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
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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