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fat error

Replace error messages with error objects that store metadata.

"But why?"

Well, by storing the metadata in a table instead of enriching the error message with it allows some neat things:

Errors can be chained e.g. for propagating out of a coroutine, so you can print the error chain in a consistent manner. Furthermore one can print the error to multiple destinations: A colored version for the terminal, a colorless version for the log file, or even aggregate it in a database for later processing/analysis.


Collecting and printing errors:

local fat_error   = require'fat_error'
local write_error = require'fat_error.writers.FancyWriter'{}

local ok, result_or_err = fat_error.pcall(unsafe_function)
if not ok then

Error messages, e.g. generated by error or assert, get implicitly converted to Error objects.

Propagating errors:

local fat_error = require'fat_error'
local Error     = require'fat_error.Error'

local coro_fn = function() ... end

local coro = coroutine.create(function()
    -- This is needed in order to collect metadata:
    local ok, result_or_err = fat_error.pcall(coro_fn)
    if not ok then

local ok, result_or_err = coroutine.resume(coro)
if not ok then
    -- Propagate error further up:
    error(Error('coroutine error', result_or_err))

Since this is quite tedious there is a simpler method:

local fat_error = require'fat_error'

local coro_fn = function() ... end

local coro = fat_error.create_coroutine_with_error_handler(coro_fn)

local result = fat_error.resume_coroutine_and_propagate_error(coro)


Module fat_error

Function .message_handler(message)

Automatically transforms error messages to Error objects and gathers metadata.

Function .pcall(fn, ...)

Just a convenient wrapper to use the fat_error.message_handler with xpcall.

Function .create_coroutine_with_error_handler(fn)

Wraps coroutine.create to generate Error objects.

Function .resume_coroutine_and_propagate_error(coro, ...)

Wraps coroutine.resume to propagate received errors.

As errors are propagated implicitly the function only returns yielded values.

Class fat_error.Error

It has these properties:

  • description: A value which describes the error - not necessarily a string.
  • message: The string representation of the error description. (Obtained using tostring)
  • parent: Error object that caused this one to be raised.
  • frames: List of Frame objects that describe the threads stack when the error occured.

Constructor Error(description[, parent])

Creates a new Error object with the given description.

You can chain errors by using the parent argument.

Static function .is_instance(value)

Test if a value is an Error instance.

Method :trace_back(level)

Collects stack frames starting at the given level.

This is normally not needed as Error.message_handler calls this already.

Class fat_error.Frame

Collects and stores information about a stack frame:

It has the same properties which debug.getinfo() returns.

Furthermore it collects information about local and upvalue variables. See the methods below for ways to access these.

Variables have these properties:

  • name: How the variable is called in the code. Note that variadic arguments have no name.
  • value: The value it had when it was analyzed. Note that tables are not cloned and therefore their contents may still change.
  • is_upvalue
  • parameter_index: If the variable is a function parameter this holds its position. Goes from 1 to nparams.
  • origin: Name of the local/upvalue/slot which probably provided the value. This may be nil.

Constructor Frame(thread, level)

Creates a new Frame object from the function running at the given stack level.

The thread defaults to the currently running one.

Static function .trace(thread, level[, max_level])

Collects stack frames starting at the given level.

Method :get_parameters()

Returns function parameters as a list.

Variables without names are variadic arguments.

Method :get_locals()

Returns all internal local variables by name.

This includes the parameters, except for the variadic ones.

Method :get_upvalues()

Returns all external local variables by name.

Method :get_named_variables()

Returns all visible variables by name.

Method :get_all_variables()

Returns all variables as a list.

Factory fat_error.writers.FancyWriter

Writes Error objects.

Factory FancyWriter{...}

Creates a new error writer instance.

  • destination: Either a file or a callback. The callback should behave like the write method of file objects. Defaults to io.stderr.

  • use_color: Whether to use ANSI escape codes to colorize the output. Defaults to true if terminal color support is detected.

  • head_location: By default the deepest frame is printed first and the error message is at the bottom.

  • show_variables:

    • none or ``: No variables are shown.
    • parameters: Show only values of function parameters. (default)
    • all: Show all variable values.
  • show_origin: Print the origin variable of values if available. Defaults to true.

License and copyright

Copyright © Henry Kielmann

fat_error is licensed under the MIT license which can be found in the LICENSE file.


Replace error messages with error objects that store metadata.





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