Serialize amount as string

[totemcaf]

The fpmoney.Amount serializes values to JSON as a floating point amount, and a currency code like this:

{
  "amount": 128.45, 
  "currency": "ARS"
}

That is used in frontends and persisted in databases as a floating point number.

Other currency packages serialize the amount as a string instead, to avoid conversions and lost of precision. For example:

{
  "amount": "128.45", 
  "currency": "ARS"
}

What do you think of having a global option or be customized by the format string stored under the "json" key (or a money key) in the struct field's tag:

type Invoice struct {
    TotalAmount fpmoney.Amount `json:"totalAmount,asString"`
}

I can provide a PR.

[nikolaydubina]

nice. that's a good one. 👍🏻

could be easily added to parser too.

Few notes:

• better not to use json tags. to avoid reflection and gencode.
• global option (package level var) - good option. please proceed draft PR if you want!
• please add fuzz/unit test for this too if you make PR

I also need to think how this package can nicely provide both versions (JSON string vs JSON number) at same time. Should we have new type? Same type name but sub-package? 🤔

One thing that may be odd here though — people use string for serialising numbers (money) to avoid using parsing, conversions (float/int/bigint) and arithmetics codebases. Here in this package we exposing parsing and arithmetics. People who use string as number in their JSON may not want to use fpmoney. So use-case may not be a strong.

However, this is interesting extension to this package!