[erlang-questions] specs for fixed length lists

Richard Carlsson <>
Sun Jul 8 21:58:58 CEST 2012

On 07/08/2012 07:47 PM, Tony Rogvall wrote:
> While thinking about this problem I wrote the following spec:
> -spec get_buttons() ->
>       {ok,{boolean(),{boolean(),{boolean(),{boolean(),[]}}}}}.
> get_buttons() ->
>      {ok,{true,{true,{false,12}}}}.
> This passed dialyzer!
> The reason (I suppose) is that the analysis is limited to depth = 2 (or 3).
> Running typer while commenting out the spec gives:
> -spec get_buttons() -> {'ok',{'true',{'true',{_,_}}}}.
> I guess this is the real reason why dialyzer can not handle fixed size
> lists (at least not a list with a length greater than 3)

Dialyzer is still pretty poor at handling recursive tuple structures, as 
you noted, and uses depth-k limitation (I don't remember the details, 
and they've probably changed since I last looked at that code). But 
lists are treated differently from tuples. It is assumed that most sane 
uses of lists will not depend on the order of elements from a type 
perspective. This lets Dialyzer use a less exact approximation, which 
still captures most of the useful information for most programs but 
without slowing down the type analysis too much.


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