[erlang-questions] Dialyzer and List Type

[Stavros Aronis]

Hi!

The most general type that foo can have is [number()] -> [number()]. The
spec restricts both number() instances to just integer(). Then, since bar's
only argument is given as is to foo, it should also be [integer()].

If you specify bar's argument as atom() then there is no way to make a
successful call to foo and Dialyzer complains. If however bar's argument is
a list of atoms, then there is a way, though a bit trivial, for a call to
succeed: an empty list is both a valid list of atoms and a valid list of
integers.

Dialyzer can see this, and does not complain. Perhaps a patch is needed,
for a warning of type "only the empty list satisfies the given spec"...

Cheers,

Stavros

On Tue, 19 Apr 2016, 08:50 PhayTsukiming, <moonsolo@REDACTED> wrote:

> Hi list, I am new to erlang and dialyzer. I have encounter a problem when
> I tried to fiddle with type specifications.
>
> Say, I have two functions:
>
> -module(t).
> -export([foo/1, bar/1]).
>
> -spec foo([integer()])->[integer()].
> foo(Arg)->
>     [I+1||I<-Arg].
>
> -spec bar(atom())->[integer()].
> bar(_Arg)->
>     foo(_Arg).
>
> Then apply dialyzer to this module and dialyzer warns:
>
> t.erl:8: Invalid type specification for function t:bar/1. The success
> typing is ([integer()]) -> [integer()]
>
> But if I change the specification of bar to:
>
> -spec bar([atom()])->[integer()].
>
> dialyzer will show no warnings. My intention is to make sure the argument
> of bar is the same type of list as the argument of foo.
>
> Is this normal? Do I miss or misunderstand something about success typing
> or dialyzer?
>
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