这是prolog代码(我跟着它).
len([],0).
len([_|T],N) :- len(T,X), N is X+1.
这是它的踪迹(即运行linux,swi)
[trace] ?- len([d,f,w,c],X).
Call: (7) len([d, f, w, c], _G314) ?
Call: (8) len([f, w, c], _L182) ?
Call: (9) len([w, c], _L201) ?
Call: (10) len([c], _L220) ?
Call: (11) len([], _L239) ?
Exit: (11) len([], 0) ?
^ Call: (11) _L220 is 0+1 ?
^ Exit: (11) 1 is 0+1 ?
Exit: (10) len([c], 1) ?
^ Call: (10) _L201 is 1+1 ?
^ Exit: (10) 2 is 1+1 …