[erlang-questions] Strange Dialyzer behavior when matching record fields with opaque types

[Nick Marino]

Hi Kostis,

Thanks for the quick and helpful response! Glad I'm not crazy after all :).
That all makes sense, and in my original use case that uncovered this, I'm
fine with just commenting out the unused code for now.

I also just uncovered a related(?) issue, in case anyone is interested: in
my example code, I found that changing #b{} to #a{} in the call to
add_element actually causes Dialyzer to crash. Here's the error message:

Proceeding with analysis...{"init terminating in
do_boot",{function_clause,[{dialyzer,message_to_string,[{opaque_match,["pattern
<{'b', Queue}, Key,
Value>","<#a{d::dict:dict(_,_)},'my_key','my_value'>"]}],[{file,"dialyzer.erl"},{line,310}]},{dialyzer,format_warning,2,[{file,"dialyzer.erl"},{line,300}]},{dialyzer_cl,'-print_warnings/1-lc$^0/1-0-',2,[{file,"dialyzer_cl.erl"},{line,818}]},{dialyzer_cl,print_warnings,1,[{file,"dialyzer_cl.erl"},{line,818}]},{dialyzer_cl,return_value,2,[{file,"dialyzer_cl.erl"},{line,715}]},{dialyzer_cl,do_analysis,4,[{file,"dialyzer_cl.erl"},{line,405}]},{dialyzer,'-cl/1-fun-0-',1,[{file,"dialyzer.erl"},{line,153}]},{dialyzer,doit,1,[{file,"dialyzer.erl"},{line,243}]}]}}

I'm tempted to try and debug this myself, but I'm not sure I have the
necessary skills and background to safely make changes to a tool like
Dialyzer. Figured I should leave this one to the experts ;). (Though if
anyone with more experience than myself would like to point me in the right
direction, I suppose I could take a stab at it - I just don't want to
accidentally end up breaking something I don't understand.)

Anyway, thanks again!
Nick

On Tue, Jan 17, 2017 at 6:07 PM, Kostis Sagonas <kostis@REDACTED> wrote:

> On 01/17/2017 11:18 PM, Nick Marino wrote:
>
>>
>> I recently encountered a rather strange, unexpected Dialyzer error. I'm
>> unsure if this might be considered a Dialyzer bug, or if I'm just
>> missing some key piece of knowledge that would help me understand what's
>> going on here.
>>
>> I took the code that generated the warning and pruned it down down to
>> this:
>>
>> -module(opaque_weirdness).
>> -export([public_func/0]).
>>
>> -record(a, {
>>     d = dict:new() :: dict:dict()
>> }).
>>
>> -record(b, {
>>     q = queue:new() :: queue:queue()
>> }).
>>
>> public_func() ->
>>     add_element(#b{}, my_key, my_value).
>>
>> add_element(#a{d = Dict}, Key, Value) ->
>>     dict:store(Key, Value, Dict);
>> add_element(#b{q = Queue}, Key, Value) ->
>>     queue:in({Key, Value}, Queue).
>>
>> Which yields the following warning when I run it through Dialyzer:
>>
>> opaque_weirdness.erl:16: The attempt to match a term of type
>> #b{q::queue:queue(_)} against the pattern {'a', Dict} breaks the
>> opaqueness of queue:queue(_)
>>
>> It seems like this warning is somehow being triggered by the presence of
>> the unused function clause in add_element. If I modify the code and add
>> a line to public_func so that we use both clauses, then Dialyzer passes
>> with no warnings.
>>
>
> Your analysis is pretty correct.  The issue is related to dead code and
> the warning you get is indeed confusing and weird.  This should be fixed in
> dialyzer.  Till then, you can suppress it either by commenting the unused
> first clause, which is dead code in your module, or by exporting function
> add_element/3.
>
>
> What roughly happens here is that Dialyzer gets confused by the fact that
> the first clause will not be used and marks its arguments as having the
> type none(), i.e. not contributing to the inferred success typing. Then the
> warning pass sees the none() in the first argument of the clause and thinks
> it has been produced due to the call to add_element being with a record
> with a different opaque subterm: these opacity violations are also denoted
> by/resulting in the none() type.  But the pattern matching failure is on
> the #a{} vs. #b{} level, not on their subterms (fields).
>
> Dialyzer _should_ issue a warning for this line, but the correct warning
> to issue here is something along the lines of:
>
> opaque_weirdness.erl:16: The attempt to match a term of type
> #b{q::queue:queue(_)} against the pattern {'a', Dict} will not succeed
>
> or that this clause will not be used since the success typing arguments of
> this function are (#b{}, _, _)
>
> Kostis
>
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