Writing idiomatic Elixir code

thousandsofthem

Trying to write good elixir code.

Input:

data = %{"a" => 0, "b" => 2, "c" => 0, "evilkey" => 666}

for specific keys i need to write some specific output if condition is true

Ruby version:

out = []
out << "text1" if data["a"] > 0
out << "text2" if data["b"] > 0
out << "text3" if data["c"] > 0
out

Ugly elixir version:

out = []
out = if data["a"] > 0 do
  out ++ ["text1"]
else
  out
end
...
...
out

Good elixir version: ???

Any thoughts here?

13 comments

#code-style #idiomatic-elixir

2 2373 13

2016-11-13 20:29:26 UTC

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taiansu

A more functional apporach

output_texts = %{
  "a" => "text1",
  "b" => "text2",
  "c" => "text3",
}

out = data
      |> Enum.filter(fn {_, v} -> v > 0 end)
      |> Enum.filter(fn {k, _} -> Enum.member(Map.keys(output_texts), k) end)
      |> Enum.map(fn {k, _} -> output_texts[k] end)
      |> Enum.join()

Post #1

michalmuskala

You can do that by using if as an expression and assembling list of values finally concatenating them together:

IO.iodata_to_binary([
  if(data["a"] > 0, do: "text1", else: ""),
  if(data["b"] > 0, do: "text2", else: ""),
  if(data["c"] > 0, do: "text3", else: "")
])

You could even extract the if to a separate function, if the pattern if very repeating:

defp if_positive(value, text), do: if(value > 0, do: text, else: "")

IO.iodata_to_binary([
  if_positive(data["a"], "text1"),
  if_positive(data["b"], "text2"),
  if_positive(data["c"], "text3")
])

Post #2

thousandsofthem

Thanks!

The solution with filters and map look about right.

Re: IO.iodata_to_binary thing - i can’t just put things together this way, but overall it looks good, just need to add |> Enum.filter(fn(x) -> !is_nil(x) end), i.e.

[
  (if data["a"] > 0, do: "text1"),
  (if data["b"] > 0, do: "text2"),
  (if data["c"] > 0, do: "text3")
] |> Enum.filter(fn(x) -> !is_nil(x) end)

Post #3

benwilson512

Author of Craft GraphQL APIs in Elixir with Absinthe

data
|> Enum.filter(&match?({_, v} when v > 0, &1))
|> Enum.map(fn 
  {"a", _} -> "text1"
  {"b", _} -> "text2"
  {"c", _} -> "text3"
end)

Post #4

benwilson512

Author of Craft GraphQL APIs in Elixir with Absinthe

Also because nobody has mentioned this yet, the “ugly elixir version” is not actually an elixir version. It would not work at all because out is immutable.

Post #5

thousandsofthem

It will, there are re-assignment for out:

out = if ...
  out ++ ...
else
  out
end

so every consequent out is different variable

Post #6

NobbZ

We do not have any guarantee of ordering in a map. So in theory the begs pour of this snippet is non deterministic, while in the OPs version we do have an explicit ordering of the keys and their transformation/concatenation into the result.

Post #7

rvirding

Creator of Erlang

I know people love |> but you don’t have to use it:

:maps.fold(fn (k,v,acc) when v > 0 -> acc ++ [output_texts[k]]
              (_,_,acc) -> acc
           end, [], data)

There is probably a function equivalent to fold in Enum but it is called something else and I could not immediately see it, hence using the one in :maps.

Post #8

thousandsofthem

Yeah, in my use-case ordering guarantees are required so maps don’t work (but for someone else it could be better approach). also there are other non-related keys as well (which should be filtered out)

plain [..., ..., ...] |> remove nils works just fine - ordering granaries, short and easy to read code

Post #9