[erlang-questions] Tricky Dialyzer type

[Erik Søe Sørensen]

Depending on exactly what tagged tuples have already been assigned meaning,
there may be other options.
If only 2-tuples are "occupied", you can use {magic, Tag, List}.
If only tuples with an initial atom are "occupied", you can use {{Tag},
List}.
Are all tuple types indeed spoken for?
On the other hand, it sounds like all list types are available. In that
case, a term like [{Tag, List}] might work better - it has different
trade-offs, at least, with respect to type analysis.

/Erik
Den 20/07/2015 11.49 skrev "Raimo Niskanen" <
raimo+erlang-questions@REDACTED>:

> On Mon, Jul 20, 2015 at 10:06:32AM +0200, Jesper Louis Andersen wrote:
> > On Mon, Jul 20, 2015 at 9:21 AM, Raimo Niskanen <
> > raimo+erlang-questions@REDACTED> wrote:
> >
> > > Ideas, anyone?
> >
> >
> > Your representation is icky at the type level. If possible, go for a
> tuple
> > instead since it has type A*B as a product and then this is fairly easy
> to
> > represent:
>
> Unfortunately that is not possible since the tagged tuple already has
> another meaning in the code I am trying to add this feature to.  This was
> the "exhausted option" I briefly mentioned.  I should have stated that
> explicitly in my question...
>
> >
> > -type tagged_list(A) :: {A, [A]}.
> >
> > This also opens up the possibility of sum-typing the tagged list:
> >
> > -type tagged_list() ::
> >     {tag_a, [..]} | {tag_b, [..]}.
> >
> > which in turn can hammer down the precision at which the dialyzer can
> work.
> >
> > In naive type systems, list are homogeneous in the sense that every
> element
> > has to be drawn from the same ground type. Properly handling lists where
> > the "first element drives the type of the remainder of the list" is
> nudging
> > itself towards the land of dependent type theory, which the dialyzer
> > definitely doesn't support.
>
> That's a pity.  For me it is just about two different kinds of wrappers.
> Obviously I am not a type expert.
>
> >
> > It may be possible to work with the notion of "list of two elements",
> > [uint8(), ty(), ...], saying that the list is a cons of uint8() followed
> by
> > one or more elements of type ty(), but I'm not sure if the dialyzer
> > understands this at all.
>
> At least list(type(), type()) and [type(), type()] both gives syntax errors
> so it seams that Dialyzer does not understand that at all.
>
> >
> >
> >
> > --
> > J.
>
> > _______________________________________________
> > erlang-questions mailing list
> > erlang-questions@REDACTED
> > http://erlang.org/mailman/listinfo/erlang-questions
>
>
> --
>
> / Raimo Niskanen, Erlang/OTP, Ericsson AB
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