Measurements are getting serialized incorrectly

[mmikhasenko]

I like very much the JSON.jl package and use all the time for serializing/deserializing Julia object in human-readable format.

When writing Measurements object, it literarily prints ±, that makes JSON invalid. It cannot be read.

using Measurements
using JSON

begin
	open("test.json", "w") do io
		JSON.print(io, Dict("value" => 1.1 ± 3.3))
	end
	# 
	open("test.json") do io
		JSON.parse(io)
	end
end

throws,

Expected ',' here
 ...when parsing byte with value '194'

This using strings works correctly,

begin
	open("test.json", "w") do io
		JSON.print(io, Dict("value" => "1.1 ± 3.3"))
	end
	# 
	open("test.json") do io
		JSON.parse(io)
	end
end

Suggestion

Treat Measurements similar to complex numbers, writing val, and err field in Dict.

[mmikhasenko]

I've looked for a dispatch ::Complex, and did not find any. It appears to be just working with a set of abstract rules.
So, I think now that the JSON.jl package can do little to solve this issue. It falls back to Measurements.jl,

JuliaPhysics/Measurements.jl#187

[quinnj]

This will be fixed with JuliaServices/StructUtils.jl#12; i.e. Measurements already has a natural -> string <- Measurement conversion setup, so we can hook into that for a natural JSON representation.