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copied!<p>This is a "keyed" operation, and can be solved quite easily with a little help from the standard library. </p> <pre><code>import qualified Data.Map as M </code></pre> <p><code>Data.Map</code> implements finite associative maps -- i.e. <code>M.Map k a</code> has a set of keys of type <code>k</code>, and to each key it associates a value of type <code>a</code>.</p> <p>The extremely useful function for this kind of problem is <a href="http://www.haskell.org/ghc/docs/6.12.1/html/libraries/containers-0.3.0.0/Data-Map.html#v%3AfromListWith"><code>fromListWith</code></a>:</p> <pre><code>M.fromListWith :: (a -> a -> a) -> [(k,a)] -> M.Map k a </code></pre> <p>The second argument, the list of <code>(k,a)</code> tuples, are simply associations, assocating a given key to a given value. The first argument is a combining function, which says what to do to the values if duplicate keys appear in the list. </p> <p>You must also use <code>M.toList :: M.Map k a -> [(k,a)]</code> which gives you back the list of associations stored in the Map. Here's an example:</p> <pre><code>ghci> M.toList (M.fromListWith (+) [(1,2), (2,3), (1,4), (3,5)]) [(1,6),(2,3),(3,5)] </code></pre> <p>Note how the keys are the <em>first</em> elements of the tuples, the opposite of how you have stated your problem. We have combined <code>(1,2)</code> and <code>(1,4)</code> into <code>(1,6)</code>. We added because we gave the combining function <code>(+)</code>.</p> <p>This one function solves the brunt of your problem -- the rest is just a little plumbing, which I will leave to you.</p>

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