View Source DRAFT: Why Lens.into invites bugs

Here I'll describe a very common mistake using Lens2.Lenses.Enum.into/2. This is sort of a digression, but it reinforces some ideas from the previous two pages.

Lens2.Lenses.Enum.all/0 turns a pointer to an Enumerable into a set of pointers to all its elements. It's most often used with lists, but it doesn't have to be.

 iex> use Lens2
 iex> Deeply.get_all(1..5, Lens.all)
 [1, 2, 3, 4, 5]

Suppose we want to increment each of the numbers. That doesn't actually make sense for a range, but let's see what happens:

iex> Deeply.update(1..5, Lens.all, & &1+1)
[2, 3, 4, 5, 6]

Or just overwrite all the values:

iex> Deeply.put(1..5, Lens.all, 1111)
[1111, 1111, 1111, 1111, 1111]

For any update operation, Lens.all/0 produces a list. Suppose we instead want a MapSet. We could do that with Enum.into/2:

iex> Deeply.put(1..5, Lens.all, 1111) |> Enum.into(MapSet.new)
MapSet.new([1111])

(Notice that collapsed all the 11111 values into one, because MapSets don't allow duplicates. Maybe that's why we wanted a MapSet.)

There is, however, a lens that is the equivalent of Enum.into/2: Lens2.Lenses.Enum.into/2:

iex> Deeply.put(1..5, Lens.all |> Lens.into(MapSet.new), 11111)
MapSet.new([11111])

Looks good. Let's even put it in a module a a predefined lens maker:

defmodule MyLenses do
  defmaker as_mapset,
    do: Lens.all |> Lens.into(MapSet.new)
end

Now it happens that we have a structure containing a range, and we want to increment the range values into a mapset. Seems easy:

iex> lens = Lens.key(:a) |> MyLenses.as_mapset
iex> Deeply.update(%{a: 1..5}, lens, & &1+1)
%{a: MapSet.new([2, 3, 4, 5, 6])}

That looks good. Time passes, and you come across a similar situation. This time you decide a named lens maker is overkill. You'll just pipe the lenses together at the point of use:

iex> lens = Lens.key(:a) |> Lens.all |> Lens.into(MapSet.new)
iex> Deeply.update(%{a: 1..5}, lens, & &1+1)
MapSet.new([a: [2, 3, 4, 5, 6]])

Look closely at that: it's not a map containing a mapset. It's a mapset containing a keyword list.

Two questions:

  1. What went wrong with this?

     Lens.key(:a) |> Lens.all |> Lens.into(MapSet.new)

  2. And why did this work...?

     lens = Lens.key(:a) |> MyLenses.as_mapset

    ...given that as_mapset is just:

     Lens.all |> Lens.into(MapSet.new)

Why the pipeline fails

What if we manually expand out the pipeline into a nested pair of Lens.seq?

Lens.seq(Lens.key(:a), Lens.seq(Lens.all, Lens.into(MapSet.new)))
** (UndefinedFunctionError) function Lens2.Lenses.into/1 is undefined or private. Did you mean:

  * into/2
  * into/3

Lens.all |>

Why the named function doesn't fail