Module:JSON

This module offers some utility methods for converting Lua values into JSON values (in UTF-8-encoded Lua strings).

Unfortunately, Lua's data model differs somewhat from JSON's, so it's not possible to write a general function that takes any Lua value and returns a JSON value, always "doing the right thing". Rather, some values cannot be converted at all, and other values have multiple possible non-equivalent representations.

The differences are:

• Lua has three types with no JSON analogues, namely function, userdata, and thread, so this module has no support for values of those types.
• Lua's concept of "metatables" has no analogue in JSON, so this module ignores metatables completely.
• Lua's number type, as implemented in Scribunto, consists of double-precision floating-point values, whereas JSON's number type consists of decimal representations. (And the end-recipient of the JSON data will likely convert the values back into some sort of floating-point notation.) This means that, aside from integers, you can't generally expect values to be converted exactly. (And even with integers, you can only expect perfect conversion in the range ±10⁹ or so.) What's more, it means that Lua has a few numeric values with no JSON analogues at all, namely positive infinity, negative infinity, and "not a number" values; so, this module does not support those values.
• Lua's string type represents strings of eight-bit bytes, whereas JSON's *string* type represents strings of Unicode characters. This module requires the Lua strings to be valid UTF-8 sequences.
• Whereas Lua has only a single table type mapping from arbitrary non-nil values to arbitrary non-nil values, JSON has separate array and object types, where an array maps from a set of integers {0,1,…,n} to arbitrary values, and an object maps from arbitrary strings to arbitrary values. As a result, this module [TBD]

(Note: the above is an attempt at an exhaustive list of differences, but it's quite possible that I missed some.)

---

local export = {}

-- Given a finite real number x, returns a string containing its JSON
-- representation, with enough precision that it *should* round-trip correctly
-- (depending on the well-behavedness of the system on the other end).
local function json_fromNumber(x)
	if type(x) ~= 'number' then
		error('Not of type "number": ' .. x .. ' (' .. type(x) .. ')')
	end
	if x ~= x or x == math.huge or x == -math.huge then
		error('Not a finite real number: ' .. x)
	end
	return string.format("%.17g", x)
end

-- This function makes an effort to convert an arbitrary Lua value to a string
-- containing a JSON representation of it. It's not intended to be very robust,
-- but may be useful for prototyping.
function export.toJSON(val, opts)
	opts = opts or {}
	
	function converter(val)
		if type(val) == 'nil' then
			return 'null'
		elseif type(val) == 'boolean' then
			return val and 'true' or 'false'
		elseif type(val) == 'number' then
			return json_fromNumber(val)
		elseif type(val) == 'string' then
			return json_fromString(val)
		elseif type(val) == 'table' then
			-- If the table has a toJSON member function, call that.
			if val.toJSON then
				return val:toJSON()
			else
				return export.json_fromTable(val, converter)
			end
		else
			error('Unsupported type: ' .. type(val))
		end
	end
	
	return converter(val)
end

-- Given a string, escapes any illegal characters and wraps it in double-quotes.
-- Raises an error if the string is not valid UTF-8.
function json_fromString(s)
	if type(s) ~= 'string' or not mw.ustring.isutf8(s) then
		error('Not a valid UTF-8 string: ' .. s)
	end
	
	s = s:gsub('[\\"]', '\\%0')
	s = s:gsub('%c', function (c)
		return string.format('\\u00%02X', c:byte())
	end)
	-- Not needed for valid JSON, but needed for compatibility with JavaScript:
	s = s:gsub('\226\128\168', '\\u2028')
	s = s:gsub('\226\128\169', '\\u2029')
	return '"' .. s .. '"'
end

-- Given a table, treats it as an array and assembles its values in the form
-- '[ v1, v2, v3 ]'. Optionally takes a function to JSONify the values before
-- assembly; if that function is omitted, then the values should already be
-- strings containing valid JSON data.
function export.json_arrayFromTable(t, f)
	f = f or function (x) return x end
	
	local ret = {}
	
	for _, elem in ipairs(t) do
		elem = f(elem)
		if elem ~= nil then
			table.insert(ret, ', ')
			table.insert(ret, elem)
		end
	end
	
	if # ret == 0 then
		return '[]'
	end
	
	ret[1] = '[ '
	table.insert(ret, ' ]')
	
	return table.concat(ret)
end

-- Given a table whose keys are all strings, assembles its keys and values in
-- the form '{ "k1": v1, "k2": v2, "k3": v3 }'. Optionally takes a function to
-- JSONify the values before assembly; if that function is omitted, then the
-- values should already be strings containing valid JSON data. (The keys, by
-- contrast, should just be regular Lua strings; they will be passed to this
-- module's jsonStringFromString.)
function export.json_fromTable(val, converter)
	converter = converter or function (x) return x end
	
	local as_array = {}
	local as_object = {}
	local string_key = false
	
	for key, value in pairs(val) do
		value = converter(value)
		
		if type(key) ~= "number" then
			string_key = true
		end
		
		if value ~= nil then
			key = json_fromString(tostring(key))
			table.insert(as_array, value)
			table.insert(as_object, key .. ' : ' .. value)
		end
	end
	
	if string_key then
		return '{' .. table.concat(as_object, ", ") .. '}'
	else
		return '[' .. table.concat(as_array, ", ") .. ']'
	end
end

return export