Elixir v1.4.5 Collectable protocol View Source
A protocol to traverse data structures.
The Enum.into/2
function uses this protocol to insert an
enumerable into a collection:
iex> Enum.into([a: 1, b: 2], %{})
%{a: 1, b: 2}
Why Collectable?
The Enumerable
protocol is useful to take values out of a collection.
In order to support a wide range of values, the functions provided by
the Enumerable
protocol do not keep shape. For example, passing a
map to Enum.map/2
always returns a list.
This design is intentional. Enumerable
was designed to support infinite
collections, resources and other structures with fixed shape. For example,
it doesn’t make sense to insert values into a range, as it has a fixed
shape where just the range limits are stored.
The Collectable
module was designed to fill the gap left by the
Enumerable
protocol. into/1
can be seen as the opposite of
Enumerable.reduce/3
. If Enumerable
is about taking values out,
Collectable.into/1
is about collecting those values into a structure.
Link to this section Summary
Functions
Returns a function that collects values alongside the initial accumulation value
Link to this section Types
Link to this section Functions
Returns a function that collects values alongside the initial accumulation value.
The returned function receives a collectable and injects a given
value into it for every {:cont, term}
instruction.
:done
is passed when no further values will be injected, useful
for closing resources and normalizing values. A collectable must
be returned on :done
.
If injection is suddenly interrupted, :halt
is passed and it can
return any value, as it won’t be used.