Help with dialyzer warning on typespec that includes anonymous function

Crowdhailer

Creator of Raxx

I have the following small bind (or flat_map) implementation for error tuples.

@spec bind({:ok, a} | {:error, reason}, function(a) :: {:ok, b} | {:error, reason}) ::
        {:ok, b} | {:error, reason}
      when a: any, b: any, reason: term
def bind({:ok, value}, func) when is_function(func, 1), do: func.(value)
def bind({:error, reason}, _func), do: {:error, reason}

When I use this function as follows

@spec safe_div(integer, integer) :: {:ok, float} | {:error, :zero_division}
def safe_div(_, 0) do
  {:error, :zero_division}
end

def safe_div(a, b) do
  {:ok, a / b}
end

OK.bind({:ok, 4}, &safe_div(3, &1))

I see the following warning.

test/integration.ex:5:no_return
Function run/0 has no local return.
________________________________________________________________________________
Please file a bug in https://github.com/jeremyjh/dialyxir/issues with this message.

Failed to parse warning:
[{:"(", 1}, {:"{", 1}, {:atom_full, 1, '\'ok\''}, {:",", 1}, {:atom_part, 1, 'a'}, {:"}", 1}, {:|, 1}, {:"{", 1}, {:atom_full, 1, '\'error\''}, {:",", 1}, {:atom_part, 1, 'r'}, {:atom_part, 1, 'e'}, {:atom_part, 1, 'a'}, {:atom_part, 1, 's'}, {:atom_part, 1, 'o'}, {:atom_part, 1, 'n'}, {:"}", 1}, {:",", 1}, {:atom_part, 1, 'f'}, {:atom_part, 1, 'u'}, {:atom_part, 1, 'n'}, {:atom_part, 1, 'c'}, {:atom_part, 1, 't'}, {:atom_part, 1, 'i'}, {:atom_part, 1, 'o'}, {:atom_part, 1, 'n'}, {:"(", 1}, {:atom_part, 1, 'a'}, {:")", 1}, {:::, 1}, {:"{", 1}, {:atom_full, 1, '\'ok\''}, {:",", 1}, {:atom_part, 1, 'b'}, {:"}", 1}, {:|, 1}, {:"{", 1}, {:atom_full, 1, '\'error\''}, {:",", 1}, {:atom_part, 1, 'r'}, {:atom_part, 1, 'e'}, {:atom_part, 1, 'a'}, {:atom_part, 1, 's'}, {:atom_part, 1, 'o'}, {:atom_part, 1, 'n'}, {:"}", 1}, {:")", 1}, {:->, 1}, {:"{", ...}, {...}, ...]


Legacy warning:
test/integration.ex:12: The call 'Elixir.OK':bind({'ok', 4},fun((_) -> {'error','zero_division'} | {'ok',float()})) breaks the contract ({'ok',a} | {'error',reason},function(a)::{'ok',b} | {'error',reason}) -> {'ok',b} | {'error',reason} when a :: any(), b :: any(), reason :: term()
________________________________________________________________________________
test/integration.ex:42:unused_fun
Function fetch_key/2 will never be called.
________________________________________________________________________________
done (warnings were emitted)

I can’t make head or tail of it. Any help would be appreciated.
You can see the full code on this PR. Add remaining type specs by CrowdHailer · Pull Request #54 · CrowdHailer/OK · GitHub

7 comments

#dialyzer #dialyxir

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2018-08-25 07:00:39 UTC

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NobbZ

You have the function type wrong. I’ll try to take a closer look into it in a couple of minutes.

Post #1

sasajuric

Author of Elixir In Action

To specify a lambda, you should use (arg1, arg2, ... -> result). With that change, the following typespec should be working:

@spec bind({:ok, a} | {:error, reason}, (a -> {:ok, b} | {:error, reason})) ::
          {:ok, b} | {:error, reason}
        when a: any, b: any, reason: term

Post #2

NobbZ

Yes exactly, that was my intuition as well, but at the time of my writing, I was only on a mobile, the following diff works for me:

diff --git a/lib/ok.ex b/lib/ok.ex
index dd333dc..b640eb0 100644
--- a/lib/ok.ex
+++ b/lib/ok.ex
@@ -44,3 +44,3 @@ defmodule OK do
   """
-  @spec bind({:ok, a} | {:error, reason}, function(a) :: {:ok, b} | {:error, reason}) ::
+  @spec bind({:ok, a} | {:error, reason}, (a -> {:ok, b} | {:error, reason})) ::
           {:ok, b} | {:error, reason}

Post #3

Crowdhailer

Creator of Raxx

Interesting. I wonder why this one was not a problem.

https://github.com/CrowdHailer/OK/blob/master/lib/ok.ex#L27-L30

Post #4

NobbZ

Because you create a parameter named function(a) which shall have the type b, which again is an alias for any. And there is no value that contradicts any.

Post #5

Crowdhailer

Creator of Raxx

Thanks.

One further question.
The parameters in the spec (a, b, reason) are just alias for anyway?
There is no way to make dialyzer realize that if given a function that returns integers the return type will be {:ok, integer} is there?

i.e.

{:ok, :my_atom} = OK.bind({:ok, 4}, &safe_div(3, &1))

That should raise an error becase the return type is really {:ok, integer} | {:error, term}. As far as I am aware there is no way to teach dialyzer that

Post #6

NobbZ

That all occurrences of a should have the same type is only a semantic and mostly of documentation also purpose. Dialyzer does not check if this is actually true.

Post #7

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