prolog 中的合并排序 - 无限递归？

Question

I am attempting to implement mergesort in prolog (swi prolog on raspberry pi). I'm very new to the language and what I have here is basically my Haskell version "ported" to Prolog as best I could:

:- use_module(library(list_util), [split_at/4, take/3, drop/3]).

merge([], [], []).
merge(Lst, [], Lst).
merge([], Lst, Lst).
merge([X|XS], [Y|YS], MergedLst) :- X < Y, MergedLst = [X|MergedLst2], merge(XS, [Y|YS], MergedLst2).
merge([X|XS], [Y|YS], MergedLst) :- X >= Y, MergedLst = [Y|MergedLst2], merge([X|XS], YS, MergedLst2).


mergesort([], []).
mergesort([Only], [Only]).
mergesort(Lst, SortedLst) :-
  length(Lst, LstLen),
  HalfWay is truncate(LstLen / 2),
  take(HalfWay, Lst, FirstHalf),
  drop(HalfWay, Lst, SecondHalf),
  mergesort(FirstHalf, A),
  mergesort(SecondHalf, B),
  merge(A, B, SortedLst).

Running this code appears to perform the sort correctly, but then goes spiraling into infinite and beyond:

?- mergesort([1,6,3,10,4,9,2,4,7,3], Y).
Y = [1, 2, 3, 3, 4, 4, 6, 7, 9|...] ;
Y = [1, 2, 3, 3, 4, 4, 6, 7, 9|...] ;
Y = [1, 2, 3, 3, 4, 4, 6, 7, 9|...]

I creeped through the debugger (trace mode) and didn't find any clues

   Call: (18) merge([10], [], _5322) ? creep
   Exit: (18) merge([10], [], [10]) ? creep
   Exit: (17) merge([10], [9], [9, 10]) ? creep
   Exit: (16) merge([10], [7, 9], [7, 9, 10]) ? creep
   Exit: (15) merge([6, 10], [7, 9], [6, 7, 9, 10]) ? creep
   Exit: (14) merge([4, 6, 10], [7, 9], [4, 6, 7, 9, 10]) ? creep
   Exit: (13) merge([4, 6, 10], [4, 7, 9], [4, 4, 6, 7, 9, 10]) ? creep
   Exit: (12) merge([3, 4, 6, 10], [4, 7, 9], [3, 4, 4, 6, 7, 9, 10]) ? creep
   Exit: (11) merge([3, 4, 6, 10], [3, 4, 7, 9], [3, 3, 4, 4, 6, 7, 9, 10]) ? creep
   Exit: (10) merge([3, 4, 6, 10], [2, 3, 4, 7, 9], [2, 3, 3, 4, 4, 6, 7, 9|...]) ? creep
   Exit: (9) merge([1, 3, 4, 6, 10], [2, 3, 4, 7, 9], [1, 2, 3, 3, 4, 4, 6, 7|...]) ? creep
   Exit: (8) mergesort([1, 6, 3, 10, 4, 9, 2, 4|...], [1, 2, 3, 3, 4, 4, 6, 7|...]) ? creep
Y = [1, 2, 3, 3, 4, 4, 6, 7, 9|...] ;

I tried it again with an empty list and this time the debugger reveals that "merge" is being called, which shouldn't be happening for that case.

?- mergesort([], Y).
Y = [] ;
Y = [] ;
Y = []
Action (h for help) ? trace
continue (trace mode)
; [trace]
   Redo: (9) merge([], [], _3038) ? creep
   Exit: (9) merge([], [], []) ? creep
   Exit: (8) mergesort([], []) ? creep
Y = []

If anyone more experienced in the language knows where the infinite loop is coming from, please let me know, thank you.

Answer 1

Your third clause of mergesort will match previous two clauses. You can cut the first two clauses or pattern match the third to ensure atleast two elements.

mergesort(Lst, SortedLst) :-
    Lst = [_, _ | _],
    ...

Or

mergesort([], []) :- !.
mergesort([Only], [Only]) :- !.