PowerQuery

Let's say you have the table:

A
10
3
4
8

You want to add up successsively each line, and produce:

A	Sum
10	10
3	13
4	17
8	25

One way is to add an index column and add a custom column which takes into its input the actual result it is calculating. The code is the following:


	    let

	          Source = Excel.CurrentWorkbook(){[Name="Table"]}[Content],

	          Typed = Table.TransformColumnTypes(Source,{{"A", Int64.Type}}),

	          Indexed = Table.AddIndexColumn(Typed, "Index", 0, 1, Int64.Type),

	          AddSumColumn = Table.AddColumn(Indexed, "Sum",

	       each [A] + (if [Index]=0 then 0 else @AddSumColumn{[Index]-1}[Sum]))

	    in

	          AddSumColumn

One key point is the @AddSumColumn where @ is mandatory for each recursive definition.

This works because a structure (list, record, table) is a bunch of lazy evaluated cells and can contain a result based on the content of an other part of the same structure. But an error will be raised if the evaluation of a cell depends on the same cell. (ex a = @a +1)

A simpler example would be let Source = {42, @Source{0},@Source{1}} which is evaluated as let Source = {42,42,42}