Compare data type values on choice of constructor only
<p>I want to compare just the constructor "tag" of a data type.</p>

<p>Is there an easy way to do this?</p>

<pre><code>data D = A | B Int | C String
check k d = ?

check A (A) -- True
check B (B 55) -- True
check C (C "xxx") -- True
</code></pre>