Folding

Scala

scala> def sum(lst: List[Int]) = (0 /: lst) ((x,y) => x+y)
sum: (List[Int])Int

scala> sum(List(1,2,3,4))
res10: Int = 10

ruby

>> def sum(array)
>> array.inject(0) {|sum,x| sum+x }
>> end
=> nil
>> sum([1,2,3,4])
=> 10

Scala

scala> def fac(n: Int)=(1 /: (1 to n)) (_ * _)
fac: (Int)Int

scala> fac(4)
res15: Int = 24

Ruby

>> def fac(n)
>> (1..n).inject(1) {|x,y| x * y }
>> end
=> nil
>> fac(4)
=> 24