[erlang-questions] [Q] dialyzer message: The pattern X can never match the type Y

Ladislav Lenart <>
Thu May 29 10:03:34 CEST 2008

Per Gustafsson wrote:
> The most likely reason you get a warning like that, which is hard to 
> understand is that you have something like this:
> -module(mod).
> -export([g/0]).
> g() ->
>   f(bar),...,f(baz).
> f(Arg) when Arg == foo; Arg == bar; Arg == baz ->
>      case Arg of
>          foo -> x();
>          _ -> y()
>      end.
> That is you never actually call f/1 with foo and f/1 is not exported.

Ok, this should be it, except that I think the code is actually
executed. Thanks to your explanation, I finally managed to create
a module that reports the warning. It is based on the real module.

Thank you both for your helpful explanations; I am already looking
forward to more enlightenment to come :-)

Ladislav Lenart

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