collatzConjecture = \number ->
    when (number % 2) is
        0 -> (number // 2)
        _ -> (3 * number + 1)

collatzConjecture = \number ->
    when (Num.isEven number) is
        Bool.true -> (number // 2)
        _ -> (3 * number + 1)

This pattern is malformed
16│          Bool.true -> (number // 2)
             ^^^^^^^^^

module [steps]

steps : U64 -> Result U64 _
steps = \number ->
    when number is
        0 -> Err "Input must be greater than 0"
        _ -> Ok (step number 0)

step = \number, count ->
    when number is
        1 -> count
        _ -> number |> collatzConjecture |> step (count + 1)

collatzConjecture = \number ->
    when (Num.isEven number) is
        Bool.true -> (number // 2)
        _ -> (3 * number + 1)