aochagavia

Idiomatic way to map a result

A common pattern in Elixir is to return {:ok, something} or {:error, something_else} when a function might fail. Sometimes, however, the caller might need to transform the result. Consider for example of a function to divide numbers, which fails when the divisor is 0, and which we want to use as part of a bigger calculation (e.g. divide x by y, then multiply by 42):

case divide(x, y) do
  {:ok, result} -> {:ok, result * 42}
  {:error, _} = error -> error
end

Given all we want is to transform the result in the :ok case, it feels a bit noisy to require a whole case expression here. Some languages provide a map or map_ok function that helps in these situations, which would simplify things down to the following:

divide(x, y)
|> map_ok(fn result -> result * 42 end)

Does Elixir provide something like this? Or would I need to implement my own helper functions? I know it is easy to implement, but it would be cleaner to be rely on the standard library for this.

9 comments

#idiomatic-elixir

0 947 9

2022-09-05 13:57:18 UTC

First Post!

hauleth

There is with co struct:

with {:ok, v} <- foo(), do: {:ok, v * 2}

Post #1

Most Liked

Eiji

As far as I know Elixir does not implements any helper function to work with pipes except Kernel.tap/2 and Kernel.then/2, but those works for all type of input - not only specific one

How about writing a simple macro based on then?

defmodule Example do
  defmacro ok_then(value, fun) do
    quote do
      case unquote(value) do
        {:ok, result} -> {:ok, unquote(fun).(result)}
        result -> result
      end
    end
  end

  def divide(_x, 0), do: {:error, :zero_divisor}
  def divide(x, y), do: {:ok, div(x, y)}
end

require Example

x
|> Example.divide(y)
|> Example.ok_then(&(&1 * 6))
|> Example.ok_then(&(&1 * 7))
|> then(fn
  {:ok, result} -> IO.inspect(result, label: "Transformed result")
  {:error, :zero_divisor} -> IO.warn("Zero divisor problem …")
  {:error, error} -> IO.warn("Unexpected error #{error}")
end)

Using this you can work only on data you are interested and optionally if something fails you can handle all errors in one place.

Post #5

hauleth

There is no strict requirement that :ok or :error will be:

tuple
with exactly 2 elements

There is plenty of examples where that do not apply:

GenServer.start_link/3 can also return :ignore
Code.fetch_docs/1 returns :docs_v1-tuple or :error tuple
IO.binread/2 can return iodata() on success, unary :error-tuple on error or :eof

Examples like that can be written and written, especially when you add Erlang libraries to the mix. It is hard to have “one catch them all” solution.

I would like to know as well, as functions fits much better there.

Post #7

Last Post!

Eiji

Honestly? I don’t remember.

I know that I read about it somewhere. I wrote the code after looking at then/2 macro code. I guess it was something about compile-time optimization like the macro is translated to case at compile time. I tried to find a guide describing it.

Post #9