Lua Multimethods (Clojure-style)

gistfile1.lua

function defmulti(keyfn)
  if not keyfn then
    keyfn = function(...) return ... end
  end
  local multi = {
    _keyfn = keyfn,
    _methods = {}
  }
  setmetatable(multi, {
    __call = function(self, ...)
      local key = self._keyfn(...)
      if self._methods[key] then
        return self._methods[key](...)
      else
        if self._methods.default then
          return self._methods.default(...)
        else
          error("no dispatch for key: " .. tostring(key))
        end
      end
    end
  })
  return multi
end

function defmethod(multi, key, fn)
  if not multi._methods then
    multi._methods = {}
  end
  multi._methods[key] = fn
end

fib = defmulti(tonumber)
defmethod(fib, 0, function() return 0 end)
defmethod(fib, 1, function() return 1 end)
defmethod(fib, "default", function(n) return fib(n-1) + fib(n-2) end)
print(fib(10))

greet = defmulti(function(user) return user.name end)
defmethod(greet, "mk", function() return "MK is 1337" end)
defmethod(greet, "mark", function() return "Mark is a lamer" end)
defmethod(greet, "default", function(user) return string.format("%s seems like a nice fellow", user.name) end)
print(greet({name = "mk"}))
print(greet({name = "mark"}))
print(greet({name = "steven"}))

This only overloads the first parameter, and only by value (not by type).