On Fri, Dec 21, 2012 at 9:27 AM, Christopher Howard wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im">Thank you for your help. An additional question, if I might: For the</div>


sake of elegance and simplicity, I modified the class and instances to<br>
avoid the &quot;tuple&quot; aspect:<br>
<br>data Socket2 a b = Socket2 a b<br>instance (Monoid a, Monoid b) =&gt; Monoid (Socket2 a b) where<br><br>
Of course, I thought it would be likely I would want other classes and<br>
instances with additional numbers of types:<br>
<br>data Socket3 a b c = Socket3 a b c<br>instance (Monoid a, Monoid b, Monoid c) =&gt; Monoid (Socket3 a b c) where<br><br>
data Socket4 a b c d = Socket4 a b c d<br>instance (Monoid a, Monoid b, Monoid c, Monoid d) =&gt; Monoid (Socket4 a b<br><br>
data Socket 5 a b c d e... et cetera<br>
--------<br>
<br>
Seeing as the pattern here is so rigid and obvious, I was wondering: is<br>
it possible to abstract this even more? So I could, for instance, just<br>
specify that I want a Socket with 8 types, and poof, it would be there?<br>
Or is this as meta as we get? (I.e., without going to something like<br>
Template Haskell.)<br></blockquote><div><br></div><div>This perhaps isn&#39;t the answer you were looking for, but just in case you weren&#39;t aware, there are already Monoid instances for tuples up to 5. You can see this at:</div>

<div><br></div><div><a href="http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Monoid.html#g:1">http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Monoid.html#g:1</a></div><div><br>
</div>
<div>Another possibility is a generic monoid class (using generic-deriving):</div><div><br></div><div><div>{-# LANGUAGE TypeOperators #-}</div><div>{-# LANGUAGE DefaultSignatures #-}</div><div>{-# LANGUAGE FlexibleContexts #-}</div>

<div><br></div><div>--------------------------------------------------------------------------------</div><div><br></div><div>import GHC.Generics</div><div><br></div><div>--------------------------------------------------------------------------------</div>

<div><br></div><div>class GMonoid&#39; f where</div><div>  gempty&#39;  :: f x</div><div>  gappend&#39; :: f x -&gt; f x -&gt; f x</div><div><br></div><div>instance GMonoid&#39; U1 where</div><div>  gempty&#39; = U1</div>

<div>  gappend&#39; U1 U1 = U1</div><div><br></div><div>instance GMonoid a =&gt; GMonoid&#39; (K1 i a) where</div><div>  gempty&#39; = K1 gempty</div><div>  gappend&#39; (K1 x) (K1 y) = K1 (x `gappend` y)</div><div><br></div>

<div>instance GMonoid&#39; f =&gt; GMonoid&#39; (M1 i c f) where</div><div>  gempty&#39; = M1 gempty&#39;</div><div>  gappend&#39; (M1 x) (M1 y) = M1 (x `gappend&#39;` y)</div><div><br></div><div>instance (GMonoid&#39; f, GMonoid&#39; h) =&gt; GMonoid&#39; (f :*: h) where</div>

<div>  gempty&#39; = gempty&#39; :*: gempty&#39;</div><div>  gappend&#39; (x1 :*: y1) (x2 :*: y2) = gappend&#39; x1 x2 :*: gappend&#39; y1 y2</div><div><br></div><div>--------------------------------------------------------------------------------</div>

<div><br></div><div>class GMonoid a where</div><div>  gempty  :: a</div><div>  gappend :: a -&gt; a -&gt; a</div><div><br></div><div>  default gempty :: (Generic a, GMonoid&#39; (Rep a)) =&gt; a</div><div>  gempty = to gempty&#39;</div>

<div><br></div><div>  default gappend :: (Generic a, GMonoid&#39; (Rep a)) =&gt; a -&gt; a -&gt; a</div><div>  gappend x y = to (gappend&#39; (from x) (from y))</div><div><br></div><div>instance (GMonoid b, GMonoid c) =&gt; GMonoid (b,c)</div>

<div>-- ...</div></div><div><br></div><div>Regards,</div><div>Sean</div></div>