[erlang-questions] Dialyzer and erlang:error/1

Tobias Lindahl <>
Wed Jul 8 15:41:32 CEST 2009

Oscar Hellström wrote:
> Hi All,
> I'm trying to express that a valid list of options will return ok while
> any invalid option would eventually throw an exception, but Dialyzer and
> I don't get along.
> Dialyzer says:
> foo.erl:14: The call foo:verify(Options::any(),[any(),...]) will never
> return since it differs in argument position 2 from the success typing
> arguments: (['bar' | 'foo'],[])
> This is entirely true, but it's not an error, it's what I want! I guess
> I'm failing to express what I want here, so could anybody help me?
I see what you are trying to do, but unfortunately it does not work this 
way. The no_return() will disappear when you make the union.

What you need to do, although I admit that it is a bit constructed, is 
to break out the clause that throws the exception to a separate 
function. In this way the no_return() allows you to accept the exception 

(Code below. Should work. Haven't tried it)

Best regards,



-spec verify([atom()]) -> ok.

verify(Options) ->
     verify(Options, []).

-spec verify([atom()], [atom()]) -> ok.

verify([foo | Options], Errors) ->
     verify(Options, Errors);
verify([bar | Options], Errors) ->
     verify(Options, Errors);
verify([Option | Options], Errors) ->
     verify(Options, [Option | Errors]);
verify([], []) ->
verify([], Errors) ->

-spec bad_options([atom()]) -> no_return().

bad_options(Errors) ->
     erlang:error({bad_options, Errors}).

> Best regards
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