From cassidoo's May 29, 2023 Newsletter

oneRow.ml

module KeyToRowMap = Map.Make (Char)

let qwertyKeyToRowMap =
  let firstRow = ['q'; 'w'; 'e'; 'r'; 't'; 'y'; 'u'; 'i'; 'o'; 'p'] in
  let secondRow = ['a'; 's'; 'd'; 'f'; 'g'; 'h'; 'j'; 'k'; 'l'] in
  let thirdRow = ['z'; 'x'; 'c'; 'v'; 'b'; 'n'; 'm'] in
    KeyToRowMap.empty 
    |> (fun m -> List.fold_left (fun acc key -> KeyToRowMap.add key 1 acc) m firstRow)
    |> (fun m -> List.fold_left (fun acc key -> KeyToRowMap.add key 2 acc) m secondRow)
    |> (fun m -> List.fold_left (fun acc key -> KeyToRowMap.add key 3 acc) m thirdRow)

let isWordInOneRow map word =
  let word = String.lowercase_ascii word in
  let firstCharRow = KeyToRowMap.find word.[0] map in
  word 
    |> String.for_all (fun c -> KeyToRowMap.find c map == firstCharRow)

let oneRow = List.filter (isWordInOneRow qwertyKeyToRowMap)