If I consider the functions head, length, elem &amp; sum, each is of them is strict, as:<br>head/length/elem x/sum _|_ are always _|_.<br><br>However:<br>head (x:_|_) is never _|_.<br>length [_|_, _|_, _|_ ...] is also never _|_.<br>

elem x [4,5,6,8,2,90,_|_,_|_ ...] is <i>only sometimes </i>_|_ (depending on x value).<br>In fact, only<br>sum [4,5,6,8,2,90,_|_,_|_ ...] is always _|_.<br><br>Which shows they don&#39;t have the same level of strictness.<br>

<br>So can you say things like &quot;all these functions are strict, but some are <i>more </i>than other&quot;, or &quot;sum is <i>deeply strict</i>&quot; ...?<br>What terms can you use to compare those functions?<br>