[SOLVED] Float format when json serializing in elixir

Float format when json serializing in elixir

Elixir uses scientific notation by default for floats greater then 1000. This causes an undesired side effect during json serialization.

iex(5)> val = 1000.00
1.0e3
iex(11)> Poison.encode(val)
{:ok, "1.0e3"}

The output I would like is

iex(11)> Poison.encode(val)
{:ok, "1000.00"}

I have seen this answer that uses :erlang.float_to_binary(0.005 * 2.7 / 100, [:compact, {:decimals, 10}]) or :io.format("~f~n",[0.005 * 2.7 / 100]), but this would require to patch Poison or to manually check all data before encoding.

Is there a cleaner way to force the default flot to binary format in elixir ?

Solution

Poison allows to implement Poison.Encoder protocol. The implementation for Float obviously exists, that’s why I’d suggest to wrap your floats into custom FloatStruct upfront:

defmodule FloatStruct do
  defstruct value: 0.0, format: [:compact, {:decimals, 10}]
end

defimpl Poison.Encoder, for: FloatStruct do
  def encode(%{value: value, format: format}, options) do
    Poison.Encoder.BitString.encode(
      :erlang.float_to_binary(value, format), options)
  end
end

I understand that this would require traversing nested terms to wrap Floats with FloatStructs, but I don’t see any easier approach. I might be wrong, though.

One might support both mentioned in the OP formats for free:

defimpl Poison.Encoder, for: FloatStruct do
  def encode(%{value: value, format: format}, options) do
    Poison.Encoder.BitString.encode(
      format(value, format), options)
  end

  defp format(value, format) when is_list(format),
    do: :erlang.float_to_binary(value, format)
  defp format(value, format) when is_binary(format),
    do: :io.format(format, [value])
end