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POAda-like types in Nimrod
 

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    <p>today I did ask in D mailing list whether it's possible to define and use custom data types in a way similar to e.g. example from Ada's <a href="http://en.wikipedia.org/wiki/Ada_%28programming_language%29">wiki</a> page:</p> <pre><code>type Day_type is range 1 .. 31; type Month_type is range 1 .. 12; type Year_type is range 1800 .. 2100; type Hours is mod 24; type Weekday is (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday); type Date is record Day : Day_type; Month : Month_type; Year : Year_type; end record; subtype Working_Hours is Hours range 0 .. 12; subtype Working_Day is Weekday range Monday .. Friday; Work_Load: constant array(Working_Day) of Working_Hours := (Friday =&gt; 6, Monday =&gt; 4, others =&gt; 10); </code></pre> <p>and the <a href="http://forum.dlang.org/thread/vqsnloitqvouoahdngzb@forum.dlang.org?page=14#post-qhneezraycclrkqxfrvh:40forum.dlang.org">reply</a> demonstrated something like:</p> <pre><code>import std.typecons; import std.exception; struct Limited(T, T lower, T upper) { T _t; mixin Proxy!_t; //Limited acts as T (almost) invariant() { enforce(_t &gt;= lower &amp;&amp; _t &lt;= upper); } this(T t) { _t = t; } } auto limited(T, T lower, T upper)(T init = T.init) { return Limited!(T, lower, upper)(init); } unittest { enum l = [-4,9]; auto a = limited!(int, l[0], l[1])(); foreach(i; l[0] .. l[1]+1) { a = i; } assertThrown({a = -5;}()); assertThrown({a = 10;}()); } </code></pre> <p>which shows it's possible, but probably misses Ada's elegance.</p> <p>Now, after reading about Nimrod recently, I wonder how it can handle similar task with the provision to ensure same Ada's type-safety?</p>
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