Sippet.Message.fetch_header_front

You're seeing just the function fetch_header_front, go back to Sippet.Message module for more information.
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fetch_header_front(message, header)

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Specs

fetch_header_front(t(), header()) :: {:ok, multiple_value()} | :error

Fetches the first value of a specific header and returns it in a tuple.

If the header does not exist, or the value is not a list, returns :error. If the header exists but it is an empty list, returns {:ok, nil}.

Examples:

iex> request =
...>   Sippet.Message.build_request(:invite, "sip:foo@bar.com")
...>   |> Sippet.Message.put_header(:content_language, ["en-US", "de-DE"])
...>   |> Sippet.Message.put_header(:max_forwards, 70)
iex> request |> Sippet.Message.fetch_header_front(:content_language)
{:ok, "en-US"}
iex> request |> Sippet.Message.fetch_header_front(:max_forwards)
:error
iex> request |> Sippet.Message.fetch_header_front(:cseq)
:error