BigDecimal.to_json/.from_json

Help & Support
1

Hi all, can someone help me understand why BigDecimal json support looks broken? I’ve a BigDecimal field in a JSON serializable class and trying a .to_json on that object results in the following error:

require "big"

class Foo
    include JSON::Serializable
  
    property bar : BigDecimal
  
    def initialize(@bar)
    end
end

puts Foo.new(bar: BigDecimal.new(3.14)).to_json
puts Foo.from_json("{\"bar\":\"3.14\"}")


Error: expected argument #1 to 'BigDecimal#to_json' to be IO, not JSON::Builder

After a bit of head scratching, I added the following:


struct BigDecimal
  def to_json(builder : JSON::Builder)
    builder.string to_s
  end

  def self.from_json(pull : JSON::PullParser) : BigDecimal
    new pull.read_string
  end
end

While the above patch fixes the first error, it now fails with:

Error: expected argument #1 to 'BigDecimal.new' to be BigDecimal, BigRational, Float, Int or String, not JSON::PullParser

Thanks in advance!

2

You have to require "big/json"

3

That’s awesome, thank you, it works! Btw, what’s the reason this needs to be manually required? Also, the documentation (BigDecimal - Crystal 1.15.1) doesn’t seem to mention this fact.

4

It’s a pattern used in various places in the stdlib. For types that have to be required manually, their JSON/YAML serializations also have to be required manually on top of them. For example: UUID and URI also need to be required manually and, if you want to send or receive them in either of those formats, you have to do things like require "uuid/json" or require "uri/yaml".

The reason is that you may be using BigDecimal but not JSON or vice versa, so loading both because you loaded one would lead to a lot of unnecessary code being loaded in a lot of use cases. Neither of them are super lightweight and, even though Crystal doesn’t fully compile code that’s never used into binary, requiring unused code isn’t free. The compiler does a fair bit of work in the parsing and semantic phases before discovering the code is not used and that can add anywhere from a few hundred milliseconds to several seconds to your compilation times. To optimize your build times, only require what you specifically depend on.

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5

I’m wondering if we couldn’t do this automatically, with double checks so require order doesn’t matter :thinking:

# big.cr
{% if @toplevel.has_constant?(:JSON) %}
  require "big/json"  
{% end %}

# json.cr
{% if @toplevel.has_constant?(:BigDecimal) %}
  require "big/json"
{% end %}
6 Likes