|
10561
|
1 |
%
|
|
|
2 |
\begin{isabellebody}%
|
|
|
3 |
\def\isabellecontext{Option{\isadigit{2}}}%
|
|
|
4 |
%
|
|
|
5 |
\begin{isamarkuptext}%
|
|
|
6 |
\indexbold{*option}\indexbold{*None}\indexbold{*Some}
|
|
|
7 |
Our final datatype is very simple but still eminently useful:%
|
|
|
8 |
\end{isamarkuptext}%
|
|
|
9 |
\isacommand{datatype}\ {\isacharprime}a\ option\ {\isacharequal}\ None\ {\isacharbar}\ Some\ {\isacharprime}a%
|
|
|
10 |
\begin{isamarkuptext}%
|
|
|
11 |
\noindent
|
|
11310
|
12 |
Frequently one needs to add a distinguished element to some existing type.
|
|
10561
|
13 |
For example, type \isa{t\ option} can model the result of a computation that
|
|
|
14 |
may either terminate with an error (represented by \isa{None}) or return
|
|
|
15 |
some value \isa{v} (represented by \isa{Some\ v}).
|
|
|
16 |
Similarly, \isa{nat} extended with $\infty$ can be modeled by type
|
|
|
17 |
\isa{nat\ option}. In both cases one could define a new datatype with
|
|
|
18 |
customized constructors like \isa{Error} and \isa{Infinity},
|
|
|
19 |
but it is often simpler to use \isa{option}. For an application see
|
|
|
20 |
\S\ref{sec:Trie}.%
|
|
|
21 |
\end{isamarkuptext}%
|
|
|
22 |
\end{isabellebody}%
|
|
|
23 |
%%% Local Variables:
|
|
|
24 |
%%% mode: latex
|
|
|
25 |
%%% TeX-master: "root"
|
|
|
26 |
%%% End:
|