The glu.json module

The glu.json module provides JSON encoding and decoding. It converts between JSON strings/files and Lua tables.

local json = require("glu.json")

Functions

decode

Parse a JSON string and return the corresponding Lua value.

local data = json.decode('{"name": "Alice", "scores": [95, 87, 92]}')
print(data.name)       --> Alice
print(data.scores[2])  --> 87

JSON types map to Lua types:

Raises a Lua error if the string is not valid JSON.

encode

Convert a Lua table to a JSON string. By default the output is compact (no whitespace):

local s = json.encode({greeting = "hello", items = {1, 2, 3}})
--> {"greeting":"hello","items":[1,2,3]}

Pass true as the second argument for indented output (2 spaces):

local pretty = json.encode({greeting = "hello"}, true)
--> {
-->   "greeting": "hello"
--> }

Pass a string to use a custom indent:

local tabs = json.encode({a = 1}, "\t")

Lua tables with consecutive integer keys (1, 2, 3, …) become JSON arrays. Tables with string keys become JSON objects. Raises a Lua error if the value cannot be serialized.

read

Read a JSON file from disk and return the parsed Lua value. This is a shorthand for reading the file and calling decode:

local config = json.read("settings.json")
print(config.version)

Raises a Lua error if the file cannot be read or contains invalid JSON.

write

Write a Lua table to a JSON file. The output is always formatted with 2-space indentation for readability:

json.write("output.json", {
    version = 2,
    tags = {"alpha", "beta"},
})

This creates output.json:

{
  "tags": [
    "alpha",
    "beta"
  ],
  "version": 2
}

Pass a string as the third argument to use a custom indent:

json.write("output.json", data, "\t")

Raises a Lua error if the file cannot be written.

Example: persistent build counter

Using glu.json together with the auxiliary file or standalone:

local json = require("glu.json")

-- Load or initialize state
local state_file = "build-state.json"
local state = { count = 0 }
local f = io.open(state_file, "r")
if f then
    f:close()
    state = json.read(state_file)
end

-- Update
state.count = state.count + 1
state.last_build = os.date()

-- Save
json.write(state_file, state)
print("Build #" .. state.count)

For data that should be tied to the document lifecycle (and trigger reruns when changed), use the _aux table instead — see Auxiliary file.