--- simple s expressions parser

local parse, parse_exp, parse_atom

-- expression: starts with ( and ends with ), contains a whietspace separated list of expressions and tokens
-- s has no leading whitespace, starts with (
-- returns exp, r (r has no leading whitespace)
-- returns nil, err
parse_exp = function(s)
	local r = s:match("^%(%s*(.*)$")
	if not r then return nil, "no expression found" end
	local exp = {}
	repeat
		local item, r_item = parse(r)
		if item then
			table.insert(exp, item)
			r = r_item
		end
	until not item
	if not r:match("^%)") then
		return nil, "expected closing )"
	end
	return exp, r:match("^%)%s*(.-)$")
end

-- atom: litteral delimited by whitespace, ), or (; and with escaping using \
-- s has no leading whitespace
-- returns exp, r (r has no leading whitespace)
-- returns nil, err
parse_atom = function(s)
	local atom = {}
	local n, r = s:match("^([^%s%(%)\\]*)(.-)$")
	if #n > 0 then table.insert(atom, n) end
	while r:match("^\\") do
		table.insert(atom, r:match("^\\(.)"))
		n, r = r:match("^\\.([^%s%(%)\\]*)(.-)$")
		if #n > 0 then table.insert(atom, n) end
	end
	if #atom == 0 then return nil, "no atom found" end
	return table.concat(atom), r:match("^%s*(.-)$")
end

-- s has no leading whitespace
-- returns exp, r (r has no leading whitespace)
-- returns nil, err
parse = function(s)
	local i, r = parse_exp(s)
	if i then return i, r end
	i, r = parse_atom(s)
	if i then return i, r end
	return nil, "no expression found"
end

local function test(s)
	local trimmed = s:match("^%s*(.-)$")
	local parsed, r = parse(s)
	if not parsed then
		print(r)
	elseif r:match(".") then
		print(("unexpected %q at end of expression"):format(r))
	else
		print(require("inspect")(parsed))
	end
end

test("((str (Hel\\)lo world) sa mère) (lol))")

test("\\lol\\ wut")

test("()")