CODEWITHSUNDEEP
elixir
Matt
Matt

Reputation: 88157

Is there a way to check a function's arity in Elixir?

If I'm writing a function that gets passed another function, is there a way to check the arity of the function I'm passed, or pattern match against different arities? I could use is_function/2 to check for specific arities, but that would be an awkward way to get the number.

Upvotes: 19

Views: 3091

Answers (2)

Cory ODaniel
Cory ODaniel

Reputation: 818

:erlang.fun_info/1 is good for finding the arity if you have a reference to the function.

If you are trying to figure out what arities a function has, but don't have a reference to it (because the arity is a part of a functions identity) there are two methods.

If you want to know if it supports a specific arity :erlang.function_exported/3 takes the module, function name (sans-arity), and arity:

:erlang.function_exported(IO, :puts, 7) #=> false

:erlang.function_exported(IO, :puts, 2) #=> true

If you want to know all of the arities a function supports you can use the module's __info__ metadata function:

:functions |> IO.__info__ |> Keyword.get_values(:puts) #=> [1,2]

Upvotes: 10

whatyouhide
whatyouhide

Reputation: 16781

You can use :erlang.fun_info/1; it returns a bunch of information about a given function, including its arity:

iex> :erlang.fun_info(fn -> :ok end)[:arity]
0
iex> :erlang.fun_info(fn(_, _, _) -> :ok end)[:arity]
3

As the documentation I linked says, this function is mainly intended for debugging purposes but it can be used to determine the arity of a function.

Upvotes: 32

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