diff --git a/lib/erl/src/thrift_base64_transport.erl b/lib/erl/src/thrift_base64_transport.erl
index 9d13151..7630333 100644
--- a/lib/erl/src/thrift_base64_transport.erl
+++ b/lib/erl/src/thrift_base64_transport.erl
@@ -29,6 +29,8 @@
 
 %% State
 -record(b64_transport, {wrapped}).
+-type state() :: #b64_transport{}.
+-include("thrift_transport_impl.hrl").
 
 new(Wrapped) ->
     State = #b64_transport{wrapped = Wrapped},
diff --git a/lib/erl/src/thrift_buffered_transport.erl b/lib/erl/src/thrift_buffered_transport.erl
index ad67510..c6df73a 100644
--- a/lib/erl/src/thrift_buffered_transport.erl
+++ b/lib/erl/src/thrift_buffered_transport.erl
@@ -35,6 +35,9 @@
 -record(buffered_transport, {wrapped, % a thrift_transport
                              write_buffer % iolist()
                             }).
+-type state() :: pid().
+-include("thrift_transport_impl.hrl").
+
 
 %%====================================================================
 %% API
diff --git a/lib/erl/src/thrift_disk_log_transport.erl b/lib/erl/src/thrift_disk_log_transport.erl
index 72f84f2..2645c67 100644
--- a/lib/erl/src/thrift_disk_log_transport.erl
+++ b/lib/erl/src/thrift_disk_log_transport.erl
@@ -35,6 +35,8 @@
                        close_on_close = false,
                        sync_every = infinity,
                        sync_tref}).
+-type state() :: #dl_transport{}.
+-include("thrift_transport_impl.hrl").
 
 
 %% Create a transport attached to an already open log.
diff --git a/lib/erl/src/thrift_file_transport.erl b/lib/erl/src/thrift_file_transport.erl
index 5ac2dbe..7ee1c12 100644
--- a/lib/erl/src/thrift_file_transport.erl
+++ b/lib/erl/src/thrift_file_transport.erl
@@ -29,6 +29,8 @@
 -record(t_file_transport, {device,
                            should_close = true,
                            mode = write}).
+-type state() :: #t_file_transport{}.
+-include("thrift_transport_impl.hrl").
 
 %%%% CONSTRUCTION   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
diff --git a/lib/erl/src/thrift_framed_transport.erl b/lib/erl/src/thrift_framed_transport.erl
index 9a29ecd..92bd588 100644
--- a/lib/erl/src/thrift_framed_transport.erl
+++ b/lib/erl/src/thrift_framed_transport.erl
@@ -36,6 +36,8 @@
                            read_buffer, % iolist()
                            write_buffer % iolist()
                           }).
+-type state() :: pid().
+-include("thrift_transport_impl.hrl").
 
 %%====================================================================
 %% API
diff --git a/lib/erl/src/thrift_http_transport.erl b/lib/erl/src/thrift_http_transport.erl
index f8c1827..f0a45fe 100644
--- a/lib/erl/src/thrift_http_transport.erl
+++ b/lib/erl/src/thrift_http_transport.erl
@@ -43,6 +43,8 @@
                          http_options, % see http(3)
                          extra_headers % [{str(), str()}, ...]
                         }).
+-type state() :: pid().
+-include("thrift_transport_impl.hrl").
 
 %%====================================================================
 %% API
diff --git a/lib/erl/src/thrift_memory_buffer.erl b/lib/erl/src/thrift_memory_buffer.erl
index b4f607a..34a3668 100644
--- a/lib/erl/src/thrift_memory_buffer.erl
+++ b/lib/erl/src/thrift_memory_buffer.erl
@@ -33,6 +33,8 @@
 -export([write/2, read/2, flush/1, close/1]).
 
 -record(memory_buffer, {buffer}).
+-type state() :: #memory_buffer{}.
+-include("thrift_transport_impl.hrl").
 
 %%====================================================================
 %% API
diff --git a/lib/erl/src/thrift_socket_transport.erl b/lib/erl/src/thrift_socket_transport.erl
index fcd6944..1a8ba81 100644
--- a/lib/erl/src/thrift_socket_transport.erl
+++ b/lib/erl/src/thrift_socket_transport.erl
@@ -29,6 +29,8 @@
 
 -record(data, {socket,
                recv_timeout=infinity}).
+-type state() :: #data{}.
+-include("thrift_transport_impl.hrl").
 
 new(Socket) ->
     new(Socket, []).
diff --git a/lib/erl/src/thrift_transport.erl b/lib/erl/src/thrift_transport.erl
index 20c4b5d..420281c 100644
--- a/lib/erl/src/thrift_transport.erl
+++ b/lib/erl/src/thrift_transport.erl
@@ -41,17 +41,20 @@
     {ok, #transport{module = Module,
                     data = Data}}.
 
-%% Data :: iolist()
+-spec write(#transport{}, iolist() | binary()) -> ok | {error, _Reason}.
 write(Transport, Data) ->
     Module = Transport#transport.module,
     Module:write(Transport#transport.data, Data).
 
+-spec read(#transport{}, non_neg_integer()) -> {ok, binary()} | {error, _Reason}.
 read(Transport, Len) when is_integer(Len) ->
     Module = Transport#transport.module,
     Module:read(Transport#transport.data, Len).
 
+-spec flush(#transport{}) -> ok | {error, _Reason}.
 flush(#transport{module = Module, data = Data}) ->
     Module:flush(Data).
 
+-spec close(#transport{}) -> ok | {error, _Reason}.
 close(#transport{module = Module, data = Data}) ->
     Module:close(Data).