Power Query M formula language: each expression

An each expression defines an anonymous function with one parameter whose name is the underscore (_).

each [XYZ] is equal to _[XYZ] which is equal to (_) => _[XYZ].

The following example evaluates to the list { 123457, 123457 }.

let incr = each _ + 1, incr_ = (_) => _ + 1, // same thing as above get_fld = each _[FLD], get_fld_ = (_) => _[FLD], // same thing as above REC = [ ABC = "abcdef", FLD = 123456 ] in { incr (get_fld (REC)), incr_(get_fld_(REC)) }

Because [X] is a shorthand for _[X], an each expression that accesses a record-field can further be abbreviated like so (which evaluates to "Hello world"):

let get_fld = each [txt], REC = [ num = 42, txt = "Hello world" ] in get_fld(REC)

Example with List.Select

The «power» of an each expression shines for example when using together with List.Select. List.Select is a function that finds elements of a list that possess a certain characteristics. This characteristics is evaluated with the second parameter of List.Select which expects a function with one parameter. If this function returns true, the element is retained in the resulting list, otherwise, it is discarded:

let
    L      = {
               [ a = 1, b = 7, c = "A" ],
               [ a = 3, b = 6, c = "B" ],
               [ a = 9, b = 7, c = "C" ],
               [ a = 8, b = 2, c = "D" ],
               [ a = 2, b = 4, c = "E" ],
               [ a = 4, b = 5, c = "F" ]
             },

    a_gt_b = List.Select(
               L,
               each [a] > [b]
             )
in
    Table.FromList(
        a_gt_b,
        Record.FieldValues,          // Apply this function on each element in a_gt_b: it turns the record into a list
      { "col a", "col b", "col c" }  // Name the columns
   )

Github repository about-Power-Query-Formula-M, path: /language/expressions/each/List.Select.M

When evaluated in Excel, for example with this VBA code, this formula evaluates to: