string.format is cool, but we can do better

fmt.lua

-- depends on https://github.com/kikito/inspect.lua
local inspect = require 'inspect'

local M = {}

local function tostring_impl(v)
    if type(v) == "table" then
      -- change to inspect(v, {newline=""}) if you want to print everything tables on one line
      return inspect(v) 
    elseif type(v) == "nil" then
      return "<nil>"
    end
    return tostring(v)
end

local function fmt_n(s, t)
  return (s:gsub('({[_%a][_%w]*})', function(w)
    local v = t[w:sub(2, -2)]
    return tostring_impl(v)
  end))
end

function M.fmt(s, ...)
  if ... == nil then -- allows to call it like this: fmt.fmt(val)
    return M.fmt("{}", s)
  end

  if type(...) == "table" and
     not string.find(s, "{}") then
    -- using named placeholders, otherwise can be fmt({}, t)
    return fmt_n(s, ...)
  end

  local t = { ... }
  local c = 0
  return (s:gsub('({})', function()
    c = c + 1
    local v = t[c]
    return tostring_impl(v)
  end))
end

function M.print(...)
  print(M.fmt(...))
end

return M

script.lua

local fmt = require 'fmt'

local line = 32
fmt.print("[WARNING] (line: {}, script: '{}', data: {}): {} = {}",
  line, "game.lua", "fmt",
  "foo", { bar=42, nice="OK!"}
)

local line = 39
fmt.print("[INFO] (line: {line}, script: '{script}', data: {data}): {foo} = {foo_cont}",
    {
      line = line,
      script = "game.lua",
      data = nil,
      foo = "foo",
      foo_cont = { bar=1, nice="wow"}
    }
)

script_output.txt

[WARNING] (line: 32, script: 'game.lua', data: fmt): foo = {
  bar = 42,
  nice = "OK!"
}
[INFO] (line: 39, script: 'game.lua', data: <nil>): foo = {
  bar = 1,
  nice = "wow"
}