Nominal type system

<h2 id="nominal-typing">Nominal typing</h2>
<p>Nominal typing means that two variables are type-compatible if and only if their declarations name the same type. For example, in <a href="/facts/C_(programming_language)/Ky2No763">C</a>, two struct types with different names in the same translation unit are never considered compatible, even if they have identical field declarations.
</p><p>However, C also allows a <a href="/facts/Typedef/kqSR4VXn">typedef</a> declaration, which introduces an alias for an existing type. These are merely <a href="/facts/Syntax/6PxSV98p">syntactical</a> and do not differentiate the type from its alias for the purpose of type checking. This feature, present in many languages, can result in a loss of type safety when (for example) the same primitive integer type is used in two semantically distinct ways. <a href="/facts/Haskell_(programming_language)/4Htv9WLu">Haskell</a> provides the C-style syntactic alias in the form of the type declaration, as well as the newtype declaration that does introduce a new, distinct type, <a href="/facts/Isomorphism/pj8KgkxU">isomorphic</a> to an existing type.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a>
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<h2 id="nominal-subtyping">Nominal subtyping</h2>
<p>In a similar fashion, nominal subtyping means that one type is a subtype of another if and only if it is explicitly declared to be so in its definition. Nominally-typed languages typically enforce the requirement that declared subtypes be structurally compatible (though <a href="/facts/Eiffel_(programming_language)/gEzPjzmn">Eiffel</a> allows non-compatible subtypes to be declared). However, subtypes which are structurally compatible "by accident", but not declared as subtypes, are not considered to be subtypes.
</p><p><a href="/facts/C%2B%2B/Et0F2qn9">C++</a>, <a href="/facts/C_Sharp_(programming_language)/UC2gxeyb">C#</a>, <a href="/facts/Java_(programming_language)/9ScgFyAL">Java</a>, <a href="/facts/Kotlin_(programming_language)/CrAqIFLn">Kotlin</a>, <a href="/facts/Objective-C/5mM557BW">Objective-C</a>, <a href="/facts/Delphi_(programming_language)/sUFUXQ8r">Delphi</a>, <a href="/facts/Swift_(programming_language)/A7qHQC32">Swift</a>, <a href="/facts/Julia_(programming_language)/AoB0PJ9C">Julia</a> and <a href="/facts/Rust_(programming_language)/8JNMV19F">Rust</a> all primarily use both nominal typing and nominal subtyping.
</p><p>Some nominally-subtyped languages, such as Java and C#, allow classes to be declared <i>final</i> (or <i>sealed</i> in C# terminology), indicating that no further subtyping is permitted.
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<h2 id="comparison">Comparison</h2>
<p>Nominal typing is useful at preventing accidental type equivalence, which allows better type-safety than structural typing. The cost is a reduced flexibility, as, for example, nominal typing does not allow new super-types to be created without modification of the existing subtypes.
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<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Structural_type_system/1JH0azFU">Structural type system</a></li>
<li><a href="/facts/Abstract_type/0XDoDYmf">Abstract type</a></li>
<li><a href="/facts/Type_system/Tz0bAMS9">Type system</a></li></ul>

<h2 id="sources">Sources</h2>
<ul><li><a href="/facts/Benjamin_C._Pierce/m9DKUgML">Pierce, Benjamin C.</a> (2002). "§19.3 Nominal and Structural Type Systems". <i><a href="/facts/Types_and_Programming_Languages/vrEw2McI">Types and Programming Languages</a></i>. <a href="/facts/MIT_Press/eTerUM3c">MIT Press</a>. pp. 253–. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-262-16209-1.</li></ul>
<h2 id="external-links">External links</h2>
<ul><li><a href="https://web.archive.org/web/20161013053206/http://c2.com/cgi/wiki?NominativeAndStructuralTyping">c2.com: Nominative and structural typing</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Haskell 2010 Report: Declarations and Bindings: User-Defined Datatypes". Archived from the original on 2017-01-04. Retrieved 2015-06-02. <a href="https://web.archive.org/web/20170104234441/https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2" target="_blank">https://web.archive.org/web/20170104234441/https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Nominal type system open-in-new

Nominal type system