Finding the predecessor / successor of a list item in Prolog -

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i having hard time translating idea lisp prolog. did find out how find predecessor , successor in lisp having hard time implementing same idea in prolog. poor syntax. please me out.

sample queries:

?- predecessor(b,[a,b,c],p). p = a.                          % expected result  ?- successor(b,[a,b,c],s). s = c.                          % expected result

here's dcg approach:

pred(x, p) --> stuff, [p,x], stuff. succ(x, s) --> stuff, [x,s], stuff. stuff --> [] | [_], stuff.  predecessor(x, l, p) :- phrase(pred(x, p), l). successor(x, l, p) :- phrase(succ(x, s), l).

sporadic trials:

| ?- predecessor(a, [a,1,2,a,3], l).  l = 2 ? ;  no | ?- predecessor(x, [1,2,3,2,5], 2).  x = 3 ?  x = 5  no | ?- successor(a, [a,1,2,a,3], l).  l = 1 ?  l = 3  no | ?- successor(x, [1,2,3,2,5], 2).  x = 1 ?  x = 3  no | ?-

per @false's comments, implementation can tidied bit:

predecessor(x, l, p) :- phrase((..., [p,x], ...), l). successor(x, l, s) :- phrase((..., [x,s], ...), l). ... --> [] | [_], ... .

with same test results.


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