Dialyzer: Cons will produce an improper list since its 2nd argument is none()

[Fernando Benavides]

I think what's happening in this case is this…
Dialyzer sees your code (using cons for the list) as…

main() -> [ #foo{x = 0} | [#foo{x = false} | []] ].

It then detects the third warning from your email (namely, that you're
using *false* for something that should be an *integer*):
foo.erl:9: Record construction
          #foo{x :: false} violates the declared type of field x ::
          integer()
Therefore, the type of that second part of the list ([#foo{x = false} |
[]]) will be *none()*.
And that leads dialyzer to complain with the second warning that you see…
foo.erl:8: Cons will produce an improper list since its 2nd argument is
          none()
Which, in time, produces the first warning…
foo.erl:7: Function main/0 has no local return

Hope this helps.

On Fri, Feb 14, 2020 at 1:45 PM Jesper Eskilson <jesper.eskilson@REDACTED>
wrote:

> Hi,
>
> When dialyzer analyzes this program:
>
>
> -module(foo).
> -export([main/0]).
> -record(foo, {x :: integer()}).
> main() ->
>   [ #foo{x = 0},
>     #foo{x = false} ].
>
>
> it says:
>
> foo.erl:7: Function main/0 has no local return
> foo.erl:8: Cons will produce an improper list since its 2nd argument is
>           none()
> foo.erl:9: Record construction
>           #foo{x :: false} violates the declared type of field x ::
>           integer()
>
>
> It seems like dialyzer assumes that any cons with second argument not
> being a list will produce an improper list, but shouldn't it treat "none()"
> differently?
>
> Is this a bug in dialyzer, or a feature whose usefulness I am unable to
> grasp?
>
> --
>
> *Jesper Eskilson*
> Senior Software Engineer
> Kred Core
> +46-72-855-8421
>
> Klarna Bank AB (publ)
> Sveavägen 46, 111 34 Stockholm
> Tel: +46 8 120 120 00 <+46812012000>
> Reg no: 556737-0431
> klarna.com
>
>

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Brujo Benavides
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