empty function declaration?

[Alex Shneyderman]

I see this pattern quite often:

  @spec send_resp(t) :: t | no_return

  def send_resp(conn)

  def send_resp(%Conn{state: :unset}) do

    raise ArgumentError, message: "cannot send a response that was not set"

  end

 

 ...

  def send_resp(%Conn{}) do

    raise AlreadySentError

  end

pattern being the first definition of the function that has no body.

What does this mean? And why does this work?

Thanks & cheers,

Alex.

[Eric Meadows-Jönsson]

It's called a function head, we use it to document the function and to define default arguments.

ExDoc will use the argument names from the function head in the generated docs, so it is way to give good names to the arguments even if you have clauses with pattern matches. It is also the only place you can define default arguments for functions with multiple clauses.

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Eric Meadows-Jönsson

[Alexei Sholik]

https://groups.google.com/d/msg/elixir-lang-talk/JCIGPvppvDA/NA_ByKwfN6IJ

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Best regards
Alexei Sholik

[Alex Shneyderman]

still, seems a bit odd of a construct. so, there are basically two case for the use:

1. to document, which IMO is a weak case since you can easily do that with the spec

@spec send_resp(conn :: t) :: t | no_return

and doing it with the spec is probably more logical since well specs are for documenting.

2. define default values. this seems like a valid case although probably not very common 

one. 

Anyway, thanks for the explanations and pointers.

Cheers,

Alex.

[Paul Schoenfelder]

Honestly I hit #2 pretty regularly. Once I started heavily using pattern matching and recursive function calls, the odds of having a multi-clause function with default values is pretty high.

Paul