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Polymorphic recursion
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<p>In <a href="/facts/Computer_science/lN3pEyPz">computer science</a>, polymorphic recursion (also referred to as <a href="/facts/Robin_Milner/oMfdsRwK">Milner</a>–<a href="/facts/Alan_Mycroft/AeQBnmhM">Mycroft</a> typability or the Milner–Mycroft calculus) refers to a <a href="/facts/Recursion_(computer_science)/2uXo18Gv">recursive</a> <a href="/facts/Parametric_polymorphism/ETb0G1zd">parametrically polymorphic</a> <a href="/facts/Function_(computer_science)/9vFJrg0A">function</a> where the type parameter changes with each recursive invocation made, instead of staying constant. <a href="/facts/Type_inference/vX9K8Qhf">Type inference</a> for polymorphic recursion is equivalent to semi-unification and therefore <a href="/facts/Undecidable_problem/QBxmXBbY">undecidable</a> and requires the use of a <a href="/facts/Semi-algorithm/lqtossYw">semi-algorithm</a> or programmer-supplied <a href="/facts/Type_annotation/psEPS0UW">type annotations</a>. </p>