digit : Parser (List U8) Nat
digit =
    input <- buildPrimitiveParser

    when input is
        ['0',..] -> Ok { val: 0, input: List.dropFirst input }
        ['1',..] -> Ok { val: 1, input: List.dropFirst input }
        ['2',..] -> Ok { val: 2, input: List.dropFirst input }
        ['3',..] -> Ok { val: 3, input: List.dropFirst input }
        ['4',..] -> Ok { val: 4, input: List.dropFirst input }
        ['5',..] -> Ok { val: 5, input: List.dropFirst input }
        ['6',..] -> Ok { val: 6, input: List.dropFirst input }
        ['7',..] -> Ok { val: 7, input: List.dropFirst input }
        ['8',..] -> Ok { val: 8, input: List.dropFirst input }
        ['9',..] -> Ok { val: 9, input: List.dropFirst input }
        _ -> Err (ParsingFailure "not a digit")

int : Parser (List U8) I64
int =
    const (\maybeDash -> \digits ->
        sign = when maybeDash is
            Ok _ -> -1
            Err _ -> 1

        List.walk digits 0 (\sum, d -> sum * 10 + d)
        |> Num.toI64
        |> Num.mul sign
    )
    |> keep (maybe (codeunit '-'))
    |> keep (oneOrMore digit)

when input is
    [ c, .. ] if c >= '0' && c <= '9' -> Ok { val: c - '0', input: List.dropFirst input }
    _ -> ...