[erlang-questions] Dyalizer warnings for too wide return type

Vincent de Phily <>
Mon Oct 18 11:48:43 CEST 2010

On Sunday 17 October 2010 18:11:59 Kostis Sagonas wrote:
> Vincent de Phily wrote:
> I could have written a much longer reply in this thread, but I'll keep
> this reply a bit short with the risk of sounding a bit impolite. This is
> not my intention.

Not at all, it's good to get an informed reply.
> The whole discussion above is a bit naive, but perhaps this is primarily
> my fault because there is no document that explains dialyzer's decisions
> and philosophy in general terms. For a long time now I wanted to write a
> "dialyzer FAQ". I hope I can come around to that pretty soon.

That'd be nice.
> Anyway, the short answer is that dialyzer will never do what users may
> initially think it would be better if it did.  The only information that
> dialyzer has to work with is the code and whenever it finds something
> that is a discrepancy between what the code does and what the programmer
> claims it does (e.g. in a spec) it spits a warning about it.  Dialyzer
> cannot be in the minds of programmers and what's their ultimate
> intention even if they add "%% TODO implement later" comments :-)
> So, my reply to Alexey's message is that in a situation like the one he
> describes the proper action is to either comment out the true case in
> foo's code or realize that now is the time for this "later".

I never said (and neither did the OP I think) that dialyzer should ignore the 
spec/code discrepancy inside a module. For the obvious reason, as you say, 
that dialyzer can't read minds.

What I thought (and it's quite possibly naive, but I'm not convinced yet) is 
that callers of the function could sometimes use the spec instead of the code 
to infer what the function returns. However, defining that "sometimes" may be 
tricky, a pandora's box we don't want to open.

> Now, having written all these, there is a dialyzer option that we have
> primarily added for internal debugging purposes, but may come in handy
> in situations like that. If you want dialyzer to do its analysis based
> on success typings only and ignore the specs of the analyzed modules,
> you can use the option '--no_spec'.
> But there is no option "trust the specs, please" for cases where
> dialyzer can determine that they do not correspond to the code.
> Whenever you want to do that, simply don't use dialyzer!

But I do want to use dialyzer, I like it (insert kiddy tantrum here). And I'm 
happy to get a dializer warning for that function, I just don't want the 
warning to propagate. If the warning is justified I'll correct it. If it isn't 
justified, I'll have less dialyzer noise to check and dismiss.
> > I'd like dializer to trust me when I declare my interface, even if the
> > implementation doesn't match that. Actually, maybe we could make the case
> > that non-exported functions must have an exact spec whereas exported
> > function can have a spec that is more allowing than the implementation ?
> Sorry, but I firmly believe that this last part is wrong. Having
> published interfaces that do not correspond to the implementation is not
> a good idea. Among other things, what sort of confidence do you have
> that the client code works ok? How do you test things in this situation?

I'll stand ground here, because it is a frequent and normal usecase (no buggy 
or unfinished code involved). Whenever you use pluggable implementations 
you're confronted with the "implementation does less that the interface 
states" problem. How many gen_server handle_call/3 did you write that could 
return all 8 possible Result types mentioned in the erlang docs ? Even for my 
own interfaces (of which I have tighter control than the standard OTP 
behaviours) I have that problem, because one module can return more types of 
results than another, but the declared interface is the same for both modules.

Unless you say that specs should always follow the code exactly, but I think 
that misses an opportunity to make them more usefull. If the spec is just 
mirroring what dializer can infer automatically anyway, why do I keep writing 
them by hand (or at all) ?

Vincent de Phily
Mobile Devices
+33 (0) 142 119 325
+353 (0) 85 710 6320 

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