How to add functions in advanced editor(Power Query)

[Jinoh]

I'm a newcomer for Power Query and I'm wondering how to add some functions in Power Query Advanced editor

Here's my current advanced editor function

---

let
Source = Table.Combine({#"Amazon May item discount Item O Ship X", #"Amazon May item discount Item O Ship O", #"Amazon May item discount Item X Ship O", #"Amazon May item discount Item X Ship X"}),
#"Replaced Value" = Table.ReplaceValue(Source,null,"",Replacer.ReplaceValue,{"promotion-ids.3"}),
#"Reordered Columns" = Table.ReorderColumns(#"Replaced Value",{"amazon-order-id", "purchase-date", "sales-channel", "sku", "quantity", "currency", "item-price", "item-promotion-discount", "promotion-ids1", "promotion-ids.2", "promotion-ids.3"}),
#"Renamed Columns" = Table.RenameColumns(#"Reordered Columns",{<!-- -->{"promotion-ids1", "promotion-ids.1"}}),
#"Split Column by Delimiter" = Table.SplitColumn(#"Renamed Columns", "sku", Splitter.SplitTextByDelimiter("_", QuoteStyle.Csv), {"sku.1", "sku.2"}),
#"Changed Type" = Table.TransformColumnTypes(#"Split Column by Delimiter",{<!-- -->{"sku.1", type text}, {"sku.2", type text}, {"promotion-ids.2", type text}, {"promotion-ids.3", type text}}),
#"Removed Columns" = Table.RemoveColumns(#"Changed Type",{"sku.2"}),
#"Split Column by Delimiter1" = Table.SplitColumn(#"Removed Columns", "sku.1", Splitter.SplitTextByDelimiter("&", QuoteStyle.Csv), {"sku.1.1", "sku.1.2", "sku.1.3", "sku.1.4", "sku.1.5", "sku.1.6"}),
#"Changed Type1" = Table.TransformColumnTypes(#"Split Column by Delimiter1",{<!-- -->{"sku.1.1", type text}, {"sku.1.2", type text}, {"sku.1.3", type text}, {"sku.1.4", type text}, {"sku.1.5", type text}, {"sku.1.6", type text}})
#
in
#"Changed Type1"

---

And I want to add this function below

---

let Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
shiftedList = {null} & List.RemoveLastN(Table.Column(Source,"promotion-ids.1"),1),
custom1 = Table.ToColumns(Source) & {shiftedList},
custom2 = Table.FromColumns(custom1,Table.ColumnNames(Source) & {"Prev"}),
x=Table.ReplaceValue(custom2, each [#"promotion-ids.1"], each if [Prev]=[#"promotion-ids.1"] then null else[#"promotion-ids.1"] ,Replacer.ReplaceValue,{"promotion-ids.1"}),

"Removed Columns" = Table.RemoveColumns(x,{"Prev"})

in #"Removed Columns"

---

The current table name is "Amazon May item discount integration" and where I'm willing to make changes is a column named "promotion-ids.1"

Attached are screenshots of Current tables and advanced editor.

I tried to paste the added functions but it said there's an error with it.

Can anybody help me solve this problem?

[Greg_Deckler · Accepted solution]

@Jinoh Here is the official documentation on using custom functions with Power Query. 

Using custom functions in Power Query - Power Query | Microsoft Docs

Custom functions have a slightly different syntax than queries as they are designed to ingest parameters.

let
    Source = () => let
        Source = ""
    in
        Source
in
    Source