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copied!<p>Try to give the expression a type.</p> <pre><code>xs = [if x < 3 then "bang" else "boom" | x <- xs] </code></pre> <p>So <code>xs</code> is a list, we don't know yet what type its elements have, so let's look at that next. The list elements are</p> <pre><code>if x < 3 then "bang" else "boom" </code></pre> <p>which is an expression of type <code>String</code> (aka <code>[Char]</code>).</p> <p>So <code>xs :: [String]</code>. Since the <code>x</code> from the expression describing the list elements is taken from the list <code>xs</code> itself, it is a <code>String</code> too, and is used in the comparison</p> <pre><code>if x < 3 </code></pre> <p>Now, <code>3</code> is an integer literal, thus it is polymorphic and has type</p> <pre><code>3 :: Num a => a </code></pre> <p>So from the expression <code>x < 3</code>, we have</p> <ul> <li>a <code>Num</code> constraint from the literal,</li> <li>the type <code>String</code> from the fact that <code>x</code> is drawn from a list of <code>String</code>s.</li> </ul> <p>Thus we need a <code>Num</code> instance for <code>String</code> to have a well-typed expression.</p> <p>Usually, there is no <code>Num</code> instance for <code>String</code> (what would a useful one look like?), so you get a type error.</p> <p>If <code>xs</code> is the argument of a function,</p> <pre><code>boom xs = [if x < 3 then "bang" else "boom" | x <- xs] </code></pre> <p>there is no reason why the type of <code>x</code> should be <code>String</code>, hence that works.</p>

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