NonEmptyList

NonEmptyList[T] is an immutable list with at least one element, enforced at construction. head, last and reduce return T directly rather than an Option.

from pyrula import NonEmptyList, IList

nel = NonEmptyList(1, 2, 3, 4)   # the first argument is the head; zero arguments raise TypeError
nel.head()                       # 1
nel.last()                       # 4
nel.reduce(lambda a, b: a + b)   # 10, seeded by head
nel.tail()                       # IList([2, 3, 4]), possibly empty

When the input may be empty, from_list returns an Option:

NonEmptyList.from_list([1, 2, 3])      # Some(NonEmptyList(1, 2, 3))
NonEmptyList.from_list([])             # Nothing
NonEmptyList.from_list(IList([10, 20]))  # Some(NonEmptyList(10, 20))

map, flat_map, append and prepend stay non-empty. flat_map needs a function that returns a NonEmptyList.

NonEmptyList(1, 2, 3).map(lambda x: x * 10)                   # NonEmptyList(10, 20, 30)
NonEmptyList(1, 2).append(3)                                  # NonEmptyList(1, 2, 3)
NonEmptyList(1, 2).flat_map(lambda x: NonEmptyList(x, x * 10))  # NonEmptyList(1, 10, 2, 20)

Convert with to_list(), to_ilist() or list(nel). fold_left(init, f) takes an explicit seed.