In Ruby it's something like:
[1,2,3].inject(0) {|a, x| a + x }
=> 6
Or just, [1,2,3].inject(:+)
Or just, [1,2,3].inject(:+)
In Haskell, you can even see it in the type signature for Monoid:
mconcat :: [a] -> a
You can see the array on the left ([1,2,3]) and the result unpacked (just 6).
What if you want to take it up one level of abstraction and have any operations on your list of numbers. You just use a different monoid called Endo.
To take it to this next level you need a more abstract append and identity.
Append needs to combine two operations:
compose = -> (f,g,*args) { f.call(g.call(*args)) }
And identity just returns what you give it:
id = -> (x) { x }
Which lets you then write:
[->(x){x + 2}, ->(x){x * 7} ].inject(id) {|x, y| compose(x, y) }.call(8)
=> 58
Or in Haskell:
Prelude> let a = foldr (.) id [(+2), (*7)]
Prelude> a 8
58
See:
