aoc_day6_part1.u

letterChar : Char -> Boolean
letterChar c = List.contains c <| toCharList "abcdefghijklmnopqrstuvwxyz"

groupAnswers : Text -> Map Char Nat
groupAnswers gr =
  combine : Map Char Nat -> Char -> Map Char Nat
  combine accMap c =
    if letterChar c then
      use Nat +
      putWith (+) c 1 accMap
    else accMap
  List.foldLeft combine Map.empty <| toCharList gr

List.splitSubSeq : [a] -> [a] -> [[a]]
List.splitSubSeq lst sep =
  go : [a] -> [a] -> [[a]] -> [[a]]
  go lst' currSeq lists =
    match stripPrefix sep lst' with
      Some rest -> go rest [] (lists :+ currSeq)
      None      ->
        match lst' with
          x +: xs -> go xs (currSeq :+ x) lists
          []      -> lists :+ currSeq
  go lst [] []

Text.splitSubSeq : Text -> Text -> [Text]
Text.splitSubSeq s sep =
  List.map fromCharList (List.splitSubSeq (toCharList s) (toCharList sep))

day6 : Text -> Nat
day6 input =
  use List map
  groups = Text.splitSubSeq input "\n\n"
  theGroupAnswers = map groupAnswers groups
  sum <| map Map.size theGroupAnswers

doDay6 : '{IO} ()
doDay6 =
  'let
    f = openFile fp Read
    contents = getText f
    printLine (Nat.toText <| day6 contents)

As a data point, it seems like .base.Text.split, a simpler version of this function, is implemented in terms of toCharList

  split : Char -> Text -> [Text]
  split separator text =
    use Universal ==
    go acc rest =
      match indexOf separator rest with
        None   -> acc :+ fromCharList rest
        Some n ->
          use Nat +
          go (acc :+ fromCharList (List.take n rest)) (List.drop (n + 1) rest)
    if text == "" then [] else go [] (toCharList text)