[erlang-questions] lists:unzip breaks dialyzer.

Kostis Sagonas <>
Tue Jun 17 15:07:41 CEST 2008

Adam Kelly wrote:
> Hi,
> I've just run into a weird problem with dialyzer.   A module (see
> below) containing a call to lists:unzip with a list of more
> than a single tuple as an argument seems to crash dialyzer.   Am I
> doing something wrong?
> Thanks,
> Adam.
> $ cat test_dia.erl
> -module(test_dia).
> -export([u/0]).
> u() ->
> 	lists:unzip([{a, 1}, {b, 2}]).
> $ erlc +debug_info test_dia.erl
> $ dialyzer -c test_dia.beam
>   Checking whether the initial PLT exists and is up-to-date... yes
>   Proceeding with analysis...
> =ERROR REPORT==== 17-Jun-2008::13:04:56 ===
> dialyzer: Internal problems were encountered in the analysis.
> $ dialyzer --version
> Dialyzer version v1.7.0

With the latest Dialyzer version (the one in R12B-3), I cannot reproduce 
the error.  I suggest you upgrade your Erlang/OTP installation to that 
one -- there are many reasons to do so, not only Dialyzer-specific.

If, for whatever reason, this is not possible, and want to solve this 
issue on your own, look at the definition of lists:unzip/1 types in your 
Erlang/OTP installation (in the file lib/hipe/cerl/erl_bif_types.erl) 
and compare them with the following definition:

type(lists, unzip, 1, Xs) ->
   strict(arg_types(lists, unzip, 1), Xs,
          fun ([Ps]) ->
              case t_is_nil(Ps) of
                true ->
                  t_tuple([t_nil(), t_nil()]);
                false -> % Ps is a proper list of pairs
                  TupleTypes = t_tuple_subtypes(t_list_elements(Ps)),
                  lists:foldl(fun(Tuple, Acc) ->
                                  [A, B] = t_tuple_args(Tuple),
                                  t_sup(t_tuple([t_list(A), t_list(B)]), 
                              end, t_none(), TupleTypes)

for possible differences.


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