elixir - how to add conditional pipe into pipeline?

Question

I have a small pipeline in elixir, it's about changing ecto model state:

model
|> cast(params, ~w(something), ~w())
|> conditional
|> Repo.update

The problem is that I have conditional pipe which can be nil sometimes, so in the case it's nil it should do nothing and can works (I presume it will be fn(x) -> x end)

So, my question is: "how can I do that"?

Answer 1

model
|> cast(params, ~w(something), ~w())
|> maybe_do_something(conditional)
|> Repo.update

defp maybe_do_something(changeset, nil), do: changeset

defp maybe_do_something(changeset, func) do
  # Do something with changeset
end

Not sure if I got your question right, but maybe thats what you are looking for.

Answer 2

Pipes are great for operations that can't fail and all of them always will be carried. In case you want to stop the pipeline, you can't. You would have to write functions like this:

maybe_repo_update(nil), do: nil
maybe_repo_update(data), do: Repo.update(data)

To solve that problem there is a new special form in Elixir 1.2 called with. It can stop the pipeline at the moment where something doesn't match:

with changeset <- cast(model, params, ~w(something), ~w())
  {:ok, changeset} <- conditional_operation(changeset)
  {:ok, model} <- Repo.insert(changeset)

This will make sure that if conditional operation returns something else than {:ok, changeset} it will not try to run the last repo insert. In Elixir 1.3 you can also use else part.

However for changesets it is more common to use solution suggested by @JustMichael:

def conditional(changeset) do
  if something_to_do do
    transform(changeset)
  else
    changeset
  end
end

This solution will always run the Repo.update part.

Answer 3

I'm Elixir newbie, so please don't be too harsh :).

Why not use anonymous functions for this purpose ?

model
|> cast(params, ~w(something), ~w())
|> (fn(n) -> conditional && n |> Repo.update || n end).()