What does reduce

 

List( 1, 2, 3, 4 ).reduce( (x,y) => x + y )   == (((1+2)+3)+4)
Step 1 : op( 1, 2 ) will be the first evaluation. 
  Start with 1, 2, that is 
    x is 1  and  y is 2
Step 2:  op( op( 1, 2 ), 3 ) - take the next element 3
  Take the next element 3: 
    x is op(1,2) = 3   and y = 3
Step 3:  op( op( op( 1, 2 ), 3 ), 4) 
  Take the next element 4: 
    x is op(op(1,2), 3 ) = op( 3,3 ) = 6    and y is 4