[erlang-questions] Dialyzer warning on pattern match with sub-record

[Russell Brown]

On 30 Mar 2017, at 09:00, Alex S. <alex0player@REDACTED> wrote:

>> #counter_state{use::'true',cnt::non_neg_integer(),lease::non_neg_integer(),lease_size::non_neg_integer(),leasing::boolean()}
> 
> Dialyzer somehow figured out that counter_state is never constructed with ‘false’ (and has no explicit contract).

Thanks for the reply, there is a code path where counter state can be updated to have `use=false`. I’m sorry but I don’t know what “no explicit contract” means in this context, can you explain?

Do you know how changing the return type tipped dialyzer off?

>> 30 марта 2017 г., в 10:04, Russell Brown <russell@REDACTED> написал(а):
>> 
>> Hi,
>> 
>> I have a dialyzer error I can’t figure out, at all, it’s truly very weird, even for dialyzer.
>> 
>> The error is: 
>> 
>> riak_kv_vnode.erl:2695: Matching of pattern State = {'state', _, _, _, _, _, VId, _, _, _, _, _, _, _, _, _, _, _, _, _, {'counter_state', 'false', _, _, _, _}, _, _} tagged with a record name violates the declared type of #state{idx::'undefined' | integer(),mod::atom() | tuple(),async_put::'false' | 'true' | 'undefined',vnodeid::'undefined' | binary(),delete_mode::'immediate' | 'keep' | 'undefined' | pos_integer(),bucket_buf_size::'undefined' | pos_integer(),index_buf_size::'undefined' | pos_integer(),key_buf_size::'undefined' | pos_integer(),async_folding::'false' | 'true' | 'undefined',in_handoff::boolean(),handoff_target::atom(),handoffs_rejected::integer(),forward::atom() | [{integer(),atom()}],hashtrees::'undefined' | pid(),upgrade_hashtree::boolean(),md_cache::atom() | tid(),md_cache_size::'undefined' | pos_integer(),counter::'undefined' | #counter_state{use::'true',cnt::non_neg_integer(),lease::non_neg_integer(),lease_size::non_neg_integer(),leasing::boolean()},status_mgr_pid::'undefined' | pid(),update_hook::atom() | tuple()}
>> riak_object:object/0
>> 
>> 
>> The code in question is:
>> 
>> %% @private generate an epoch actor, and update the vnode state.
>> -spec new_key_epoch(#state{}) -> {NewEpoch::boolean(), EpochActor :: binary(), #state{}}.
>> new_key_epoch(State=#state{vnodeid=VId, counter=#counter_state{use=false}}) ->
>>   {false, VId, State};
>> new_key_epoch(State) ->
>>   NewState=#state{counter=#counter_state{cnt=Cntr}, vnodeid=VId} = update_counter(State),
>>   EpochId = key_epoch_actor(VId, Cntr),
>>   {true, EpochId, NewState}.
>> 
>> 
>> What is really freaky (to me) is that this code (the previous version) does not provoke a dialyzer warning:
>> 
>> %% @private generate an epoch actor, and update the vnode state.
>> -spec new_key_epoch(#state{}) -> {EpochActor :: binary(), #state{}}.
>> new_key_epoch(State=#state{vnodeid=VId, counter=#counter_state{use=false}}) ->
>>   {VId, State};
>> new_key_epoch(State) ->
>>   NewState=#state{counter=#counter_state{cnt=Cntr}, vnodeid=VId} = update_counter(State),
>>   EpochId = key_epoch_actor(VId, Cntr),
>>   {EpochId, NewState}.
>> 
>> The record definition for #counter_state is:
>> 
>> -record(counter_state, {
>>         use = true :: boolean(),
>>         cnt = 0 :: non_neg_integer(),
>>         lease = 0 :: non_neg_integer(),
>>         lease_size = 0 :: non_neg_integer(),
>>         leasing = false :: boolean()
>>        }).
>> 
>> I have missed out the record definition for #state as it is huge, but crucially, unchanged between the two definitions of new_key_epoch/1 above.
>> 
>> If the match in the function head, and the record, are exactly the same between the two versions of the code, why does the one that returns a three tuple provoke a warning?
>> 
>> Many thanks in advance if you can help
>> 
>> Cheers
>> 
>> Russell
>> 
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