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GenerativeDatatype
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In <a href="StandardML">Standard ML</a>, datatype declarations are said to be <em>generative</em>, because each time a datatype declaration is evaluated, it yields a new type. Thus, any attempt to mix the types will lead to a type error at compile-time. The following program, which does not type check, demonstrates this.
<pre class=code>
<B><FONT COLOR="#5F9EA0">functor</FONT></B> F () <B><FONT COLOR="#5F9EA0">=</FONT></B>
<B><FONT COLOR="#5F9EA0">struct</FONT></B>
<B><FONT COLOR="#A020F0">datatype</FONT></B> t <B><FONT COLOR="#5F9EA0">=</FONT></B> T
<B><FONT COLOR="#A020F0">end</FONT></B>
<B><FONT COLOR="#5F9EA0">structure</FONT></B> S1 <B><FONT COLOR="#5F9EA0">=</FONT></B> F ()
<B><FONT COLOR="#5F9EA0">structure</FONT></B> S2 <B><FONT COLOR="#5F9EA0">=</FONT></B> F ()
<B><FONT COLOR="#A020F0">val</FONT></B> _: S1.t <B><FONT COLOR="#5F9EA0">-</FONT></B><B><FONT COLOR="#5F9EA0">></FONT></B> S2.t <B><FONT COLOR="#5F9EA0">=</FONT></B> <B><FONT COLOR="#A020F0">fn</FONT></B> x <B><FONT COLOR="#5F9EA0">=</FONT></B><B><FONT COLOR="#5F9EA0">></FONT></B> x
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Generativity also means that two different datatype declarations define different types, even if they define identical constructors. The following program does not type check due to this.
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<B><FONT COLOR="#A020F0">datatype</FONT></B> t <B><FONT COLOR="#5F9EA0">=</FONT></B> A <B><FONT COLOR="#5F9EA0">|</FONT></B> B
<B><FONT COLOR="#A020F0">val</FONT></B> a1 <B><FONT COLOR="#5F9EA0">=</FONT></B> A
<B><FONT COLOR="#A020F0">datatype</FONT></B> t <B><FONT COLOR="#5F9EA0">=</FONT></B> A <B><FONT COLOR="#5F9EA0">|</FONT></B> B
<B><FONT COLOR="#A020F0">val</FONT></B> a2 <B><FONT COLOR="#5F9EA0">=</FONT></B> A
<B><FONT COLOR="#A020F0">val</FONT></B> _ <B><FONT COLOR="#5F9EA0">=</FONT></B> <B><FONT COLOR="#A020F0">if</FONT></B> true <B><FONT COLOR="#A020F0">then</FONT></B> a1 <B><FONT COLOR="#A020F0">else</FONT></B> a2
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Last edited on 2005-01-26 20:34:48 by <span title="cfs36.cs.cornell.edu"><a href="MatthewFluet">MatthewFluet</a></span>.
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