lists.rb

head     = ->(list) { list.call {|h, _, _| h } }
tail     = ->(list) { list.call {|_, t, _| t } }
is_empty = ->(list) { list.call {|_, _, e| e } }

empty = ->(&selector) do
    selector[nil, nil, true]
end

prepend = ->(el, list) do
    ->(&selector) { selector[el, list, false] }
end

map = ->(list, &fn) do
    is_empty[list] ? empty : prepend[fn[head[list]], map[tail[list], &fn]]
end

filter = ->(list, &fn) do
    return empty if is_empty[list]
    out = filter[tail[list], &fn] || empty
    prepend[head[list], out] if fn[head[list]]
end

append = ->(el, list) do
    return prepend[el, empty] if is_empty[list]
    prepend[head[list], append[el, tail[list]]]
end

last = ->(list) do
    return head[list] if is_empty[tail[list]]
    last[tail[list]]
end

list = prepend["a", prepend["b", prepend["c", empty]]]

list = append["d", list]

map.call(list) {|i| p i }