gistfile1.txt

split :: [a] -> [([a], [a])]
split []     = []
split [_]    = []
split (x:xs) = ([x], xs) : [(x:ls, rs) | (ls, rs) <- split xs]

-- auxillary function combine in exprs:

combine :: Expr -> Expr -> [Expr]
combine l r = [App o l r | o <- ops]

ops :: [Op]
ops = [Add,Sub,Mul,Div]
---------------------------------------
exprs :: [Int] -> [Expr]
exprs []  = []
exprs [n] = [Val n]
exprs ns  = [ e | (ls, rs) <- split ns,
                  l        <- exprs ls,
                  r        <- exprs rs,
                  e        <- combine l r] 

solutions :: [Int] -> Int -> [Expr]
solutions ns n =
        [e | ns' <- choices ns, e <- exprs ns', eval e == [n]]