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copied!<p>To answer your second question, you can use type equivalence constraint (from TypeFamilies extension) to relax your <code>RList</code> instance definition:</p> <pre><code>class HList l => RList r l instance RList r HNil instance (RList r1 l, r1 ~ r2) => RList r1 (HCons (R r2 a) l) </code></pre> <p>Now your <code>rc</code> will be inferred to the desired type.</p> <p>I don't think I can 'clearly explain' the phenomenon though (somebody surely will), but it's obvious that the difference between <code>rPair</code> and <code>rCons</code> is that while the former binds <code>r</code> type of both arguments to the same type variable, the later doesn't: the second argument is just <code>l</code> constrained with that there should be some instance of <code>RList</code> for that <code>l</code>). Since there is no type signature for <code>rc</code> (note that if you provide one your original example typechecks) and r1 and r2 have polymorphic, <em>not equivalent</em>, <code>r</code>'s , compiler is trying to find an instance definition for <code>RList r (HCons (R r1 Double) HNil)</code> (<code>r</code> comes from the 1st argument and <code>r1</code> - from the 2nd) and fails to do so. With type equivalence constraint we define an instance of RList with two distinct <code>r1</code> and <code>r2</code> with the only condition that these needs to be equivalent, so it looks like GHC binds them to the same polymorphic type variable when resolving instance of <code>RList</code> for <code>l</code>.</p>

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