title: readCol = map fromSql non type-variable argument
body: <pre><code>:i fromSql 
  fromSql ::
    convertible-1.1.1.0:Data.Convertible.Base.Convertible SqlValue a =&gt;
    SqlValue -&gt; a
          -- Defined in ‘Database.HDBC.SqlValue’
  </code></pre>
  <p>If I run e.g <code>map fromSql (EXAMPLESQL)</code> it outputs ok. But if I redefine it as <code>readCol = map fromSql</code> I get the error:</p>
  <pre><code>readCol = map fromSql
  
  &lt;interactive&gt;:23:1: error:
      • Non type-variable argument
          in the constraint: convertible-1.1.1.0:Data.Convertible.Base.Convertible
                               SqlValue b
        (Use FlexibleContexts to permit this)
      • When checking the inferred type
          readCol :: forall b.
                     convertible-1.1.1.0:Data.Convertible.Base.Convertible SqlValue b =&gt;
                     [SqlValue] -&gt; [b]
  </code></pre>
  <p>How to solve this?</p>