Dialyzer does not catch errors on returned functions

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Answer

As far as I can understand, Dialyzer wont perform checks on input and output types of returned anonymous functions. The following example, much like mine, is incorrect, but the tool won’t complain:

# no error
@spec add(integer()) :: (String.t() -> String.t())
def add(x) do
  fn y -> x + y end
end

It will however point out a mismatch in arity, e.g.

# invalid_contract
# The @spec for the function does not match the success typing of the function.

@spec add2(integer()) :: (integer(), integer() -> integer())
def add2(x) do
  fn y -> x + y end
end

If however, we try to call the function, Dialzyer might detect that the input and output types are incorrect, but if it does so there is a good chance it will default to the "Function has no local return" error message.

There is an excellent reply in StackOverflow (which I have quickly summarized here) that explains this behavior in more detail and even though we (I and the person replying) were not able to find a specific reason to the “why” of this happening, the empirical tests and samples are still of considerable value.

Upon skimming through the Dialyzer paper I found I was still unable to understand why the algorithm behaves this way, but I will leave it here in case someone else has a different take on it and is able to better understand this specific case:

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