append(+*List1, +*List2, +*List3)

True when all three arguments are lists, and the members of List3 are the members of List1 followed by the members of List2.


List1 term
a list
List2 term
a list
List3 term
a list consisting of List1 followed by List2


Appends lists List1 and List2 to form List3:

     | ?- append([a,b], [a,d], X).
     X = [a,b,a,d]
     | ?- append([a], [a], [a]).
     | ?- append(2, [a], X).

Takes List3 apart:

     | ?- append(X, [e], [b,e,e]).
     X = [b,e]
     | ?- append([b|X], [e,r], [b,o,r,e,r]).
     X = [o,r]
     | ?- append(X, Y, [h,i]).
     X = [],
     Y = [h,i] ;
     X = [h],
     Y = [i] ;
     X = [h,i],
     Y = [] ;


Suppose L is bound to a proper list (see lib-lis-prl). That is, it has the form [T1,...,Tn] for some n. In that instance, the following things apply:

  1. append(L, X, Y) has at most one solution, whatever X and Y are, and cannot backtrack at all.
  2. append(X, Y, L) has at most n+1 solutions, whatever X and Y are, and though it can backtrack over these it cannot run away without finding a solution.
  3. append(X, L, Y), however, can backtrack indefinitely if X and Y are variables.


The following examples are perfectly ordinary uses of append/3:

To enumerate adjacent pairs of elements from a list:

     next_to(X, Y, /*in*/ List3) :-
             append(_, [X,Y|_], List3).

To check whether Word1 and Word2 are the same except for a single transposition. (append/5 in library(lists) would be better for this task.)

     one_transposition(Word1, Word2) :-
             append(Prefix, [X,Y|Suffix], Word1),
             append(Prefix, [Y,X|Suffix], Word2).
     | ?- one_transposition("fred", X).
     X = "rfed" ;
     X = "ferd" ;
     X = "frde" ;

Given a list of words and commas, to backtrack through the phrases delimited by commas:

     comma_phrase(List3, Phrase) :-
             append(F, [','|Rest], List3),
             (   Phrase = F
             ;   comma_phrase(Rest, Phrase)
     comma_phrase(List3, List3).
     | ?- comma_phrase([this,is,',',um,',',an,
                            example], X).
     X = [this,is] ;
     X = [um] ;
     X = [an,example] ;

See Also

length/2 lib-lis library(lists)