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    <p>The second problem is not a whitespace problem, you have to bracket the complex patterns <code>a:as</code> and <code>b:bs</code> so you'd write <code>add res (a:as) (b:bs)</code></p> <p>To put the point about whitespace in another way, the way it looks in the <code>where</code> clause is the way it would look at the top level. You would write: </p> <pre><code>addNums1 key num = add [] key num add res (a:as) (b:bs) | as == [] = res | otherwise = add (res ++ [a+b]) as bs </code></pre> <p>So, adding indentation, you'd write</p> <pre><code> addNums2 key num = add [] key num where add res (a:as) (b:bs) | as == [] = res | otherwise = add (res ++ [a+b]) as bs </code></pre> <p>But we can't unindent your where clause so it would be on the left margin. (I modify it to something equivalent)</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>since the guards are to the left of <code>add</code>; here they actually wind up on the left margin. I recommend lining up subordinate definitions with the governing <code>where</code>:</p> <pre><code>woof xs = foldr moo baaaah xs where moo :: Integer -&gt; Integer -&gt; Integer moo x 0 = 17 moo x y = x * x + y * (y + x + 1) baaaah :: Integer baaaah = 3 -- *Main&gt; woof [1,2] -- 529 </code></pre> <p>It's not as lovely as some things, but less error prone since it decreases the cognitive load of thinking more about indentation. (And Oleg does it!) It would immediately have averted this difficulty too. I think it isn't suitable everywhere, but this is more attractive and maybe makes indentation questions clearer:</p> <pre><code> woof xs = foldr moo baaaah xs where moo :: Integer -&gt; Integer -&gt; Integer moo x 0 = 17 moo x y = x * x + y * (y + x + 1) baaaah :: Integer baaaah = 3 </code></pre> <p>Then we can see that the list of definitions in the where clause are like the list of definitions in a Haskell module, lined up with a 'left margin'.</p>
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    1. COAnother style that avoids ambiguity (and is the style I use) is to simply always have the first token of an indentation-sensitive block in-line with whatever precedes it, then use vertical alignment afterwards. So `add res (a:as) (b:bs) | as == [] = res` would be the first line. Of course, this also encourages concision and careful decomposition into smaller functions, in order to avoid slamming into the right margin at 40m/s...
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    2. COI don't do that, maybe I should -- but it does seem to have the clear merit for beginners who are lost in Haskell whitespace that it reduces anxiety about placing the first guard.
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