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copied!<p>No, it was not "easy", as it doesn't work, unfortunately. <code>halve([1,2,3]-[],A,B)</code> doesn't work; <code>halve_pom([1,2,3]-[],A,B)</code> doesn't work either. Also, it is unclear which of the splitting schemes you prefer, <code>[1,2,3,4,5,6,7] -> ([1,3,5,7] , [2,4,6])</code> or <code>-> ([1,2,3,4] , [5,6,7])</code>.</p> <p>If your <code>halve</code> predicate worked, all you'd have left to do is to define <code>merge</code>, which would merge the two halves of a list, <em>in order</em>. Then,</p> <pre><code>mergesort(A,S):- halve(A,B-[],C-[]), mergesort(B,SB), mergesort(C,SC), merge(SB,SC,S-[]). </code></pre> <p>Note that you probably would call it with a normal list <code>A</code>, i.e. the <code>halve</code> predicate should expect its first argument as a normal list (i.e. not a difference-list).</p> <p>See also <a href="https://stackoverflow.com/questions/15456014/what-does-the-symbol-mean-in-prolog-when-dealing-with-lists">What does the "-" symbol mean in Prolog when dealing with lists?</a> . The <code>'-'</code> is unnecessary; instead, its two constituents should be used as two separate arguments to a predicate.</p> <hr> <p>So, one way to code <code>halve</code> is</p> <pre><code>halve([A,B|C],[A|X]-ZX,[B|Y]-ZY):- halve(C,X-ZX,Y-ZY). halve([A],[A|X]-X,Y-Y). halve([],X-X,Y-Y). </code></pre> <p>The same approach can be used to code <code>merge</code>:</p> <pre><code>merge(SB,SC,S-Z):- SB=[A|B], SC=[C|D], A=<C, S=[A|T], merge(B,SC,T-Z). merge(SB,SC,S-Z):- SB=[A|B], SC=[C|D], A>C, ... , ... . merge(SB,SC,S-Z):- SB=[A|B], SC=[], ... , ... . merge(SB,SC,S-Z):- SB=[], SC=[C|D], S=[C|T], ... . merge([],[],Z-Z). </code></pre>

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