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-type header() ::
'Cache-Control' |
'Connection' |
'Date' |
'Pragma'|
'Transfer-Encoding' |
'Upgrade' |
'Via' |
'Accept' |
'Accept-Charset'|
'Accept-Encoding' |
'Accept-Language' |
'Authorization' |
'From' |
'Host' |
'If-Modified-Since' |
'If-Match' |
'If-None-Match' |
'If-Range'|
'If-Unmodified-Since' |
'Max-Forwards' |
'Proxy-Authorization' |
'Range'|
'Referer' |
'User-Agent' |
'Age' |
'Location' |
'Proxy-Authenticate'|
'Public' |
'Retry-After' |
'Server' |
'Vary' |
'Warning'|
'Www-Authenticate' |
'Allow' |
'Content-Base' |
'Content-Encoding'|
'Content-Language' |
'Content-Length' |
'Content-Location'|
'Content-Md5' |
'Content-Range' |
'Content-Type' |
'Etag'|
'Expires' |
'Last-Modified' |
'Accept-Ranges' |
'Set-Cookie'|
'Set-Cookie2' |
'X-Forwarded-For' |
'Cookie' |
'Keep-Alive' |
'Proxy-Connection' |
binary() |
string().

-type headers() :: [{header(), iodata()}].

-type method() :: string() | atom().

-type pos_timeout() :: pos_integer() | 'infinity'.

-type bodypart() :: iodata() | 'http_eob'.

-type socket() :: _.

-type port_num() :: 1..65535.

-type poolsize() :: non_neg_integer() | atom().

-type invalid_option() :: any().

-type pool_id() :: pid() | atom().

-type destination() :: {string(), pos_integer(), boolean()}.

-type raw_headers() :: [{atom() | binary() | string(), binary() | string()}].

-type partial_download_option() ::
{'window_size', window_size()} |
{'part_size', non_neg_integer() | 'infinity'} |
invalid_option().

-type option() ::
{'connect_timeout', timeout()} |
{'send_retry', non_neg_integer()} |
{'partial_upload', non_neg_integer() | 'infinity'} |
{'partial_download', [partial_download_option()]} |
{'connect_options', socket_options()} |
{'proxy', string()} |
{'proxy_ssl_options', socket_options()} |
{'pool', pid() | atom()} |
invalid_option().

-type options() :: [option()].

-type host() :: string() | {integer(), integer(), integer(), integer()}.

-type http_status() :: {integer(), string() | binary()} | {'nil', 'nil'}.

-type socket_options() :: [{atom(), term()} | atom()].

-type window_size() :: non_neg_integer() | 'infinity'.

-type upload_state() :: {pid(), window_size()}.

-type body() :: binary()    |
'undefined' | % HEAD request.
pid().        % When partial_download option is used.

-type result() ::
{ok, {{pos_integer(), string()}, headers(), body()}} |
{ok, upload_state()} |
{error, atom()}.