[erlang-questions] Dialyzer cannot catch gen_server callback return type error

Maxim Treskin <>
Fri Jul 6 10:39:58 CEST 2012

I just really not understand. Function handle_cast returns, and returns
incorrect type according to -spec signature, right?
Why Dialyzer lose this information? It is sort of missbehaviour i think.

On 6 July 2012 00:12, Maxim Treskin <> wrote:

> Thank you, Stavros!
> On 5 July 2012 21:36, Stavros Aronis <> wrote:
>> Hi again,
>> your example shows another case of the "might also be correct" rationale
>> of Dialyzer. Your full code is:
>> -spec handle_cast(term(), #state{}) -> {noreply, NewState :: #state{}}.
>> handle_cast({asd}, _State) ->
>>     NewState = {bugotak},
>>     {noreply, NewState};
>> handle_cast(_Msg, State) ->
>>     {noreply, State}.
>> It is true that the return value of the first clause violates the spec,
>> but this information is lost within Dialyzer as the "general" type
>> signature that Dialyzer will find and use
>> is my_module:handle_cast(term(),term()) -> {'noreply',term()} due to the
>> second clause.
> Why signature `my_module:handle_cast(term(),term()) ->
> {'noreply',term()}`  used? Is it taken from -callback signature, all right?
>>  This more general type signature "absorbs" and hides the incorrect
>> result. The function "can work" if you only call with a message different
>> than {asd} and provide a correct #state{} as input, so Dialyzer remains
>> silent. If, on the other hand, you had just the first clause, or both
>> clauses returned a value that couldn't be #state{}, you would get a warning.
>> Remember that Dialyzer does not promise to find all the bugs in your
>> code. It might be interesting and possible to check the return value of
>> each separate clause of a function, if there exists a spec, however...
>> Thanks for the example and idea!
>> Stavros
>> On Thu, Jul 5, 2012 at 4:31 PM, Maxim Treskin <> wrote:
>>> Hello!
>>> File attached.
>>> Just compile it with
>>> erlc +debug_info my_module.erl
>>> then
>>> dialyzer . --plt ~/.r15b01_dialyzer.plt --no_native -Werror_handling
>>> -Wrace_conditions -Wunderspecs
>>> and I see:
>>>   Checking whether the PLT /home/zert/.r15b01_dialyzer.plt is
>>> up-to-date... yes
>>>   Proceeding with analysis... done in 0m0.39s
>>> done (passed successfully)
>>> On 5 July 2012 20:22, Stavros Aronis <> wrote:
>>>> Hi!
>>>> Your description is not very clear, so it might be better to send me
>>>> some code snippet off-list, but let me put some facts here to make sure
>>>> that the expectations from Dialyzer are 'reasonable'!
>>>> - Dialyzer first uses the -callback attibutes of the behaviour and will
>>>> always warn if the arguments or return value of a callback function are not
>>>> subtypes of those attributes. If e.g. the return value of a callback is
>>>> expected to be {noreply, term()} and the callback always returns {reply,
>>>> term(), term()}, Dialyzer will warn about this.
>>>> - As long as your implementation can possibly return an acceptable
>>>> value, Dialyzer is happy. You might have a callback that either returns a
>>>> correct value or something else that cannot be handled by the behaviour,
>>>> but Dialyzer will assume that the correct term will always be returned.
>>>> This happens because Dialyzer should never emit a false warning and this
>>>> additional bad value might have been inferred because Dialyzer's analysis
>>>> was not strong enough to exclude it.
>>>> - Spec attributes are also taken into account when anayzing callbacks
>>>> in the following manner:
>>>> -- First the specs themselves are checked for compatibility with the
>>>> -callback attributes of the behaviour. Here it is the user (and not
>>>> Dialyzer) that writes these specs, so if these specs allow more values you
>>>> get a warning. The rationale is that if you are providing a spec it should
>>>> be at least as restrictive as the callback attribute.
>>>> -- Second, the spec is used to check the arguments and return value of
>>>> the function, as is the case with any spec.
>>>> For gen_server, the -callback attributes can be seen here<https://github.com/erlang/otp/blob/maint/lib/stdlib/src/gen_server.erl#L123>.
>>>> There, the State and NewState can be any terms. If you also have a spec
>>>> saying that this callback function should return {noreply, State ::
>>>> #state{}} and you return {noreply, {bugotak}} then you are violating your
>>>> own spec (but not the callback attribute of gen_server) and you should get
>>>> a warning. Is this not the case when you run Dialyzer?
>>>> Stavros
>>>> _______________________________________________
>>>> erlang-questions mailing list
>>>> http://erlang.org/mailman/listinfo/erlang-questions
>>> --
>>> Max Treskin
> --
> Max Treskin

Max Treskin
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