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-- Allow a calling function to be passed a mix of positional arguments with
-- keyword arguments. Reference them as fields of a table.
-- Don't use this for a function that can accept a single table argument.
-- mapargs() assumes that a single table argument means its caller was called
-- with f{table constructor} syntax, and maps that table to the specified names.
-- Usage:
-- function f(...)
-- local a = mapargs({'a1', 'a2', 'a3'}, ...)
-- ... a.a1 ... etc.
-- end
-- f(10, 20, 30) -- a.a1 == 10, a.a2 == 20, a.a3 == 30
-- f{10, 20, 30} -- a.a1 == 10, a.a2 == 20, a.a3 == 30
-- f{a3=300, a1=100} -- a.a1 == 100, a.a2 == nil, a.a3 == 300
-- f{1, a3=3} -- a.a1 == 1, a.a2 == nil, a.a3 == 3
-- f{a3=3, 1} -- a.a1 == 1, a.a2 == nil, a.a3 == 3
local function mapargs(names, ...)
local args = table.pack(...)
local posargs = {}
local keyargs = {}
-- For a mixed table, no Lua operation will reliably tell you how many
-- array items it contains, if there are any holes. Track that by hand.
-- We must be able to handle f(1, nil, 3) calls.
local maxpos = 0
-- For convenience, allow passing 'names' as a string 'n0,n1,...'
if type(names) == 'string' then
names = string.split(names, ',')
end
if not (args.n == 1 and type(args[1]) == 'table') then
-- If caller passes more than one argument, or if the first argument
-- is not a table, then it's classic positional function-call syntax:
-- f(first, second, etc.). In that case we need not bother teasing
-- apart positional from keyword arguments.
posargs = args
maxpos = args.n
else
-- Single table argument implies f{mixed} syntax.
-- Tease apart positional arguments from keyword arguments.
for k, v in pairs(args[1]) do
if type(k) == 'number' then
posargs[k] = v
maxpos = math.max(maxpos, k)
else
if table.find(names, k) == nil then
error('unknown keyword argument ' .. tostring(k))
end
keyargs[k] = v
end
end
end
-- keyargs already has keyword arguments in place, just fill in positionals
args = keyargs
-- Don't exceed the number of parameter names. Loop explicitly over every
-- index value instead of using ipairs() so we can support holes (nils) in
-- posargs.
for i = 1, math.min(#names, maxpos) do
if posargs[i] ~= nil then
-- As in Python, make it illegal to pass an argument both positionally
-- and by keyword. This implementation permits func(17, first=nil), a
-- corner case about which I don't particularly care.
if args[names[i]] ~= nil then
error(string.format('parameter %s passed both positionally and by keyword',
tostring(names[i])))
end
args[names[i]] = posargs[i]
end
end
return args
end
return mapargs
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