An exercise in map, reduce, length for lists

list_ops.exs

defmodule ListOps do
  
  def length(list) do
    reduce(list,0,fn(item,acc) -> acc + 1 end)
  end

  def reduce([], acc, _) do
    acc
  end

  def reduce([h|t], acc, fun) do
    reduce(t,fun.(h,acc),fun)
  end

  def map(list, fun) do
    reduce(list,[],fn(item, acc) ->
      [fun.(item)|acc]
    end) |> Enum.reverse
  end
end

ExUnit.start

defmodule ListOpsTest do
  use ExUnit.Case, async: true

  test "length of empty list" do
    assert 0 == ListOps.length([])
  end

  test "length of list 1" do
    assert 1 == ListOps.length([1])
  end

  test "length of list 10" do
    assert 10 == ListOps.length([1,2,3,4,5,6,7,8,9,10])
  end

  test "reduce int list" do
    assert 55 == ListOps.reduce([1,2,3,4,5,6,7,8,9,10],0,fn(item, acc) ->
      acc + item
    end)
  end

  test "treat reduce like map" do
    assert [2,4,6] == ListOps.reduce([1,2,3],[],fn(item, acc) ->
      acc ++ [item * 2]
    end)
  end

  test "map ints + 1" do
    assert [2,3,4] == ListOps.map([1,2,3], &(&1 + 1))
  end
end