Getting Bad function Error in Elixir

Question

Just messing with elixir result got an error that I cannot figure out.Here is my snippet,I implemented a simple Parallel map function for some api calls. func is the function where the actual calls are made and it returns {:ok,result} or {:error,reason} which I handle after the mapping in a different function

Originally

def pmap(collection,func,limit \\ 5000) do
    collection
    |> Enum.map(&Task.async(func.(&1)))
    |> Enum.map(&Task.await(&1,limit))

Got the error so changed it to this for readability

def pmap(collection,func,limit) do
  collection
  |>Enum.map(fn(x) -> Task.async(func.(x)) end)
  |>Enum.map(fn(task) -> Task.await(task,limit) end)

The error I am getting states

[error] Task #PID<0.197.0> started from #PID<0.187.0> terminating
** (BadFunctionError) expected a function, got: {:ok,result}
erlang.apply/2
(elixir) lib/task/supervised.ex:85: Task.Supervised.do_apply/2
(elixir) lib/task/supervised.ex:36: Task.Supervised.reply/5
(stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Function: &:erlang.apply/2

From what I gather it is assuming that the actual task collection element is being passed as the function

So I modified the function to

    IO.puts "PMAP BEGUN"
    tasks = collection
        |> Enum.map(fn(x) -> Task.async(func.(x)) end)
    answer = Enum.map(tasks,fn(task) -> Task.await(task,limit) end)
    IO.puts "PMAP DONE"
    answer

The IO.puts were for debugging.So I guess the error happens on the second map as PMAP DONE is never displayed. I still have the same error.

What exactly is wrong here? I have written the same function before almost verbatim and it worked.

Paweł Dawczak · Accepted Answer · 2017-10-02 20:02:24Z

7

The problem you're experiencing happens on line:

  |> Enum.map(fn(x) -> Task.async(func.(x)) end)

to make it work you need to wrap function execution with anonymous function, like:

  |> Enum.map(fn(x) -> Task.async(fn -> func.(x) end) end)

This due to fact if you run func.(x) this will evaluate, and the result of evaluation will be passed to Task.async/1, while when you wrap it with function, it will be up to Task to execute it.

The hint is indicated in error message:

** (BadFunctionError) expected a function, got: {:ok,result}

Hope this helps!

edited Oct 2, 2017 at 20:02

answered Oct 2, 2017 at 19:56

Paweł Dawczak

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