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    <p>There are a number of problems here.</p> <p>First of all, the result of a list comprehension is a list, so you're calling <code>Data.Vector.map</code> on a list, which won't work. And the <code>x</code> inside the comprehension is a <code>Vector</code>, which is another type mismatch. Either use a list instead of a Vector (along with <code>Prelude.map</code>) or convert the list to a <code>Vector</code> (in which case you can't use a list comprehension).</p> <p>Secondly, ignoring the list/<code>Vector</code> problem, <code>[i !! 2 | i &lt;- x]</code> will give you a list containing only the elements at position 2 from each sub-list. Using your example, the comprehension would yield <code>["12", "34"]</code>. Then when you map <code>read</code> over it, you'll get <code>[12, 34]</code>, rather than the output you're shooting for.</p> <p>Finally, the output you're wanting to see is not valid for lists or for <code>Vectors</code> in Haskell. Both types of container must be homogeneous, i.e. they cannot contain values of more than one type. A <code>[Int]</code> cannot contain <code>String</code>s, nor can a <code>[String]</code> contain <code>Int</code>s, but your desired output contains both. There are ways you can get around this using existential types, but chances are there's a better solution for your underlying problem than to try to build heterogeneous collections.</p> <p>Edit: You edited the last part of your post to use tuples, so the above paragraph no longer applies. The first two problems I mentioned still exist, though.</p> <p>If you start with a list of 4-tuples (<code>[(String, String, String, String)]</code>), you can get what you want like this:</p> <pre><code>&gt; let x = [("a", "b", "12", "d"), ("e", "f", "34", "g")] &gt; map (\(a, b, c, d) -&gt; (a, b, read c :: Int, d)) x [("a", "b", 12, "d"), ("e", "f", 34, "g")] </code></pre>
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