# Poison
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[](https://hex.pm/packages/poison)
[](https://hex.pm/packages/poison)
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Poison is a new JSON library for Elixir focusing on wicked-fast **speed**
without sacrificing **simplicity**, **completeness**, or **correctness**.
Poison takes several approaches to be the fastest JSON library for Elixir.
Poison uses extensive [sub binary matching][1], a **hand-rolled parser** using
several techniques that are [known to benefit HiPE][2] for native compilation,
[IO list][3] encoding and **single-pass** decoding.
Preliminary benchmarking has sometimes put Poison's performance closer to
`jiffy`, and almost always faster than existing Elixir libraries.
## Installation
First, add Poison to your `mix.exs` dependencies:
```elixir
def deps do
[{:poison, "~> 3.0"}]
end
```
Then, update your dependencies:
```sh-session
$ mix deps.get
```
## Usage
```elixir
defmodule Person do
@derive [Poison.Encoder]
defstruct [:name, :age]
end
Poison.encode!(%Person{name: "Devin Torres", age: 27})
#=> "{\"name\":\"Devin Torres\",\"age\":27}"
Poison.decode!(~s({"name": "Devin Torres", "age": 27}), as: %Person{})
#=> %Person{name: "Devin Torres", age: 27}
Poison.decode!(~s({"people": [{"name": "Devin Torres", "age": 27}]}),
as: %{"people" => [%Person{}]})
#=> %{"people" => [%Person{age: 27, name: "Devin Torres"}]}
```
Every component of Poison -- the encoder, decoder, and parser -- are all usable
on their own without buying into other functionality. For example, if you were
interested purely in the speed of parsing JSON without a decoding step, you
could simply call `Poison.Parser.parse`.
If you use Poison 1.x, you have to set a module to `as` option in order to
decode into a struct. e.g. `as: Person` instead of `as: %Person{}`. The change
was introduced in 2.0.0.
## Parser
```iex
iex> Poison.Parser.parse!(~s({"name": "Devin Torres", "age": 27}))
%{"name" => "Devin Torres", "age" => 27}
iex> Poison.Parser.parse!(~s({"name": "Devin Torres", "age": 27}), keys: :atoms!)
%{name: "Devin Torres", age: 27}
```
Note that `keys: :atoms!` reuses existing atoms, i.e. if `:name` was not
allocated before the call, you will encounter an `argument error` message.
You can use the `keys: :atoms` variant to make sure all atoms are created as
needed. However, unless you absolutely know what you're doing, do **not** do
it. Atoms are not garbage-collected, see
[Erlang Efficiency Guide](http://www.erlang.org/doc/efficiency_guide/commoncaveats.html)
for more info:
> Atoms are not garbage-collected. Once an atom is created, it will never be
> removed. The emulator will terminate if the limit for the number of atoms
> (1048576 by default) is reached.
## Encoder
```iex
iex> IO.puts Poison.Encoder.encode([1, 2, 3], [])
"[1,2,3]"
```
Anything implementing the Encoder protocol is expected to return an
[IO list][4] to be embedded within any other Encoder's implementation and
passable to any IO subsystem without conversion.
```elixir
defimpl Poison.Encoder, for: Person do
def encode(%{name: name, age: age}, options) do
Poison.Encoder.BitString.encode("#{name} (#{age})", options)
end
end
```
For maximum performance, make sure you `@derive [Poison.Encoder]` for any struct
you plan on encoding.
### Encoding only some attributes
When deriving structs for encoding, it is possible to select or exclude specific
attributes. This is achieved by deriving `Poison.Encoder` with the `:only` or
`:except` options set:
```elixir
defmodule PersonOnlyName do
@derive {Poison.Encoder, only: [:name]}
defstruct [:name, :age]
end
defmodule PersonWithoutName do
@derive {Poison.Encoder, except: [:name]}
defstruct [:name, :age]
end
```
In case both `:only` and `:except` keys are defined, the `:except` option is
ignored.
### Key Validation
According to [the JSON spec](https://tools.ietf.org/html/rfc7159#section-4) keys
in a JSON object should be unique. This is enforced and resolved in different
ways in other libraries. In the Ruby JSON library for example, the output
generated from encoding a hash with a duplicate key (say one is a string, the
other an atom) will include both keys. When parsing JSON of this type, Chromium
will override all previous values with the final one.
Like Ruby, Poison will also generate JSON with duplicate keys. If you'd like to
ensure that your generated JSON doesn't have this issue, you can pass the
`strict_keys: true` option when encoding. This will force the encoding to fail.
Note that validating keys can cause a small performance hit.
```iex
iex> Poison.encode!(%{:foo => "foo1", "foo" => "foo2"}, strict_keys: true)
** (Poison.EncodeError) duplicate key found: "foo"
```
## Benchmarking
```sh-session
$ mix deps.get
$ MIX_ENV=bench mix compile
$ MIX_ENV=bench mix bench
```
## License
Poison is released under [CC0-1.0][5] (see `LICENSE`).
[1]: http://www.erlang.org/euc/07/papers/1700Gustafsson.pdf
[2]: http://www.erlang.org/workshop/2003/paper/p36-sagonas.pdf
[3]: http://jlouisramblings.blogspot.com/2013/07/problematic-traits-in-erlang.html
[4]: http://prog21.dadgum.com/70.html
[5]: https://creativecommons.org/publicdomain/zero/1.0/
