Function capturing

Throughout this tutorial, we have been using the notation name/arity to refer to functions. It happens that this notation can actually be used to retrieve a named function as a function type. Start iex, running the math.exs file defined above:

$ iex math.exs

iex> Math.zero?(0)
true
iex> fun = &Math.zero?/1
&Math.zero?/1
iex> is_function(fun)
true
iex> fun.(0)
true

Local or imported functions, like is_function/1, can be captured without the module:

iex> &is_function/1
&:erlang.is_function/1
iex> (&is_function/1).(fun)
true

Note the capture syntax can also be used as a shortcut for creating functions:

iex> fun = &(&1 + 1)
#Function<6.71889879/1 in :erl_eval.expr/5>
iex> fun.(1)
2

The &1 represents the first argument passed into the function. &(&1+1) above is exactly the same as fn x -> x + 1 end. The syntax above is useful for short function definitions.

If you want to capture a function from a module, you can do &Module.function():

iex> fun = &List.flatten(&1, &2)
&List.flatten/2
iex> fun.([1, [[2], 3]], [4, 5])
[1, 2, 3, 4, 5]

&List.flatten(&1, &2) is the same as writing fn(list, tail) -> List.flatten(list, tail) end which in this case is equivalent to &List.flatten/2. You can read more about the capture operator & in the Kernel.SpecialForms documentation.