Defining function with same arity but different map values in one of param

Questions & Help Questions
,
1

Hi! I might be asking a silly one, sorry for that and thanks for your time!
my aim is to implement quite simple logic:
the get_or_create_movie could be called with first param with “type” = “movie”, in that case I do not care about any other params and the code stays the same
if the first param contains “type” => “tv-series”, then the logic start to differ according to values. I feel that the straightforward approach I took looks not really pretty :wink:

  def get_or_create_movie(%{ "type" => "movie" } = movie) do
    # same movie code
  end
  def get_or_create_movie(%{ "type" => "movie" } = movie, _like_time) do
    # same movie code
  end
  def get_or_create_movie(%{ "type" => "movie" } = movie, _like_time, _review_text) do
    # same movie code
  end


  def get_or_create_movie(%{ "type" => "tv-series" } = series) do
    # only series code
  end
  def get_or_create_movie(%{ "type" => "tv-series" } = series, like_time) do
    # like_time only series code
  end
  def get_or_create_movie(%{ "type" => "tv-series" } = series, like_time, review_text) do
    # like_time and review text only series code
  end

the dyalizer complains (warning) that the clauses /2 and /3 was previously defined.
the function could be called with “type” = “movie” in the first param and with second and/or third params present

2

Hello :slight_smile:
Are you certain this is a dialyzer warning ?

It seems to me the problem is about mixing different arity and function clauses.
If you define the same function clause with the same number of arguments together it should be fine, like so:

  def get_or_create_movie(%{ "type" => "movie" } = movie) do
    # same movie code
  end
  def get_or_create_movie(%{ "type" => "tv-series" } = series) do
    # only series code
  end

  def get_or_create_movie(%{ "type" => "movie" } = movie, _like_time) do
    # same movie code
  end
  def get_or_create_movie(%{ "type" => "tv-series" } = series, like_time) do
    # like_time only series code
  end

  def get_or_create_movie(%{ "type" => "movie" } = movie, _like_time, _review_text) do
    # same movie code
  end
  def get_or_create_movie(%{ "type" => "tv-series" } = series, like_time, review_text) do
    # like_time and review text only series code
  end
3

Ah, right! That is correct. One problem is the repetitive code in “movie”-case. is there a more compact, DRY way to match any arity if the “type” => “movie”?

4

Can you normalize to just /3 functions? E.g. by using a default value if like_time or review_text are not passed.

5

I think @LostKobrakai’s suggestion is the better idea, but if you can’t find a neutral element for like_time and review_text you can at least use a helper function

6

Not sure what the rest of your code is doing, but is something like the below not possible?

If the like_time/review_text logic is identical for each type, then you can just match on the params as a whole rather than targetting the types specifically. Then handle the “type” within the functions like the below or by using a conditional statement.

  def get_or_create_movie(params) do
    handle_type(params["type"])
    # /1 code 
  end

  def get_or_create_movie(params, _like_time) do
    handle_type(params["type"])
    # /2 code
  end
  
  def get_or_create_movie(params, _like_time, _review_text) do
    handle_type(params["type"])
    # /3 code
  end

  def handle_type("movie"),     do: # Movie code
  def handle_type("tv-series"), do: # TV Code
7

I’ve tried that:

  def get_or_create_movie(%{ "type" => "tv-series" } = series, like_time) do
  end

  def get_or_create_movie(%{ "type" => "movie" } = movie, _time \\ nil, _text \\ nil) do
  end

  def get_or_create_movie(%{ "type" => "tv-series" } = series, like_time, review_text) do
  end

one small hicup I had with this is that I can’t pass the function itself as &get_or_create_movie\3 to a Enum stuff as it will raise an exception when it will be passed with one param so I needed to wrap it to anonymous fun and call it with one param like &(Movie.get_or_create_movie(&1)) so the defaults would work. But that is okay.

after that the Elixir warning complains about clauses and defaults:

def get_or_create_movie/3 has multiple clauses and also declares default values.
In such cases, the default values should be defined in a header. Instead of:

    def foo(:first_clause, b \\ :default) do ... end
    def foo(:second_clause, b) do ... end

one should write:

    def foo(a, b \\ :default)
    def foo(:first_clause, b) do ... end
    def foo(:second_clause, b) do ... end

which I find as not correct logic for my case, is it?

8

Each arity (the number after the /) is effectively a separate function. You can capture one with the defaults by using a smaller number:

defmodule Foo do
  def bar(x, y \\ 1, z \\ 2) do
    {x, y, z}
  end
end

Enum.map([:a, :b, :c], &Foo.bar/1)
# => [{:a, 1, 2}, {:b, 1, 2}, {:c, 1, 2}]

Enum.scan([:a, :b, :c], &Foo.bar/2)
# => [:a, {:b, :a, 2}, {:c, {:b, :a, 2}, 2}]

(The result for Enum.scan isn’t terribly relevant, it was just the first function that came to mind that takes a callback that expects two arguments.)

In both cases, the arguments that aren’t passed to the lower-arity versions Foo.bar/1 and Foo.bar/2 are filled in with the defaults.

It’s common in Erlang code to see a long series of function heads with short heads filling in parameters and calling longer ones. This is the same as what Elixir’s compiler builds:

def bar(x, y \\ 1, z \\ 2), do: ...

# is equivalent to

def bar(x), do: bar(x, 1, 2)
def bar(x, y), do: bar(x, y, 2)
def bar(x, y, z), do: ...

which is why you can capture bar/1 and bar/2 distinctly from bar/3

9

thank you for the detailed explanation. That was really eyes opening insight. Now I got the functions much deeper (still a lot to go I believe). elixir community is amazing