dialyzer: user-defined types just synonyms?

Dmitry Belyaev <>
Thu Apr 22 13:42:24 CEST 2010

I want to define my type as a synonym for built-in type. But I want 
dialyzer to warn me if a function is called with wrong type.
I have following code and dialyzer says everything is nice. But I want 
it to warn about "StrFromStr = my_to_str(Str)" line.
Can I make it? How?

I remember Haskell's "newtype" would be of desired behaviour.



-type my_str() :: string().

-spec start(string()) -> tuple().
start(Str0) ->
    MyStr = str_to_my(Str0),
    Str = my_to_str(MyStr),
    MyFromMy = str_to_my(MyStr),
    StrFromStr = my_to_str(Str),
    {Str, MyFromMy, StrFromStr}.

-spec str_to_my(string()) -> my_str().
str_to_my(Str) ->

-spec my_to_str(MyStr) -> string() when is_subtype(MyStr, my_str()).
my_to_str(MyStr) ->

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