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PO'Multiple declarations of' error when pattern matching with wildcard
 

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    <p>For learning purposes I am trying to write my own implementation of the <code>zipWith</code> function. However, I am hitting an issue with pattern matching on edge cases with <code>_</code>. First I will describe the good case, then the bad case. Hopefully someone will be able to explain why they behave differently. Thanks</p> <p>If I write the <code>zipWith</code> function as follows, it works (Note the order of the edge cases matching empty list on lines 2 &amp; 3):-</p> <pre><code>zipWith' :: (a -&gt; b -&gt; c) -&gt; [a] -&gt; [b] -&gt; [c] zipwith' _ [] _ = [] zipWith' _ _ [] = [] zipWith' f (x:xs) (y:ys) = f x y : zipWith' f xs ys </code></pre> <p>Compiling in GHCI:-</p> <pre><code>ghci&gt; :l ZipWith.hs [1 of 1] Compiling Main ( ZipWith.hs, interpreted ) </code></pre> <p>Ok, the above is fine, yet if I swap the pattern matching for the edge cases around GHCI throws 'Multiple declarations of' error for lines 2 and 4.</p> <pre><code>zipWith' :: (a -&gt; b -&gt; c) -&gt; [a] -&gt; [b] -&gt; [c] zipWith' _ _ [] = [] zipwith' _ [] _ = [] zipWith' f (x:xs) (y:ys) = f x y : zipWith' f xs ys </code></pre> <p>Compiling in GHCI:-</p> <pre><code>ZipWith.hs:4:0: Multiple declarations of `Main.zipWith'' Declared at: ZipWith.hs:2:0 ZipWith.hs:4:0 Failed, modules loaded: none. </code></pre> <p>I'm stumped...</p> <ol> <li>Looking at the patterns on lines 2 and 4 they seem mutually exclusive but i'm obviously missing something fundamental here</li> <li>Why would switching the patterns on lines 2 and 3 cause the compilation error to go away.</li> </ol>
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