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Quelle Option2.thySprache: Isabelle |
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(*<*) theory Option2 imports Main begin hide_const None Some hide_type option (*>*) text‹\indexbold{*option (type)}\indexbold{*None (constant)}% indexbold{*Some (constant)} final datatype is very simple but still eminently useful: › datatype 'a option = None | Some 'a text‹\noindent one needs to add a distinguished element to some existing type. example, type ‹t option› can model the result of a computation that either terminate with an error (represented by const‹None›) or return value term‹v› (represented by term‹Some v›). , typ‹nat› extended with $\infty$ can be modeled by type typ‹nat option›. In both cases one could define a new datatype with constructors like term‹Error› and term‹Infinity›, it is often simpler to use ‹option›. For an application see S\ref{sec:Trie}. › (*<*) (* definition infplus :: "nat option \<Rightarrow> nat option \<Rightarrow> nat option" where "infplus x y \<equiv> case x of None \<Rightarrow> None | Some m \<Rightarrow> (case y of None \<Rightarrow> None | Some n \<Rightarrow> Some(m+n))" *) end (*>*)
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2026-07-09