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pattern-match on the input and, depending on the data constructor, either recur on a smaller input or terminate the recursion with the base case. When you write the larger case, you can pretend you already have the definition written. Remember that the cons operator adds something to the begining of the list. Here's a simpler example:-- not tail recursive sum1 [] = 0 sum1 (x:xs) = x + sum1 xs What should we add? Yes, once you call again f with a new value of n, it has no way to reference the old value of n unless you pass it explicitly. ._2a172ppKObqWfRHr8eWBKV{-ms-flex-negative:0;flex-shrink:0;margin-right:8px}._39-woRduNuowN7G4JTW4I8{border-top:1px solid var(--newCommunityTheme-widgetColors-lineColor);margin-top:12px;padding-top:12px}._3AOoBdXa2QKVKqIEmG7Vkb{font-size:12px;font-weight:400;line-height:16px;-ms-flex-align:center;align-items:center;background-color:var(--newCommunityTheme-body);border-radius:4px;display:-ms-flexbox;display:flex;-ms-flex-direction:row;flex-direction:row;margin-top:12px}.vzEDg-tM8ZDpEfJnbaJuU{color:var(--newCommunityTheme-button);fill:var(--newCommunityTheme-button);height:14px;width:14px}.r51dfG6q3N-4exmkjHQg_{font-size:10px;font-weight:700;letter-spacing:.5px;line-height:12px;text-transform:uppercase;display:-ms-flexbox;display:flex;-ms-flex-pack:justify;justify-content:space-between}._2ygXHcy_x6RG74BMk0UKkN{margin-left:8px}._2BnLYNBALzjH6p_ollJ-RF{display:-ms-flexbox;display:flex;margin-left:auto}._1-25VxiIsZFVU88qFh-T8p{padding:0}._3BmRwhm18nr4GmDhkoSgtb{color:var(--newCommunityTheme-bodyText);-ms-flex:0 0 auto;flex:0 0 auto;line-height:16px} Let's start with this incorrect definition: How do you calculate the length of a list? This is a loose fork of Edward Kmett's recursion-schemes library. Recursion Function. When the tail gets to an empty list, the base case will be invoked and recursion will stop. Another factor I found is that a lot of times it's explained badly. 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