F# join string option list

Question

How do I concatenate a list of string option?

let m = [ ""; "12"; "a"; "b"] 
// I can join these with 
m |> List.toSeq |> String.concat "\n" 

// now I got a list of string option list 
let l = [Some ""; None; Some "a"; Some "b"] 
l |> List.toSeq |> ????

Answer 1

you first use List.choose to "extract" the Some values of the list :

l |> List.choose id |> String.concat "\n"

Note you don't need List.toSeq as seq is an alias for IEnumerable and 't list implement it already.