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%
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\begin{isabellebody}%
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\def\isabellecontext{Option{\isadigit{2}}}%
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%
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\begin{isamarkuptext}%
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\indexbold{*option (type)}\indexbold{*None (constant)}%
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\indexbold{*Some (constant)}
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Our final datatype is very simple but still eminently useful:%
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\end{isamarkuptext}%
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\isacommand{datatype}\ {\isacharprime}a\ option\ {\isacharequal}\ None\ {\isacharbar}\ Some\ {\isacharprime}a%
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\begin{isamarkuptext}%
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\noindent
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Frequently one needs to add a distinguished element to some existing type.
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For example, type \isa{t\ option} can model the result of a computation that
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may either terminate with an error (represented by \isa{None}) or return
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some value \isa{v} (represented by \isa{Some\ v}).
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Similarly, \isa{nat} extended with $\infty$ can be modeled by type
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\isa{nat\ option}. In both cases one could define a new datatype with
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customized constructors like \isa{Error} and \isa{Infinity},
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but it is often simpler to use \isa{option}. For an application see
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\S\ref{sec:Trie}.%
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\end{isamarkuptext}%
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\end{isabellebody}%
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%%% Local Variables:
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%%% mode: latex
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%%% TeX-master: "root"
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%%% End:
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