[erlang-questions] Dialyzer

[Stavros Aronis]

On Sun, Jan 20, 2019 at 6:31 PM Frans Schneider <fchschneider@REDACTED> wrote:
>
> Yep, it were the nonempty lists which caused the problem. I used
> nonempty because _I_ know that the function never is called with an
> empty list, but that is not relevant for the specs of the function.

Good, but this is not what Fred pointed out!

It is likely that this function is *never* called with values that
satisfy the specified type for `Seq_nums`, either because the calls
themselves are bad (not likely if the code is working) or because the
type definition for `Seq_nums` is wrong.

By removing the 'nonempty', you're **silencing** the warning that the
only way for the function to succeed as specified, is if passed an
empty list.

> On 1/20/19 5:34 PM, Fred Hebert wrote:
> > On 01/20, Frans Schneider wrote:
> >> Dear lsit,
> >>
> >> I have the following function plus specification:
> >>
> >> -spec new(Seq_nums, DDS_filter) -> Cfrs when
> >>      Seq_nums :: nonempty_list(rtps:sequence_number()),
> >>      DDS_filter :: fun((rtps:sequence_number()) -> boolean()),
> >>      Cfrs :: nonempty_list(cfr()).
> >>
> >> new(Seq_nums, DDS_filter) ->
> >>    [#cfr{sequence_number = N, status = unsent,
> >>          is_relevant = not DDS_filter(N)}
> >>     || N <- Seq_nums].
> >>
> >> Dialyzer complains with:
> >>
> >> Invalid type specification for function rtps_cfr:new/2. The success
> >> typing is ([],_) -> []
> >>
> >> The filter is an external function,
> >> What is wrong with the spec?
> >>
> >
> > You should look at the call-site. There's a possibility that the only
> > places that call the function use arguments that are lists of terms that
> > don't match the sequence number type. In doing so, dialyzer "removes"
> > the types with these as crashing (i.e. DDS_filter would raise an
> > exception).
> >
> > What is left is the set of empty lists (once all the filters are done)
> > which only gives ([],_) -> []
> >
> > That's at least my guess without seeing the calling code.
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