Applying schema programatically

[smpa01]

I have a source table from which I have retrievd schema. How can I apply the same schema programatically from a source table to a target table.

@AlexisOlson 

For example,

//src
let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUXIEYiMDIxNdQyNdQ0Ol2FgA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [colA = _t, colB = _t, colC = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"colA", Int64.Type}, {"colB", type text}, {"colC", type date}}),
    Custom1 = Table.Schema(#"Changed Type"),
    #"Removed Columns" = Table.RemoveColumns(Custom1,{"Kind", "IsNullable", "NumericPrecisionBase", "NumericPrecision", "NumericScale", "DateTimePrecision", "MaxLength", "IsVariableLength", "NativeTypeName", "NativeDefaultExpression", "NativeExpression", "Description", "IsWritable", "FieldCaption"})
in
    #"Removed Columns"

//target
let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUXIEYiMDIxNdQyNdQ0Ol2FgA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [colA = _t, colB = _t, colC = _t]),
    Custom1 = Table.ColumnNames(Source),
    #"Converted to Table" = Table.FromList(Custom1, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Merged Queries" = Table.NestedJoin(#"Converted to Table", {"Column1"}, src, {"Name"}, "src", JoinKind.LeftOuter),
    #"Expanded src" = Table.ExpandTableColumn(#"Merged Queries", "src", {"TypeName"}, {"TypeName"}),
    #"Added Custom" = Table.AddColumn(#"Expanded src", "Custom", each {[Column1],[TypeName]}),
    Custom = #"Added Custom"[Custom],
    Custom2 = Table.TransformColumnTypes(Source,Custom)
in
    Custom2

//desired
let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUXIEYiMDIxNdQyNdQ0Ol2FgA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [colA = _t, colB = _t, colC = _t]),
    Custom1 = Table.TransformColumnTypes(Source,{{"colA", Int64.Type}, {"colB", Text.Type}, {"colC", Date.Type}})
in
    Custom1

Thank you in advance.

[m_dekorte · Accepted solution]

Hi @smpa01 

You can use Expression evaluate and specify an environment, to illustrate:

     #"Added Custom" = Table.AddColumn(#"Expanded src", "Custom", each 
            { [Column1], Expression.Evaluate([TypeName], 
                [   #"Any.Type" = Any.Type,
                    #"Date.Type" = Date.Type,
                    #"DateTime.Type" = DateTime.Type,
                    #"DateTimeZone.Type" = DateTimeZone.Type,
                    #"Day.Type" = Day.Type,
                    #"Duration.Type" = Duration.Type,
                    #"Logical.Type" = Logical.Type,
                    #"Currency.Type" = Currency.Type,
                    #"Decimal.Type" = Decimal.Type,
                    #"Double.Type" = Double.Type,
                    #"Int16.Type" = Int16.Type,
                    #"Int32.Type" = Int32.Type,
                    #"Int64.Type" = Int64.Type,
                    #"Int8.Type" = Int8.Type,
                    #"Number.Type" = Number.Type,
                    #"Percentage.Type" = Percentage.Type,
                    #"Single.Type" = Single.Type,
                    #"Text.Type" = Text.Type,
                    #"Time.Type" = Time.Type
                ] )}
            ),

I hope this is helpful