Modules, Functions and Guards

Functions are the unit of code

Code lives in modules (defmodule Shop.Pricing do ... end). Public functions use def and private ones defp. A function is identified by its name and arity (number of arguments), written total/1, so total/1 and total/2 are different functions. Functions can have multiple clauses: Elixir tries each clause top to bottom and runs the first whose patterns and guards match, which replaces many if statements. Guards (when is_integer(qty) and qty > 0) allow only a limited set of side-effect-free expressions; defguard defines reusable ones. Default arguments use \\ (def greet(name, greeting \\ "Hello")). Anonymous functions are written fn x -> x * 2 end and called with a dot, double.(4); the capture operator & gives shorthand: &(&1 * 2) or &String.upcase/1. Functions return the last expression; there is no return keyword. Use @doc and @moduledoc for documentation and @spec for type specifications. Module attributes such as @gst_rate 0.18 act as compile-time constants.

Multi-clause functions with guards

Pattern matching in function heads replaces conditionals.

defmodule Shop.Pricing do
  @moduledoc "Price calculations in paise."
  @gst_rate_percent 18

  @doc "Shipping fee for an order total in paise."
  @spec shipping_fee(non_neg_integer(), keyword()) :: non_neg_integer()
  def shipping_fee(total, opts \\ [])
  def shipping_fee(total, _opts) when total >= 100_000, do: 0
  def shipping_fee(_total, opts) do
    if Keyword.get(opts, :express, false), do: 9_900, else: 4_900
  end

  def with_gst(paise) when is_integer(paise) and paise >= 0 do
    paise + div(paise * @gst_rate_percent + 50, 100)
  end

  def describe({:ok, total}), do: "Total: #{format(total)}"
  def describe({:error, :empty_cart}), do: "Your cart is empty"
  def describe({:error, reason}), do: "Could not price order: #{inspect(reason)}"

  defp format(paise), do: :erlang.float_to_binary(paise / 100, decimals: 2)
end

IO.puts(Shop.Pricing.shipping_fee(150_000))              # 0
IO.puts(Shop.Pricing.shipping_fee(20_000, express: true)) # 9900
IO.puts(Shop.Pricing.describe({:ok, Shop.Pricing.with_gst(10_000)}))   # Total: 118.00

double = fn x -> x * 2 end
IO.inspect(Enum.map([1, 2, 3], double))
IO.inspect(Enum.map(["pune", "delhi"], &String.capitalize/1))

Clause order matters

Clauses are tried top to bottom, so put specific clauses first. The compiler warns when a later clause can never match because an earlier one is more general.

त्वरित जाँच: In Elixir, how are total/1 and total/2 related?

• They are the same function
• They are different functions because arity is part of a function's identity
• total/2 overrides total/1
• Defining both is an error