Elixir anonymous function as a function parameter

Question

Trying to pass a custom function to the following function but I have no luck.

   defmodule M do
     def custom_fn([head|tail],fnce) do
        fnce.(head)
        custom_fn(tail,fnce)
      end

     def custom_fn([]), do: nil

   end

Trying to call it like this:

M.custom_fn([1,2,3], (fn(x) -> x * 10 end))

Getting this

** (FunctionClauseError) no function clause matching in M.custom_fn/2    

The following arguments were given to M.custom_fn/2:

    # 1
    []

    # 2
    #Function<6.99386804/1 in :erl_eval.expr/5>

test.ex:11: M.custom_fn/2

Also, what does the following message say? M.custom_fn/2
To be more specific, I don't understand /2.

Aleksei Matiushkin · Accepted Answer · 2018-03-08 12:09:19Z

2

/2 is a function arity.

The issue is not with a function at all.

def custom_fn([head|tail],fnce) do
  fnce.(head)
  # ⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓ HERE
  custom_fn(tail,fnce)
end

The last call on an empty array is not matched. You should update the second clause to accept the second argument:

def custom_fn([], _), do: nil

answered Mar 8, 2018 at 12:09

Aleksei Matiushkin

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Comments

Dogbert · Accepted Answer · 2018-03-08 12:09:11Z

2

You forgot to add the second argument to the base case. Like the error says, now you don't have any clause that matches ([], <function>).

def custom_fn([]), do: nil

should be

def custom_fn([], _), do: nil

answered Mar 8, 2018 at 12:09

Dogbert

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Comments

chris · Accepted Answer · 2018-03-08 12:10:42Z

2

As other answers gives the explanation, I will give the right version.

defmodule M do
  def custom_fn([], _), do: nil
  def custom_fn([head|tail],fnce) do
    fnce.(head)
    custom_fn(tail,fnce)
  end
end

IO.inspect M.custom_fn([1,2,3], (fn(x) -> x * 10 end))

answered Mar 8, 2018 at 12:10

chris

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3 Comments

Onorio Catenacci Over a year ago

And make sure the function heads are in the order in which YongHao gives them. Order is important on that particular construct.

Aleksei Matiushkin Over a year ago

@OnorioCatenacci no, in this particular case the order in not important. [] and [h|t] are never matched simultaneously. Otherwise OP would experience infinite recursion instead of match error.

Onorio Catenacci Over a year ago

Thanks for the correction; I'm just accustomed to order of function heads being important in general so I assumed it'd matter here too.

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