Multiple conditional map/struct modification

Questions / Help
1

Let’s say there is a map like:

gin = %{a: "valueA", b: "valueB", […]}

if I need to check and possibly modify values of some keys plus add key/value pairs if some of them are missing I do it like:

gin = if gin[:a], do: gin, else: Map.put_new(gin, :a, …
gin = if gin[:b], do: Map.update!(gin, :b, …), else: Map.put(gin, :b, …)
[… etc.]

Sure it works but neither “looks” nor “feels” :slight_smile: Can I streamline it somehow so that I don’t repeat gin = all the time?

I guess I could change each line into an anonymous function and pipe the outputs through but that would make this code block even larger I presume. Any better ways?

How do Elixir experts handle such situations?

2

To be a little bit more concrete, say the requirement is to set key :a to 1 unless already set, and multiply key :b by 2 if set and otherwise set it to 2.

gin = Map.put_new(gin, :a, 1)
gin = Map.update(gin, :b, 2, & &1 * 2)

or using a pipe:

gin =
  gin
  |> Map.put_new(:a, 1)
  |> Map.update(:b, 2, & &1 * 2)
3

The piped variant looks very nice (the other is basically what I do now) but – I am sorry – I should have explained that the conditions/actions can also be more complex than simple “add if not present”, “add or update” . For example most ugly are things like “if x is set to a_value then set y to another_value and remove z”. While setting y and removing z can be done in separate steps - how does one reference the piped-in map so that checking one key and setting another (or checking one and removing another) could be done in one go. I see this possibility only with inline anon functions but maybe I am missing something here?

4

A common way to reference the piped map is by using then/2 but when I reach that point I tend to break up the pipe into multiple expressions and/or functions as I find following the logic that way easier.

3 Likes
5

Roger, thank you. I’ll have to rethink it as currently I don’t like the way it looks no matter how I rewrite it :slight_smile:

6

I would just go one step further: don’t use anonymous functions, promote them to normal functions. This also gives you the advantage of having a pipe chain that’s more or less self-documenting.

2 Likes
7

Yes, that’s what I have in mind now too. I didn’t like the idea initially because those functions are not going to be “generic” enough for my taste. ATM this seems like the best option though. Thank you for confirming this.

8

Hmm, do you want to handle the case of a missing key differently from when an existing key’s value is nil or false? I generally prefer using the is_nil/1 and is_map_key/2 guards to make that distinction explicit in helper functions.

By the way, if you’re just concerned with missing keys, you might not even need conditional if statements. Map.put_new/3 already passes the map through unchanged when the key already exists while Map.update/4 accepts a default value when the key is missing.

Map.put_new(%{a: 1, b: 2}, :a, 3) => %{a: 1, b: 2}
Map.update(%{a: 1}, :b, 11, fn existing_value -> existing_value * 2 end) => %{a: 1, b: 11}