Condition handling in Lua

cond.lua

local M = {}

local error, unpack = error, unpack or table.unpack
local running = coroutine.running
local stderr = io.stderr
local exit = os.exit
local insert, remove = table.insert, table.remove

-- conditions

local handlers = setmetatable({}, {
	__mode = 'k', -- do not retain dead coroutines
	__index = function (self, key) -- no handlers by default
		self[key] = {}; return self[key]
	end,
})

local function removing(xs, x, ok, ...)
	assert(remove(xs) == x)
	if ok then return ... else error(...) end
end

-- establish a handler during call
function M.hcall(h, f, ...)
	local hs = handlers[running()]
	insert(hs, h)
	return removing(hs, h, pcall(f, ...))
end

-- signal the given condition to currently active handlers
function M.signal(...)
	local hs = handlers[running()]
	for i = #hs, 1, -1 do hs[i](...) end
end
local signal = M.signal

function M.error(...)
	signal(...)
	stderr:write("error: " .. tostring(...) .. "\n")
	exit(1)
end

function M.warn(...)
	signal(...)
	stderr:write("warning: " .. tostring(...) .. "\n")
end

-- restarts

-- invoke the given restart
function M.restart(r, ...)
	local n = select('#', ...); r.n = n
	for i = 1, n do r[i] = select(i, ...) end
	error(r)
end

local function continue(r, ok, ...)
	if ok then return ok, ... end
	if ... == r then return false, unpack(r, 1, r.n) end
	error(...)
end

-- establish a restart during call
function M.rcall(f, ...)
	local r = {}
	return continue(r, pcall(f, r, ...))
end

return M

example.lua

local cond = require 'cond'

-- common condition types (XXX should use proper dynamic variables instead)

local retry, use = nil, nil

-- I/O library

local function _gets()
	if math.random() < 0.5 then cond.error 'lossage' end
	return 'user input'
end

local function gets()
	local ok, value = cond.rcall(function (_use)
		use = _use
		local ok, value
		repeat ok, value = cond.rcall(function (_retry)
			retry = _retry
			return _gets()
		end) until ok
		return value
	end)
	-- ok or not, we got a value either way
	return value
end

-- application (knows nothing about errors)

local function app()
	for i = 1, 5 do print(string.format("got: %q", gets())) end
	return "success"
end

-- shell

local ok, value = cond.rcall(function (abort)
	return cond.hcall(function (err)
		io.stderr:write("I/O error: " .. err .. "\n")
		while true do
			io.stderr:write("[a]bort, [r]etry, [u]se value? ")
			local answer = io.read('*l')
			if answer == 'a' then cond.restart(abort, "aborted") end
			if answer == 'r' then cond.restart(retry) end
			if answer == 'u' then
				io.stderr:write("value? ")
				cond.restart(use, io.read('*l'))
			end
		end
	end, app)
end)
print(ok, value)