Elixir条件语句 #

iex(1)> if true do
...>   "True branch"
...> end
"True branch"

iex(2)> if false do
...>   "True branch"
...> end
nil

iex(1)> if false do
...>   "True branch"
...> else
...>   "False branch"
...> end
"False branch"

iex(1)> if true, do: "True", else: "False"
"True"

iex(2)> if true, do: "True"
"True"

iex(1)> unless false do
...>   "Not false"
...> end
"Not false"

iex(2)> unless true do
...>   "Not true"
...> else
...>   "Is true"
...> end
"Is true"

iex(1)> if 0, do: "0 is truthy"
"0 is truthy"

iex(2)> if "", do: "empty string is truthy"
"empty string is truthy"

iex(3)> if [], do: "empty list is truthy"
"empty list is truthy"

iex(4)> if nil, do: "nil is truthy"
nil

iex(1)> case {:ok, "Hello"} do
...>   {:ok, result} -> result
...>   {:error, reason} -> "Error: #{reason}"
...> end
"Hello"

iex(1)> case {1, 2, 3} do
...>   {1, x, 3} -> "Matched with x = #{x}"
...>   {4, 5, 6} -> "Won't match"
...>   _ -> "Match anything"
...> end
"Matched with x = 2"

iex(1)> case {1, 2, 3} do
...>   {1, x, 3} when x > 0 -> "x is positive: #{x}"
...>   {1, x, 3} when x < 0 -> "x is negative: #{x}"
...>   _ -> "No match"
...> end
"x is positive: 2"

iex(1)> case 5 do
...>   x when x < 0 or x > 10 -> "Out of range"
...>   x when x >= 0 and x <= 10 -> "In range"
...> end
"In range"

iex(1)> case :unknown do
...>   :ok -> "OK"
...> end
** (CaseClauseError) no case clause matching: :unknown

def process_file(path) do
  case File.read(path) do
    {:ok, content} ->
      process_content(content)

    {:error, :enoent} ->
      {:error, "File not found"}

    {:error, reason} ->
      {:error, "Read error: #{reason}"}
  end
end

def handle_response(response) do
  case response do
    %{status: 200, body: body} ->
      {:ok, parse_body(body)}

    %{status: 404} ->
      {:error, :not_found}

    %{status: status} when status >= 500 ->
      {:error, :server_error}

    %{status: status} when status >= 400 ->
      {:error, :client_error}
  end
end

iex(1)> cond do
...>   1 + 1 == 3 -> "Wrong"
...>   2 * 2 == 4 -> "Correct"
...>   true -> "Fallback"
...> end
"Correct"

def grade(score) do
  cond do
    score >= 90 -> "A"
    score >= 80 -> "B"
    score >= 70 -> "C"
    score >= 60 -> "D"
    true -> "F"
  end
end

def check_permission(user) do
  cond do
    user.role == :admin -> :full_access
    user.role == :moderator -> :moderate_access
    user.active == false -> :no_access
    user.subscription == :premium -> :premium_access
    true -> :basic_access
  end
end

iex(1)> cond do
...>   1 + 1 == 3 -> "Wrong"
...> end
** (CondClauseError) no cond clause evaluated to a true value

defmodule Guard do
  def type_check(x) when is_integer(x), do: "Integer"
  def type_check(x) when is_float(x), do: "Float"
  def type_check(x) when is_binary(x), do: "String"
  def type_check(x) when is_list(x), do: "List"
  def type_check(x) when is_map(x), do: "Map"
  def type_check(x) when is_atom(x), do: "Atom"
  def type_check(_), do: "Unknown"
end

defmodule Math do
  def abs(x) when x < 0, do: -x
  def abs(x), do: x

  def divide(_, 0), do: {:error, :division_by_zero}
  def divide(a, b) when is_number(a) and is_number(b), do: {:ok, a / b}
end

def handle_event(event) do
  case event do
    %{type: :click, target: target} ->
      handle_click(target)

    %{type: :keypress, key: key} when key in ["Enter", "Space"] ->
      handle_action_key(key)

    %{type: :keypress, key: key} ->
      handle_key(key)

    %{type: :scroll, direction: direction} ->
      handle_scroll(direction)
  end
end

def calculate_discount(user, order) do
  cond do
    user.membership == :platinum -> 0.20
    user.membership == :gold -> 0.15
    user.membership == :silver -> 0.10
    order.total > 1000 -> 0.05
    true -> 0
  end
end

def format_name(name) do
  if name do
    String.trim(name)
  else
    "Anonymous"
  end
end

def bad_example(user) do
  if user do
    if user.active do
      if user.admin do
        "Admin access"
      else
        "User access"
      end
    else
      "Inactive"
    end
  else
    "Guest"
  end
end

def good_example(user) do
  case user do
    nil -> "Guest"
    %{active: false} -> "Inactive"
    %{admin: true} -> "Admin access"
    _ -> "User access"
  end
end

if condition do
  statement1
  statement2
end

if condition, do: statement

if condition do
  statement1
else
  statement2
end

if condition,
  do: statement1,
  else: statement2