benlime
How to encode Decimal with Jason Library to float?
Hey guys,
I have a short and simple question, which I am currently unable to figure out using the documentation of the Jason library.
I have a struct which contains some keys with values of the Decimal struct. I would like to have the Decimal struct (I am using the Decimal Library for it) always converted to float when encoding to JSON. So far I only know that I would have to implement the Json.Encoder protocol and have the encode method to call Decimal.to_float(value). But I am not sure how to do that? How do I implement the protocol?
I have tried to just make a file like this:
defimpl Jason.Encoder, for: Decimal do
def encode(struct, opts) do
Jason.Encode.map(Decimal.to_float(struct), opts)
end
end
However, this code never runs. Can anybod help? 
Thanks guys.
First Post!
NobbZ
Please do not use floats. You will use all the precision you initially got by using decimal. Please do either use a String or dump into a JSON object.
Aside of that, you should use Jason.Encode.float/1. From what I can see in the snippet you posted, the code should crash rather than not being run. Can you therefore reproduce the problem in a fresh project and put it on github?
Most Liked
NobbZ
Floating point numbers might have their right to exist in JSON, but definitively not when your datasource is a Decimal, as Decimal allows for very exact values, which float doesn’t.
This is good.
But to be honest, in JSON the only way to represent numbers in a safe way, as JSON does not differentiate between integers and floats, but knows only about numbers. So numbers that look integral might get coerced to float without you beeing able to control.
From jason:
https://github.com/michalmuskala/jason/blob/master/lib/encoder.ex#L204-L210
So you can not change it.
And here we already have a problem. What is encoded as 1 Euro and 5 cent might get rendered as 1.05 in the JSON, but as this probably not an exact value in Float this might get loaded as 1.04999999 (or similar) and displayed as such. Or even worse, the client truncates or rounds, or does other things out of your control.
Your user has the right to get the same (or at least equivalent) information that you are working with, especially when its the users money!
NobbZ
There are enough, just take 0.3, you can not represent that value as float32, the same is probably true for float64, but you can at least have a closer approximation.
Play around with the float converter, it shows you what is possible and what is not.
Also, just as an exercise. Try to find n and m: 0.3 = m / (2 ** n)
Even though that is not the real representation, it will give you an estimate what the problem is.
benwilson512
Sure, here’s a concrete example:
Suppose you have a service where people want to send you numbers, and you’re going to do some kind of processing with those numbers, and then send back a result. Suppose it’s super simple, like a math test for kids where they’re asked to round a number.
The task is: Round “2.675” to the nearest hundredths place.
You take that value, and parse it to a float:
iex(7)> {value, _} = Float.parse("2.675")
{2.675, ""}
Yay! that looks right!
iex(8)> value |> Float.round(2)
2.67
Oh no…
This happens because it isn’t actually 2.675, it’s 2.67499999999999982236431605997495353221893310546875 and Elixir is nice and displays something more friendly.
Yes, if you care about precision at all. To be clear, plenty of times you don’t care about precision. Maybe you’re doing geolocation stuff where precision is carried as a separate term anyway, maybe you’re doing something in a GPU with floating point math. But if people want you to treat the number as it is written, you can’t convert it to a float, at all.
Last Post!
wojtekmach
Thanks for writing down what worked for you, always appreciated!
My only suggestion would be to add the following check to catch values that’d be considered invalid:
+ if Decimal.inf?(value) or Decimal.nan?(value) do
+ raise "cannot encode #{inspect(value)} to JSON"
+ end
Decimal.to_string(value, :normal)
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