About

Functions may declare default values for one or more arguments.

def number(n \\ 13), do: "That's not my favorite"

• When compiling the above example, Elixir creates a function definition for number/0 (no arguments), and number/1 (one argument).
• If more than one argument has default values, the default values will be applied to the function from left to right to fill in for missing arguments.
• If the function has multiple clauses, it is required to write a function header for the default arguments.
• Any expression can serve as the default value.
• Anonymous functions cannot have default arguments.

def guess(number \\ 5)
def guess(number) when number != 5, do: false
def guess(number) when number == 5, do: true

guess()
# => true

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Originally from Exercism elixir concepts