[erlang-questions] Tricky Dialyzer type

[Raimo Niskanen]

On Mon, Jul 20, 2015 at 03:19:02PM +0200, Jesper Louis Andersen wrote:
> On Mon, Jul 20, 2015 at 11:49 AM, Raimo Niskanen <
> raimo+erlang-questions@REDACTED> wrote:
> 
> > That's a pity.  For me it is just about two different kinds of wrappers.
> > Obviously I am not a type expert.
> >
> 
> The reason it is somewhat uncommon to want a list in a programming language
> with type systems is that you can't get it to compile if the type check
> fails. So in such a language, you would have been forced into the product
> type with a tuple {uint8(), [ty()]} as the only way to represent this. The
> wrong program would never have existed. Hence it would not be a problem.
> 
> Digging further, you eventually hit granite in the sense that you do need a
> heterogenous list type. When that happens you are either out of luck, or
> the type system is expressive enough to let you model it.
> 
> It is, generally, hard to graft a type system unto old code. You may have
> to fall back to say that your data is [any()]. It is unsatisfactory, but at
> least it is correct. I think that if you ask the 'typer' program, you get
> the most specific type in the type lattice which dialyzer can figure out
> from looking at the code base. If you use this type, you are at least not
> incorrect, and you are at the limit of what the dialyzer understands,
> currently.

Ok.  Thank you for your time.  I now know a bit more than before.

Best Regards
/ Raimo


> 
> 
> 
> 
> -- 
> J.

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-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB