Create a table where each row summarises column data from another table

Daniel, I'm afraid  I don't understand the meaning of "col" in that. I want to use a column name, but List.Transform expects a list as the first argument. Both of the two "result" lines here give a table of "Error" for the two columns.

let
fn=(tbl,col)=>#table(Table.ColumnNames(tbl),{List.Transform(col,each List.Sum(Table.Column(tbl,_)))}),
col="First Column",
tbl = #table({"First Column", "Second Column"}, {{1,2},{3,4}}),
result = fn(tbl,"First Column")
//result = fn(tbl,Table.Column(tbl,"First Column"))
in result

I'd better give an example. Suppose I have a fn that returns a list of three items from a given list of values.

let
fn=(listofvalues)=>{List.Count(listofvalues),List.First(listofvalues),List.Last(listofvalues)},
col="First Column",
tbl = #table({"First Column", "Second Column"}, {{1,2},{1,4},{5,3}}),
result = fn(Table.Column(tbl,col))
in result
/* the fn gives me a list for ONE column's list of values:
List
3
1
5

I would like an expression which calls fn for each column in the table and returns a Table with one row for each source column and as many columns as values in the returned summary list, like this:
ColumnName       Count First Last
First Column         3        1       5
Second Column    3        2       3

If it helps I could write fn as

fn=(listofvalues)=>[Count=List.Count(listofvalues),First=List.First(listofvalues),Last=List.Last(listofvalues)],

*/