confused about Prolog Response -


consider:

bar([], ws, ws). bar([x|xs], ys, [x|zs]) :- bar(xs, ys, zs).

1) how have no solutions 

bar([1,2], [], [x,x]).

2) how have 1 solution when z=5?

bar([5,y], [y,5], [z,z|_]).

3)how have 1 solution when z=5 , y = 5?

bar([_,y], [5,5], [z,z,z|_]).

first, consider meaning of bar/3: rule has 3 arguments representing lists. bar/3 returns solution when last list concatenation of first two:

bar([1,2,3], [4,5], x)       // x=[1,2,3,4,5] bar([1,2,3], x, [1,2,3,4,5]) // x=[4,5] bar(x, [4,5], [1,2,3,4,5])   // x=[1,2,3]

knowing this, can figure answers questions:

  • bar([1,2], [], [x,x]). needs x unify 1 , 2 @ same time, impossible
  • bar([5,y], [y,5], [z,z|_]). unifies 5 first z, , second z y, single solution
  • bar([_,y], [5,5], [z,z,z|_]). unifies y second z, , third z 5, single solution.

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