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copied!<blockquote> <p>How do I structure this kind of code into one which won't have a stack space problem and/or be lazy?</p> </blockquote> <p>You can't make this function process the list lazily, monads or no. Here's a direct translation of <code>countDigitsForList</code> to use pattern matching instead of <code>do</code> notation:</p> <pre><code>countDigitsForList [] = return [] countDigitsForList (x:xs) = case countDigits x of Left e -> Left e Right q -> case countDigitsForList xs of Left e -> Left e Right qs -> Right (q:qs) </code></pre> <p>It should be easier to see here that, because a <code>Left</code> at any point in the list makes the whole thing return that value, in order to determine the outermost constructor of the result, the entire list must be traversed and processed; likewise for processing each element. Because the final result potentially depends on the last character in the last string, this function as written is <em>inherently strict</em>, much like summing a list of numbers.</p> <p>Given that, the thing to do is ensure that the function is strict <em>enough</em> to avoid building up a huge unevaluated expression. A good place to start for information on that is discussions on the difference between <code>foldr</code>, <code>foldl</code> and <code>foldl'</code>.</p> <blockquote> <p>An accumulator and tail recursion seems like it may solve the problem but I repeatedly read that neither are necessary in Haskell because of lazy evaluation.</p> </blockquote> <p>Both are unnecessary when you can instead generate, process, and consume a list lazily; the simplest example here being <code>map</code>. For a function where that's not possible, strictly-evaluated tail recursion is precisely what you want.</p>

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