Editing Abstraction inversion

{{Short description|Computer programming anti-pattern}}
{{multiple issues|
{{example farm|date=October 2013}}
{{Original research|date=May 2009}}
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In [[computer programming]], '''abstraction inversion''' is an [[anti-pattern]] arising when users of a construct need functions implemented within it but not exposed by its [[interface (computing)|interface]]. The result is that the users re-implement the required functions in terms of the interface, which in its turn uses the internal implementation of the same functions.  This may result in implementing lower-level features in terms of higher-level ones, thus the term 'abstraction inversion'.

Possible ill-effects are:
* The user of such a re-implemented function may seriously underestimate its running-costs.
* The user of the construct is forced to obscure their implementation with complex mechanical details.
* Many users attempt to solve the same problem, increasing the risk of error.

== Examples ==
Alleged examples from professional programming circles include:
* In [[Ada programming language|Ada]], choice of the ''rendezvous'' construct as a synchronisation primitive forced programmers to implement simpler constructs such as [[semaphore (programming)|semaphores]] on the more complex basis.<ref name="pipeline_on_ada">[http://www.pipeline.com/~hbaker1/CritLisp.html#fn2 Critique of DIN Kernel Lisp Definition Version 1.2, footnote 2] {{Webarchive|url=https://web.archive.org/web/20180506220654/http://www.pipeline.com/~hbaker1/CritLisp.html#fn2 |date=2018-05-06 }} - says (without references) that the term derives from critiques of the Ada ''rendezvous'', appears to be one of the earliest uses.</ref>
* [[Applesoft BASIC]] and [[Lua (programming language)|Lua]] (when configured for desktop computers) have a floating-point as the base numeric type.<ref>[http://lua-users.org/wiki/FloatingPoint lua-users c2: Floating Point] Accessed 2009-10-12.</ref><ref>[http://www.lua.org/pil/2.3.html Programming in Lua : 2.3 - Numbers] Accessed 2009-10-12.</ref> While Applesoft did implement [[integer]] arithmetic, it was implemented on top of [[floating point|floating-point arithmetic]], and neither had any [[bitwise operation|bitwise operators]] (Lua added support in Lua 5.2)<ref>[http://lua-users.org/wiki/BitwiseOperators lua-users c2: Bitwise Operators] Accessed 2013-01-15.</ref> Applesoft also had no support for [[Bit blit|blit]]ting of [[raster graphics]] (even though the language supported vector graphics on the Apple II's raster hardware). This caused games and other programs written in Applesoft to run slower.
* Creating an object to represent a function is cumbersome in [[object-oriented]] languages such as [[Java (programming language)|Java]] and [[C++]] (especially prior to C++11 and Java 8), in which functions are not [[first-class object]]s. In C++ it is possible to make an object 'callable' by overloading the <code>()</code> operator, but it is still often necessary to implement a new class, such as the ''[http://www.sgi.com/tech/stl/functors.html Functors in the STL]''. ([[C++11]]'s lambda function makes it much easier to create an object representing a function.)
* Tom Lord has suggested that [[Subversion (software)|Subversion]] version control system pays for the abstraction inversion of implementing a write-only database on a read/write database with poor performance.<ref name="lord_on_svn">[https://archive.today/20130222124420/http://sourcefrog.net/weblog/software/vc/subversion/lord-on-svn.html sourcefrog : Tom Lord on Subversion<!-- Bot generated title -->]</ref>
* Using [[stored procedure]]s to manipulate data in a relational database, without granting programmers right to deploy such procedures, leads to reimplementing queries outside the database. For example, large datasets (in extreme cases - whole tables) are fetched and actual filtering takes place in application code. Alternatively, thousands of rows are updated (inserted or even fetched) one by one instead of running a multiple row query.
* Microsoft's [[WinUI 3]] systematically replaces the title bar of the windows it creates with a custom one that ignores the end-user's color settings, always appearing gray instead. Applying the end-user's chosen color to the title bar requires using further customization code on Windows 11, and completely replacing the custom title bar with another custom one on Windows 10.<ref>[https://learn.microsoft.com/en-us/windows/apps/develop/title-bar Title bar customization - Windows apps | Microsoft Learn]</ref>

Examples that are common outside professional programming circles include:

* Using spreadsheet lookup functions to replicate the functionality of a database
* Using variant data types as loop counters in Microsoft Visual Basic where an integer type is also available.

== See also ==
*[[Leaky abstraction]]

== References ==
{{reflist}}

==External links==
* [[c2:AbstractionInversion|Abstraction Inversion]] at Portland Pattern Repository - extensive discussion, much of it taking "abstraction inversion" in the sense of "concealed complexity"

[[Category:Anti-patterns]]