Sample code to prove for -callback

README.md

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+### Does -callback work with Dialyzer?
+
+I'm not so sure at this point. I've run the following using Erlang R15B02 (we're on an old version).
+```bash
+> dialyzer --build_plt --apps erts stdlib kernel
+  Compiling some key modules to native code... done in 0m17.77s
+  Creating PLT /Users/Ceryni/.dialyzer_plt ...
+Unknown functions:
+  compile:file/2
+  compile:forms/2
+  compile:noenv_forms/2
+  compile:output_generated/1
+  crypto:des3_cbc_decrypt/5
+  crypto:start/0
+Unknown types:
+  compile:option/0
+ done in 0m27.91s
+done (passed successfully)
+> dialyzer dude.erl runner.erl no_spec_dude.erl spec_dude.erl
+  Checking whether the PLT /Users/Ceryni/.dialyzer_plt is up-to-date... yes
+  Proceeding with analysis...
+runner.erl:7: Function run/0 has no local return
+runner.erl:9: The call spec_dude:hey_dude(42) breaks the contract (Say::string()) -> Res::string()
+ done in 0m0.31s
+done (warnings were emitted)
+```
+
+So as you see, dialyzer issues a warning on line 9 but not line 8 as I would have expected. The `-spec` defined in `spec_dude.erl` is the same as the `-callback` defined on the behavior `dude.erl`. I'm not sure why but it seems like dialyzer is not applying the `-callback` to the implementing callback module `no_spec_dude.erl`.

dude.erl

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+-module(dude).
+
+-callback hey_dude(Say :: string()) -> Res :: string().

no_spec_dude.erl

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+-module(no_spec_dude).
+
+-behaviour(dude).
+
+-export([hey_dude/1]).
+
+hey_dude(Say) ->
+  lists:flatten(io_lib:format("Hey Dude, ~p", [Say])).

runner.erl

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+-module(runner).
+
+
+-export([run/0]).
+
+-spec run() -> ok.
+run() ->
+  io:format("~p", [no_spec_dude:hey_dude(42)]),
+  io:format("~p", [spec_dude:hey_dude(42)]).

spec_dude.erl

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+-module(spec_dude).
+
+-behaviour(dude).
+
+-export([hey_dude/1]).
+
+-spec hey_dude(Say :: string()) -> Res :: string().
+hey_dude(Say) ->
+  lists:flatten(io_lib:format("Hey Dude, ~p", [Say])).