I believe an apples-to-apples comparison would be:
# before
x
|> then(&Map.put_new_lazy(&1, :file_path, fn -> Path.join([&1.base_path,
&1.filename]) end))
# after
x
|> Map.put_new_lazy(:file_path, &Path.join([&1.base_path, &1.filename]))
And I think it does read better.
That being said, personally I don’t think I have commonly needed something like
this though and I’d just break out of a pipe and move on. :)
If we add this precedent, that the function receives the input as one of the
arguments, should we update other functions for consistency too? Do we have an
Map.update accepting a 2-arity function? (Is the map the first or the second
argument?!)
> On 19 May 2023, at 13:30, Christian Trosclair
> <christian.troscl...@dockyard.com> wrote:
>
> It is sometimes if not often desirable to build up some Map or Keyword list
> and add or update values in such a way that takes into account the current
> state of the data, yet some of the current functions do not allow this to be
> done from a pipe.
>
> A trivial example for instance:
> ```
> map = %{filename: “thing.txt”}
> map = Map.put_new(map, :base_path, "/")
> map = Map.put_new(map, :file_path, Path.join([map.base_path, map.filename]))
>
> ```
> This is begging to be piped, but Map.put_new_lazy/3’s function argument does
> not receive the current map.
>
> For reference Map.put_new_lazy/3
> https://github.com/elixir-lang/elixir/blob/a64d42f5d3cb6c32752af9d3312897e8cd5bb7ec/lib/elixir/lib/map.ex#L381
>
> <https://github.com/elixir-lang/elixir/blob/a64d42f5d3cb6c32752af9d3312897e8cd5bb7ec/lib/elixir/lib/map.ex#L381>
> It is true we could simply use Kernel.then/2, but I think we can do better
> and make code easier to scan.
>
> An example using pipes and Kernel.then/2
> ```
> %{filename: “thing.txt”}
> |> Map.put_new(:base_path, "/")
> |> then(&(Map.put_new(&1, :file_path, Path.join([&1.base_path,
> &1.filename]))))
>
> ```
> Kernel.then/2 is breaking up what could be a quick scan of the code.
>
> We can smooth this out.
>
> Here is an implementation on Map.put_new_lazy/3
>
> ```
> @spec put_new_lazy(map, key, ((map) -> value)) :: map
> def put_new_lazy(map, key, fun) when is_function(fun, 1) do
> case map do
> %{^key => _value} ->
> map
>
> %{} ->
> put(map, key, fun.(map))
>
> other ->
> :erlang.error({:badmap, other})
> end
> end
>
> ```
> Then we could rewrite the original example like:
>
> ```
> %{filename: “thing.txt”}
> |> Map.put_new(:base_path, "/")
> |> Map.put_new_lazy(:file_path, &(Path.join([&1.base_path, &1.filename]))))
>
> ```
> If we wanted to go even further, we could even accept an arity 2 function and
> pass in both the map and the key.
>
> Thoughts?
>
>
>
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