Elixir: Module: Guard

‘when’ used in conditional expression evaluation as a ‘guard’

In the code example above, the when keyword is used to evaluate each expression to prevent a negative value (or bad argument) from being passed to the :math module’s sqrt() function. For example, passing the value -1 to :math.sqrt() would return a FunctionClauseError because a function clause that would work with a negative argument would not be found. And execution breaks, before continuing to :math.sqrt(2 * 9.8 * distance). Passing a negative number—without a guard—would otherwise return an ArithmeticError. So (as demonstrated both above and below), when is used as an Elixir guard to prevent a negative number from being passed to the :math.sqrt() function.

iex(1)> Drop.fall_velocity(:earth, -1)
** (FunctionClauseError) no function clause matching in Drop.fall_velocity/2
    drop.ex:2: Drop.fall_velocity(:earth, -1)
iex(2)> Drop.fall_velocity(:earth, 23)
21.23205124334434
iex(3)> Drop.fall_velocity(:earth, 0) 
0.0

Introducing Elixir