[erlang-questions] specs for fixed length lists

Tony Rogvall <>
Sun Jul 8 20:42:14 CEST 2012

Ah, you are right (of course)
Stupid mistake trying to use | for dotted pair, sorry about that.

I was suppose to "write" something like:

-type buttons() :: 

Several attempts writing this kind of specs, crashed dialyzer:

-type buttons() :: 
        maybe_improper_list(boolean(), integer()).

-spec get_buttons() -> {ok,buttons()}.

get_buttons() ->

dialyzer list_type.erl
  Checking whether the PLT /Users/tony/.dialyzer_plt is up-to-date... yes
  Proceeding with analysis...
=ERROR REPORT==== 8-Jul-2012::20:41:14 ===
Error in process <0.29.0> with exit value: {{badmatch,false},[{erl_types,t_maybe_improper_list,2,[{file,"erl_types.erl"},{line,1361}]},{erl_types,t_from_form,5,[{file,"erl_types.erl"},{line,3665}]},{erl_types,t_from_form,5,[{file,"erl_types.erl"},{line,3724}]},{erl_types,list_from_form... 

dialyzer: Analysis failed with error:
Last messages in the log cache:
  Reading files and computing callgraph... done in 0.13 secs
  Removing edges... done in 0.00 secs


On 7 jul 2012, at 21:53, Richard Carlsson wrote:

> On 2012-07-07 13:57, Tony Rogvall wrote:
>> Try this:
>>     -type buttons() :: [boolean() | [boolean() | [boolean() |
>> [boolean() | []]]]].
> You certainly can _write_ it like that, just like you can write "1+1" instead of "2". To a tool like Dialyzer, it will still just be interpreted as a (nonempty) list of boolean(). The exact number of elements is not tracked. And if you for example try to say "[atom() | [integer() | []]]", it will just be the same as saying "nonempty list of atom()|integer()" - the order of occurrence of the element types is also not part of the list type.
>    /Richard
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