concat_str : List(Str), Str -> Str
concat_str = |items, seperator| {
  items.fold_with_index("", |state, item, index|{
    if index == 0 state.concat(item) else state.concat(seperator).concat(item)
  })
}
expect [] |> concat_str(",") |> Str.is_eq("")
expect [ "A" ] |> concat_str(",") |> Str.is_eq("A")
expect [ "A", "B" ] |> concat_str(",") |> Str.is_eq("A,B")
expect [ "A", "B", "C" ] |> concat_str(",") |> Str.is_eq("A,B,C")

keep_until : List(item), (item -> Bool) -> List(item)
keep_until = |items, predicate| {
    var $result = []
    for item in items {
        if predicate(item) == False {
            $result = $result.append(item)
        } else return $result
    }
    $result
}
expect [1, 2, 3] |> keep_until(|item| item == 2) |> List.is_eq([1])
expect [1, 2, 3] |> keep_until(|item| item > 2) |> List.is_eq([1, 2])
expect [1, 2, 3] |> keep_until(|item| item > 0) |> List.is_empty()

keep_while : List(item), (item -> Bool) -> List(item)
keep_while = |items, predicate| {
    var $result = []
    for item in items {
        if predicate(item) {
            $result = $result.append(item)
        } else return $result
    }
    $result
}
expect [1, 2, 3] |> keep_while(|item| item == 1) |> List.is_eq([1])
expect [1, 2, 3] |> keep_while(|item| item < 3) |> List.is_eq([1, 2])
expect [1, 2, 3] |> keep_while(|item| item < 1) |> List.is_empty()