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With

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.9", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

Navigation

With

In Elixir, with is a control structure that provides a convenient way to handle multiple expressions and pattern match on their results. It allows you to chain together a sequence of expressions and evaluate them one by one, stopping if any of them return an error or a pattern match fails.

with {:ok, v1} <- check1(),
     {:ok, v2} <- check2(v1),
     {:ok, v3} <- check3(v2),
     {:ok, v4} <- check4(v3),
     {:ok, _} <- check5(v4) do
  # code that requires successful results above
end

The with construct simplifies error handling and makes code more readable by avoiding nested case statements or lengthy if conditions. It provides a concise way to handle a series of expressions while gracefully handling potential errors or failed pattern matches.

Here's a version of the above code written using nested case statements to demonstrate how much more concise with statements can be.

case action1() do
  {:ok, v1} ->
  case action2(v1) do
    {:ok, v2} ->
      case action3(v2) do
        {:ok, v3} -> 
          case action4(v3) do
            {:ok, v4} -> 
              case check5(v4) do
                {:ok, _} -> 
                  # code that requires successful results above.
              end
          end
      end
  end
end

We've use {:ok, value} tuples above, but with statements can pattern match on any Elixir term.

student = %{name: "Einar", is_admin: true}

with %{name: name, is_admin: true} <- student do
  IO.puts("admin #{name} is authorized.")
end

Your Turn

with statements can include an else clause for handling errors. The else clause can match on many different patterns.

Change the value of user below to hit every case in the else clause (one at a time.)

user = %{name: "Jon", is_admin: false}

with %{name: name, is_admin: true} <- user do
  IO.puts("admin #{name} is authorized.")
else
  %{is_admin: false, name: name} ->
    IO.puts("#{name} is not an admin")

  %{is_admin: false} ->
    IO.puts("Unknown user is not an admin")

  _ ->
    IO.puts("Something went wrong!")
end

Handling Errors

Some projects use tuples to annotate which statement in a with expression failed.

Some members of the community argue it's preferable to use custom error types instead of using with statements. See Chris Keathley: Good and Bad Elixir for more information.

However, we want you to be aware of this pattern so that you are familiar with it if a future team you're on uses it.

Your Turn

Change value to trigger every else clause.

value = "example"

with {:not_binary, true} <- {:not_binary, is_binary(value)},
     {:too_long, true} <- {:too_long, String.length(value) <= 10},
     {:too_short, true} <- {:too_short, 2 <= String.length(value)} do
  IO.puts("success")
else
  {:not_binary, _} -> IO.puts("value is not binary")
  {:too_long, _} -> IO.puts("value is too long")
  {:too_short, _} -> IO.puts("value is too short")
end

Commit Your Progress

DockYard Academy now recommends you use the latest Release rather than forking or cloning our repository.

Run git status to ensure there are no undesirable changes. Then run the following in your command line from the curriculum folder to commit your progress.

$ git add .
$ git commit -m "finish With reading"
$ git push

We're proud to offer our open-source curriculum free of charge for anyone to learn from at their own pace.

We also offer a paid course where you can learn from an instructor alongside a cohort of your peers. We will accept applications for the June-August 2023 cohort soon.

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