How do I fix a “will never return since the success typing is […] and the contract is..” from Dialyzer?

2
votes

I am using Dialyzer to fix errors in Erlang code.

io:format(IoDevice, "[]");

This line produces the following error:

The call io:format(IoDevice::pid(),[91 | 93,...]) 
  will never return since the success typing is
  (atom() | binary() | string(),[any()]) -> 'ok' 
  and the contract is (Format,Data) -> 'ok' 
  when Format :: format(), Data :: [term()]

I am not able to understand what the problem is can anyone explain it?

Thank you

1
erlangdialyzer

1 Answers

5
votes

I recommend to read io manual page. Its usage is simple: 

1> io:format("hello ~p~n", [world]). % ~n means newline
hello world
ok
2> io:format("hello ~p~n", [<<"world">>]).             
hello <<"world">>
ok
3> io:format("hello ~s~n", [<<"world">>]).
hello world
ok

In above dialyzer told you that io:format/2 (format/2 means function format which accepts 2 arguments) accepts an atom() or string() or binary() as 1st argument and a list with zero or more elements as 2nd argument. According to your code, dialyzer detects that IoDevice is an Erlang pid() not an string() or binary().