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    <p>There are two code paths, comparing items by index [the "yes" condition] and then comparing them without regard to index [the "maybe" path]. Using recursion and a helper function [or two] allows both code paths to be followed:</p> <pre><code>val ls1 = ["duck", "goose", "swan", "gull"] val ls2 = ["duck", "swan", "goose", "pigeon"] fun checker (list1,list2) = (* Zipping the two lists together creates a list of pairs that can be searched for "yes" values as the first part of the aux function. It might be worth checking to see if ListPair.zipEq is more suited to the needs of a particular appliation. *) let val zipped = ListPair.zip(list1, list2) (* find_in_list is called if there is no "yes" match. It recurses down list2 with the string from list1 which did not return "yes". *) fun find_in_list (x, xs) = case xs of [] =&gt; "no" | x'::xs' =&gt; if x' = x then "maybe" else find_in_list (x, xs') (* This function could be the main body of checker, but instead it trampolines. First it checks for "yes". Either appends "yes" onto a recursive call to itself or otherwise appends a call to find_in_list onto a recursive call to itself. The type checker wants an explicit type for lop because the record is being accessed with #1 and #2 *) fun aux (lop : (string * string) list) = case lop of [] =&gt; [] | x'::xs' =&gt; if #1 (hd lop) = #2 (hd lop) then "yes"::aux (tl lop) else (find_in_list (#1 (hd lop), list2))::(aux (tl lop)) in aux(zipped) end checker (ls1,ls2) (* returns ["yes", "maybe", "maybe", "no"] *) </code></pre>
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    1. CONo reason to use `hd` and `tl` on `lop` - you have already split it into `x'` and `xs'`, and using these would be clearer. Additionally, instead of using `#1 (hd lop)` (or `#1 x'`), you can pattern match that too: replace `x'::xs'` with `(a, b)::xs'`.
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