Editing Monad (functional programming) (section)

=== Usage ===
The value of the monad pattern goes beyond merely condensing code and providing a link to mathematical reasoning.
Whatever language or default [[programming paradigm]] a developer uses, following the monad pattern brings many of the benefits of [[purely functional programming]].
By [[reification (computer science)|reifying]] a specific kind of computation, a monad not only [[encapsulation (computer programming)|encapsulates]] the tedious details of that computational pattern, but it does so in a [[declarative programming|declarative]] way, improving the code's clarity.
As monadic values explicitly represent not only computed values, but computed ''effects'', a monadic expression can be replaced with its value in [[referential transparency|referentially transparent positions]], much like pure expressions can be, allowing for many techniques and optimizations based on [[rewriting]].<ref name="Wadler1992" />

Typically, programmers will use {{mvar|bind}} to chain monadic functions into a sequence, which has led some to describe monads as "programmable semicolons", a reference to how many [[imperative programming|imperative]] languages use semicolons to separate [[Statement (computer programming)|statements]].<ref name="RealWorldHaskell" /><ref name="GentleIntroHaskell" />
However, monads do not actually order computations; even in languages that use them as central features, simpler function composition can arrange steps within a program.
A monad's general utility rather lies in simplifying a program's structure and improving [[separation of concerns]] through abstraction.<ref name="Wadler1992" /><ref name="MonadsAreNot">{{cite web | title = What a Monad is not | url = https://wiki.haskell.org/What_a_Monad_is_not | date = 7 October 2018}}</ref>

The monad structure can also be seen as a uniquely mathematical and [[compile time]] variation on the [[decorator pattern]].
Some monads can pass along extra data that is inaccessible to functions, and some even exert finer control over execution, for example only calling a function under certain conditions.
Because they let application programmers implement [[domain logic]] while offloading boilerplate code onto pre-developed modules, monads can even be considered a tool for [[aspect-oriented programming]].<ref name="deMeuter1997">{{cite conference | last = De Meuter | first = Wolfgang | title = Monads as a theoretical foundation for AOP | conference = International Workshop on Aspect Oriented Programming at ECOOP | date = 1997 | location = Jyväskylä, Finland | url = http://soft.vub.ac.be/Publications/1997/vub-prog-tr-97-10.pdf | citeseerx = 10.1.1.25.8262}}</ref>

One other noteworthy use for monads is isolating side-effects, like [[input/output]] or mutable [[state (computer science)|state]], in otherwise purely functional code.
Even purely functional languages ''can'' still implement these "impure" computations without monads, via an intricate mix of function composition and [[continuation-passing style]] (CPS) in particular.<ref name="Wadler1990" />
With monads though, much of this scaffolding can be abstracted away, essentially by taking each recurring pattern in CPS code and bundling it into a distinct monad.<ref name="Wadler1992" />

If a language does not support monads by default, it is still possible to implement the pattern, often without much difficulty.
When translated from category-theory to programming terms, the monad structure is a [[concept (generic programming)|generic concept]] and can be defined directly in any language that supports an equivalent feature for [[parametric polymorphism#Bounded parametric polymorphism|bounded polymorphism]].
A concept's ability to remain agnostic about operational details while working on underlying types is powerful, but the unique features and stringent behavior of monads set them apart from other concepts.<ref name="MonadSansMetaphors">{{cite web | url = https://wiki.haskell.org/Monad_(sans_metaphors) | title = Monad (sans metaphors) | website = HaskellWiki | date = 1 November 2009 | access-date = 24 October 2018}}</ref>