(reduce f val coll)
In this animation, the function reduce takes 3 arguments:
f: a function that takes two arguments and returns a valueval: an initial valuecoll: a collection of items
reduce calls function f with val and the first item in coll. The value returned by f is then used to call f again, but this time with the second item in coll. The value returned by f is then used to call f again, but this time with the third item in coll … and so on until all items in coll have been used. The last value returned by f is the final value that reduce returns as result: it is the result of the reduce opération.