_fold

Applies a QL expression iteratively to the elements of a List -- Function

This function is advanced, but sometimes enormously useful. It is Querki's version of the common functional-programming function fold().

Basically, this receives a List, and takes two parameters: an initial value and an expression. It runs through the List, one element at a time, building up an "accumulator". Each time through, you get the current accumulator and the next element; the expression should result in a new value, which will become the new accumulator.

The accumulator is passed into the expression as the bound name $acc; the next element as $next.

It is best understood through an example. This is how you would implement "sum" using _fold:

<1, 2, 3, 4> -> _fold(0, $acc -> _plus($next))

That is, you can think of a sum as simply adding all of the elements together one at a time. So you start with 0 (the first parameter). The first time it calls the expression, $acc is 0 and $next is 1 (the first element of the List), so it results in 1. Next time around, $acc is 1 and $next is 2 (the next element), so it results in 3. Then it adds 3 and 3 to get 6; then it adds 6 and 4, to get 10.

For convenience, the value of $acc is also given at the context of the expression. So you could actually express the above even more concisely as:

<1, 2, 3, 4> -> _fold(0, _plus($next))