> validate the structure of tuples as parameters to
  > functions.

Dialyzer can definitely do that, you just need to specify a type that is a tuple:

    -type something() :: {Thing  :: thingtype(),
                          Thing2 :: thing2type()}.
    %% ...
    -spec foo(something()) -> thingtype().

But I agree that Dialyzer can feel incomplete. It's been a little while now but I recall having to use `term()' in specs (basically, untyped) to suppress false positives in some cases. It was also quite slow.

Still, a much better situation than Ruby.