haskell code for http://programmingpraxis.com/2013/01/01/happy-new-year/

combineNumbers.hs

import Data.List(foldl')

data Operator = Plus | Minus | Div | Mult | None | Id Double deriving(Eq,Show)
operators = [Plus,Minus,Div,Mult,None]

higher :: Operator -> Operator -> Bool
higher None None = False
higher None _ = True
higher Div Plus = True
higher Div Minus = True
higher Mult Plus = True
higher Mult Minus = True
higher _ _ = False

apply :: Operator -> Double -> Double -> Double
apply None a b = 10*a+b
apply Mult a b = a*b
apply Div a b = a/b
apply Plus a b = a+b
apply Minus a b = a-b

eval ::  [Operator] -> Double
eval xs = shunting xs [] []
  where shunting [] [v] _ = v
        shunting (Id n:ns) as os = shunting ns (n:as) os -- push operand
        shunting [] (a:b:bs) (o:os) = shunting [] (apply o b a:bs) os
        shunting (n:ns) as [] = shunting ns as [n] -- push to empty operator stack
        shunting (n:ns) (a:b:bs) (o:os)
          | n `higher` o = shunting ns (a:b:bs) (n:o:os)
          | otherwise = shunting (n:ns) (apply o b a:bs) os

combinations ::  Int -> [a] -> [[a]]
combinations n = sequence . replicate n

interleave (n:ns) cs = reverse $ foldl' (\acc (a,b) -> a:b:acc) [n] (zip ns cs)

solve n = [combo | combo <- map (interleave ids) (combinations 9 operators)
                 , eval combo == n]
  where ids = [Id n | n <- reverse [1..10]]

main = print $ solve 2013

combineNumbers.lua

local ops = {"","+","-","*","/"}
function check(n,e,res)
  e = e..n
  if n == 1 then
    if luaeval(e) == 2013 then table.insert(res,e) end
  else
    for i,v in ipairs(ops) do check(n-1, e..v,res) end
  end
  return res
end

function luaeval(e)
  return assert(loadstring("return "..e))()
end

local res = check(10,"",{})
for i,v in ipairs(res) do print(v,",") end