Prolog Type Error -

i'm working on prolog algorithm perform "swap" on list.

example:

input: [1,2,3,4] -> output: [3,4,1,2] input: [1,2,3,4,5] -> output: [4,5,3,1,2]

the first half , second half of list swap places, if there odd number center element retains it's position. have come algorithm, getting error:

?- swap([1,2,3,4],l). error: length/2: type error: `integer' expected, found `round(4/2)' 

my code follows:

swap(l, s) :-     length(l, length),     reverse(l, l2),     mixing(l, length, a),     trim(l2, length/2 , b),     append(a,b,s).  trim(l,n,s) :-     length(p,n),     append(p,s,l).  mixing(l, length, a) :-     (  mod(length, 2) == 0     -> trim(l, round(length/2), a)     ;  trim(l, round(length/2), a)     ). 

the problem in 'mixing' when call trim (l, round(length/2), a) type not integer? understand length/2 not integer (most float) , thought round equivalent integer(expr) rounds , transforms data type integer. tried replacing round truncate(expr) , integer(expr), receiving same errors. can explain i'm doing wrong?

round not function, it's predicate. haven't looked @ rest of code, line should be

round(length/2, r), trim(l, r, a) 

edit: btw, you're overthinking it.

swap([], []). swap([x], [x]). swap([x, y | a], [y, x | b]) :- swap(a, b).