Erlang dialyzer demo

dial.erl

dial.erl
-module(dial).
-export([dial/0]).

addthree(X) ->
    X + 3.

dial() ->
   Y = addthree(2),
   io:fwrite("~p~n", [Y]).

When run:
2> dial:dial().
5

Add a bad call:
_ = addthree("test"),

Compiles without error.
1> c(dial).

2> dial:dial().
** exception error: an error occurred when evaluating an arithmetic expression
     in function  dial:addthree/1 (dial.erl, line 5)
     in call from dial:dial/0 (dial.erl, line 9)

Alternately, run typEr on original code. Due to our trivial example, an overly specified function is suggested.

$ /usr/lib/erlang/bin/typer dial.erl

%% File: "dial.erl"
%% ----------------
-spec addthree(2) -> 5.
-spec dial() -> none().

Paste output into code.
Now add bad function call, see this on compile:

dial.erl:10: Function dial/0 has no local return
dial.erl:12: The call dial:addthree([101 | 115 | 116,...]) will never return since the success typing is (2) -> 5 and the contract is (2) -> 5
 done in 0m0.17s