DAX EARLIER function in Power Query

Question

I have a table with lots of different dates in column DATE and a smaller number of letters in column LETTER. I now want the maximum date for each letter.

In DAX, I would use something like 

CALCULATE(MAX([Date]),FILTER(ALL(Table),[Letter]=EARLIER([Letter])).

Is there an equivalent to EARLIER in M/Power Query?

Answer 1

Solutions in the code below. Notice that each uses "PreviousStep" as a basis, so these are separate solutions.

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    PreviousStep = Table.TransformColumnTypes(Source,{{"Date", type date}, {"Letter", type text}}),

    // 1. Add a column to the original table with the MaxDate for each letter
    //    "earlier"  is just the name of a function parameter; it could as well have been "x" or "MarcelBeug"
    AddedMaxDate = Table.AddColumn(PreviousStep, "MaxDate", (earlier) => List.Max(Table.SelectRows(PreviousStep, each [Letter] = earlier[Letter])[Date])),

    // 2. Group by letter and get the MaxDate for each letter
    GroupedOnLetter = Table.Group(PreviousStep, {"Letter"}, {{"MaxDate", each List.Max([Date]), type date}})
in
    GroupedOnLetter