diff --git a/docs/reference/list.rst b/docs/reference/list.rst index df19597f..6d1896f3 100644 --- a/docs/reference/list.rst +++ b/docs/reference/list.rst @@ -82,7 +82,47 @@ In general, lists are written with square brackets. Disco> 2 ∈ [1,2,3] T -Inductive definition of lists ------------------------------ +Recursive list processing +------------------------- -Write more here. +Fundamentally, lists in Disco are built from two ingredients: + +- The empty list ``[]`` +- The "cons" operator ``::`` which adds one more element to the start + of a list. That is, ``x :: rest`` is a list which has a first element ``x`` + and where ``rest`` is the rest of the list. + +For example, ``1 :: [2,3,4,5]`` represents adding the element ``1`` to +the beginning of the list ``[2,3,4,5]``; it is the same as +``[1,2,3,4,5]``. + +:: + + Disco> 1 :: [2,3,4,5] + [1, 2, 3, 4, 5] + +In fact, any list can be written using only ``[]`` and ``::``. + +- The empty list is ``[]``; +- ``x :: []`` represents a list with one element; +- ``x :: (y :: [])`` is a list with two elements; +- and so on. + +:: + + Disco> 1 :: [] + [1] + Disco> 1 :: (2 :: []) + [1, 2] + Disco> 1 :: (2 :: (3 :: [])) + [1, 2, 3] + +We can write recursive functions to process lists by :doc:`pattern +matching ` on ``[]`` and ``::``. For example, below is a +function to add all the elements of a list of natural numbers: + +:: + + sum : List(N) -> N + sum([]) = 0 + sum(n :: ns) = n + sum(ns)