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copied!<p>The main indendation rule in Haskell is that if you want to continue a definition on another line, it has to be further indented than the the thing you're defining. In this case the guards for your <code>add</code> function are <em>less</em> indented, so that's what the compiler is complaining about.</p> <p>Disregarding the other errors in your code, the indentation should be something like this:</p> <pre><code>addNums key num = add [] key num where add res a:as b:bs | a == [] = res | otherwise = add res:(a+b) as bs </code></pre> <p>Also note that the exact amount of indentation does not matter, only the indentation of the continuing lines relative to the thing being defined.</p> <p>Another syntactic problem with your code is that you seem to have misunderstood the precedence rules of Haskell. In Haskell, function application binds <em>tighter than any operator</em>, so <code>add res a:as b:bs</code> is parsed as <code>(add res a):(as b):bs</code>, while you meant <code>add res (a:as) (b:bs)</code>.</p> <p>The final problems are type errors. The <code>(:)</code> operator has the type <code>a -> [a] -> [a]</code>, which means that it takes <em>an element</em> and <em>a list</em>, and produces <em>a list</em>. In your code <code>res:(a+b)</code>, you appear to have this reversed, as <code>res</code> is a list and <code>a+b</code> is the element. Since there is no operator in Haskell to append a single element to the <em>end</em> of a list, you'll have to use the list concatenation operator <code>(++) :: [a] -> [a] -> [a]</code> instead: <code>res ++ [a+b]</code>.</p> <p>You're also comparing the <em>element</em> <code>a</code> to the <em>list</em> <code>[]</code> in your guard. This is probably not what you meant, and the pattern <code>(a:as)</code> would not match if the list was empty. The easiest solution to this is to add another pattern instead of your guard.</p> <p>Putting all of this together, this code should hopefully do what you intended:</p> <pre><code>addNums key num = add [] key num where add res [] _ = res add res (a:as) (b:bs) = add (res ++ [a+b]) as bs </code></pre> <p>P.S. Repeatedly appending to the end of a list is not very efficient. In fact, it's O(n<sup>2</sup>). You might want to add to the front instead and reverse the list when you're done. This is O(n).</p> <hr> <p>References:</p> <ul> <li><a href="http://en.wikibooks.org/wiki/Haskell/Indentation" rel="nofollow">Indentation</a></li> <li><a href="http://en.wikibooks.org/wiki/Haskell/Pattern_matching" rel="nofollow">Pattern matching</a></li> </ul>

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