What

Both have a base value:

• Number: 0.

• List: Empty list.

Both have an operation to "step" up a value that can cover the whole value universe of the type:

• Number: +1.

• List: cons with a value.

With this, we can encode numbers and lists as:

• Number: $\lambda a.\lambda z.a(a(a...z))$.

  • $a$ stands for increment.

  • $z$ stands for zero.

    -> A number $n$ roughly means applying increment $n$ times to zero.

• List: $\lambda c.\lambda l.c{e}_0(c{e}_1(c{e}_2...l))$ .

  • $c$ stands for cons.

  • $l$ stands for empty list.

    -> A list roughly means applying cons with a value $n$ times to the empty list.