How to use "with" statement?

Questions / Help
1

Hi,

Is there any documentation or description of best practice on how to use the “with” statement ?
Or at least proper syntax description ?

Using some examples from online and this forum, I wrote the below case.

The problem is that when I run an app I see that something is going on, and then nothing, no success result and no errors.

Simply, empty terminal. Any suggestions of what could be wrong ?

with {:ok, url}         <- do_get_url(id, lang, url),
     {:ok, json}        <- do_get_json(url),
     {:ok, map}         <- do_convert_json_to_map(json),
     {:ok, models}      <- do_get_models(map, amount),
     {:ok, flat_map}    <- do_flatten_map(models),
     {:ok, models}      <- do_download_model_images(flat_map, %{download_dir: dir, image_max_size: size})
   do   
     {:ok, models}
   else
     {:error, reason} -> {:error, reason}
     error -> {:error, "#{inspect error}"}
end
1 Like
2

So just to be clear, you get neither {:ok, models} nor {:error, reason} back?

3

except the last statement error -> {:error, "#{inspect error}"} is anything else supposed to print something on the terminal?

Maybe try

with {:ok, url}         <- do_get_url(id, lang, url),
     {:ok, json}        <- do_get_json(url),
     {:ok, map}         <- do_convert_json_to_map(json),
     {:ok, models}      <- do_get_models(map, amount),
     {:ok, flat_map}    <- do_flatten_map(models),
     {:ok, models}      <- do_download_model_images(flat_map, %{download_dir: dir, image_max_size: size})
   do   
     {:ok, models} |> IO.inspect
   else
     {:error, reason} -> {:error, reason} |> IO.inspect
     error -> {:error, "#{inspect error}"}
end

Also in this line

{:ok, models}      <- do_get_models(map, amount),

the amount variable is declared somewhere above the with statement?

1 Like
4

Hi, thank your for your replays!
I did check step by step the code, and the problem was in one do_flatten_map(models),

There, was guard is_map(models) ,but by mistake the list was passed to this function.
Therefore, the pattern matching did work. The strange thing is that there was no errors…

Currently the problem solved, but the main question was about documentation - is there any official documentation about:

with

statement ?

5

https://hexdocs.pm/elixir/Kernel.SpecialForms.html#with/1

Just by way of education with, like basically everything in Elixir, is an expression not a statement. Expressions return values statements do not. An example of the latter would be a C style if block.

5 Likes
6

Hi benwilson512,

Thank you for clarification and for the link.
This is again exactly that I needed !