Objetos de erro estruturados

Por que usar erros estruturados?

-- Plain string: hard to handle programmatically
error("database error: connection refused")

-- Structured: type + data
error({
  type = "DatabaseError",
  code = "CONN_REFUSED",
  host = "localhost",
  port = 5432,
  message = "connection refused"
})

Padrão de construtor de erros

local ErrorMT = {__tostring = function(e)
  return string.format("[%s] %s", e.type, e.message)
end}

local function makeError(errType, msg, data)
  local e = {type=errType, message=msg}
  if data then for k,v in pairs(data) do e[k]=v end end
  return setmetatable(e, ErrorMT)
end

local E = {
  notFound = function(name) return makeError("NOT_FOUND","not found: "..name,{name=name}) end,
  badInput = function(msg,field) return makeError("BAD_INPUT",msg,{field=field}) end,
}

local ok, err = pcall(error, E.notFound("user:42"))
print(tostring(err))   -- [NOT_FOUND] not found: user:42

Verificação de tipo de objetos de erro

local function handleRequest(fn)
  local ok, err = pcall(fn)
  if ok then return true end
  if type(err) == "table" then
    if err.type == "NOT_FOUND" then
      print("404: " .. err.message)
    elseif err.type == "BAD_INPUT" then
      print("400: " .. err.message .. " (field: " .. (err.field or "?") .. ")")
    else
      print("500: unhandled error: " .. tostring(err))
    end
  else
    print("500: " .. tostring(err))
  end
  return false
end

Hierarquia de erros

local function isError(e, errType)
  if type(e) ~= "table" then return false end
  return e.type == errType or e.parentType == errType
end

local function makeDbError(code, msg)
  return {type="DbError:"..code, parentType="DbError", code=code, message=msg}
end

local err = makeDbError("TIMEOUT","query timed out")
print(isError(err, "DbError"))          -- true
print(isError(err, "DbError:TIMEOUT"))  -- true
print(isError(err, "NetworkError"))     -- false

Envolvimento de erros

local function wrapError(msg, cause)
  return {
    type = "WrappedError",
    message = msg,
    cause = cause,
  }
end

local function loadConfig(path)
  local ok, err = pcall(function()
    local f = assert(io.open(path,"r"))
    local content = f:read("a")
    f:close()
    return content
  end)
  if not ok then
    error(wrapError("failed to load config: "..path, err))
  end
end

local ok2, e = pcall(loadConfig, "missing.cfg")
if not ok2 then
  print(e.message)
  print("Caused by:", tostring(e.cause))
end

Códigos de erro versus tipos de erro

local STATUS = {OK=200, NOT_FOUND=404, SERVER_ERROR=500, BAD_REQUEST=400}

local function makeStatusError(status, msg)
  return {status=status, message=msg, type="HTTPError"}
end

local function handleError(e)
  if e.status == STATUS.NOT_FOUND then
    print("Resource not found:", e.message)
  elseif e.status >= 500 then
    print("Server error:", e.message)
  else
    print("Error", e.status, e.message)
  end
end

handleError(makeStatusError(404, "user not found"))

Pilha de erros (cadeia de causas)

local function unwindCause(e, depth)
  depth = depth or 0
  local pad = string.rep("  ", depth)
  if type(e) == "table" then
    print(pad .. (e.type or "Error") .. ": " .. (e.message or "?"))
    if e.cause then unwindCause(e.cause, depth+1) end
  else
    print(pad .. tostring(e))
  end
end

local inner = {type="IoError", message="permission denied"}
local outer = {type="ConfigError", message="cannot load config", cause=inner}
unwindCause(outer)
-- ConfigError: cannot load config
--   IoError: permission denied

Erro no contexto de uma função de retorno

local function runCallbacks(callbacks, data)
  local errors = {}
  for name, fn in pairs(callbacks) do
    local ok, err = pcall(fn, data)
    if not ok then
      errors[#errors+1] = {callback=name, error=err}
    end
  end
  return errors
end

local cbs = {
  validate = function(d) assert(d.name, "name required") end,
  transform = function(d) d.name = d.name:upper() end,
}

local errs = runCallbacks(cbs, {})
for _, e in ipairs(errs) do
  print(e.callback, "->", e.error)
end

Exibição dos detalhes do erro

local function printError(err, prefix)
  prefix = prefix or "Error"
  if type(err) ~= "table" then
    print(prefix .. ": " .. tostring(err))
    return
  end
  print(prefix .. " [" .. (err.type or "unknown") .. "]")
  print("  Message: " .. (err.message or "?"))
  for k, v in pairs(err) do
    if k ~= "type" and k ~= "message" and k ~= "cause" then
      print("  " .. k .. ": " .. tostring(v))
    end
  end
  if err.cause then printError(err.cause, "  Caused by") end
end

Asserções com erros estruturados

local function assertT(cond, errType, msg, data)
  if not cond then
    local e = {type=errType, message=msg}
    if data then for k,v in pairs(data) do e[k]=v end end
    error(e, 2)
  end
  return cond
end

local function createUser(name, age)
  assertT(type(name)=="string", "BAD_INPUT", "name must be string", {field="name"})
  assertT(age >= 0 and age <= 150, "BAD_INPUT", "invalid age", {field="age", value=age})
  return {name=name, age=age}
end

local ok, err = pcall(createUser, "Alice", -5)
if not ok then print(err.type, err.field, err.value) end

Verificação rápida

Recapitulação: erros estruturados