method of managing mutable state. [[Clojure]], for example, uses managed references that can be updated by applying pure functions to the current state. This kind of approach enables mutability while still promoting the use of pure functions as the preferred way to express computations.{{citation needed|date=July 2018}}
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Alternative methods such as [[Hoare logic]] and [[uniqueness type|uniqueness]] have been developed to track side effects in programs. Some modern research languages use [[effect system]]s to make the presence of side effects explicit.<ref>{{cite book |last1=Hartmanis |first1=Juris |date=1986 |pages=123–135 |url=https://doi.org/10.1007/3-540-16761-7_62 |access-date=2024-12-12 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |isbn=978-3-540-16761-7 |last2=Hemachandra |first2=Lane|title=Automata, Languages and Programming |chapter=Complexity classes without machines: On complete languages for UP |series=Lecture Notes in Computer Science |volume=226 |doi=10.1007/3-540-16761-7_62 }}</ref>
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