Control Structures

if and unless

Chances are you’ve encountered if/2 before, and if you’ve used Ruby you’re familiar with unless/2. In Elixir they work much the same way but they are defined as macros, not language constructs. You can find their implementation in the Kernel module.

It should be noted that in Elixir, the only falsey values are nil and the boolean false.

iex> if String.valid?("Hello") do
...>   "Valid string!"
...> else
...>   "Invalid string."
...> end
"Valid string!"

iex> if "a string value" do
...>   "Truthy"
...> end
"Truthy"

Using unless/2 is like if/2 only it works on the negative:

iex> unless is_integer("hello") do
...>   "Not an Int"
...> end
"Not an Int"

case

If it’s necessary to match against multiple patterns we can use case/2:

iex> case {:ok, "Hello World"} do
...>   {:ok, result} -> result
...>   {:error} -> "Uh oh!"
...>   _ -> "Catch all"
...> end
"Hello World"

The _ variable is an important inclusion in case/2 statements. Without it, failure to find a match will raise an error:

iex> case :even do
...>   :odd -> "Odd"
...> end
** (CaseClauseError) no case clause matching: :even

iex> case :even do
...>   :odd -> "Odd"
...>   _ -> "Not Odd"
...> end
"Not Odd"

Consider _ as the else that will match “everything else”.

Since case/2 relies on pattern matching, all of the same rules and restrictions apply. If you intend to match against existing variables you must use the pin ^/1 operator:

iex> pie = 3.14
 3.14
iex> case "cherry pie" do
...>   ^pie -> "Not so tasty"
...>   pie -> "I bet #{pie} is tasty"
...> end
"I bet cherry pie is tasty"

Another neat feature of case/2 is its support for guard clauses:

This example comes directly from the official Elixir Getting Started guide.

iex> case {1, 2, 3} do
...>   {1, x, 3} when x > 0 ->
...>     "Will match"
...>   _ ->
...>     "Won't match"
...> end
"Will match"

Check the official docs for Expressions allowed in guard clauses.

cond

When we need to match conditions rather than values we can turn to cond/1; this is akin to else if or elsif from other languages:

This example comes directly from the official Elixir Getting Started guide.

iex> cond do
...>   2 + 2 == 5 ->
...>     "This will not be true"
...>   2 * 2 == 3 ->
...>     "Nor this"
...>   1 + 1 == 2 ->
...>     "But this will"
...> end
"But this will"

Like case/2, cond/1 will raise an error if there is no match. To handle this, we can define a condition set to true:

iex> cond do
...>   7 + 1 == 0 -> "Incorrect"
...>   true -> "Catch all"
...> end
"Catch all"

with

The special form with/1 is useful when you might use a nested case/2 statement or situations that cannot cleanly be piped together. The with/1 expression is composed of the keywords, the generators, and finally an expression.

We’ll discuss generators more in the list comprehensions lesson, but for now we only need to know they use pattern matching to compare the right side of the <- to the left.

We’ll start with a simple example of with/1 and then look at something more:

iex> user = %{first: "Sean", last: "Callan"}
%{first: "Sean", last: "Callan"}
iex> with {:ok, first} <- Map.fetch(user, :first),
...>      {:ok, last} <- Map.fetch(user, :last),
...>      do: last <> ", " <> first
"Callan, Sean"

In the event that an expression fails to match, the non-matching value will be returned:

iex> user = %{first: "doomspork"}
%{first: "doomspork"}
iex> with {:ok, first} <- Map.fetch(user, :first),
...>      {:ok, last} <- Map.fetch(user, :last),
...>      do: last <> ", " <> first
:error

Now let’s look at a larger example without with/1 and then see how we can refactor it:

case Repo.insert(changeset) do
  {:ok, user} ->
    case Guardian.encode_and_sign(user, :token, claims) do
      {:ok, token, full_claims} ->
        important_stuff(token, full_claims)

      error ->
        error
    end

  error ->
    error
end

When we introduce with/1 we end up with code that is easy to understand and has fewer lines:

with {:ok, user} <- Repo.insert(changeset),
     {:ok, token, full_claims} <- Guardian.encode_and_sign(user, :token, claims) do
  important_stuff(token, full_claims)
end

As of Elixir 1.3, with/1 statements support else:

iex> import Integer
Integer
iex> m = %{a: 1, c: 3}
%{a: 1, c: 3}
iex> a =
...>   with {:ok, number} <- Map.fetch(m, :a),
...>     true <- is_even(number) do
...>       IO.puts "#{number} divided by 2 is #{div(number, 2)}"
...>       :even
...>   else
...>     :error ->
...>       IO.puts("We don't have this item in map")
...>       :error
...> 
...>     _ ->
...>       IO.puts("It is odd")
...>       :odd
...>   end
It is odd
:odd

It helps to handle errors by providing case-like pattern matching in it. The value passed is the first non-matched expression.

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