Here's the revised version of Main.hs program combining the implementations of the functions in the specified order:
```haskell
module Main (main) where

import TestUtils
import EvalIntExpr (testEvalIntExpr)
import EvalIdExpr (testEvalIdExpr)
import EvalMaybeExpr (testEvalMaybeExpr)
import PostfixExpr (testPostfixExpr)

-- Implement toSingletonLists using map
toSingletonLists :: [e] -> [[e]]
toSingletonLists xs = map (:[]) xs

-- Implement listMap using list comprehension
listMap :: (a -> b -> c) -> a -> [b] -> [c]
listMap f a xs = [f a x | x <- xs]

-- Implement member using foldl
member :: Eq e => e -> [e] -> Bool
member e xs = goFalse xs
  where
    goFalse ys = foldl (\acc y -> acc || (y == e)) False ys

-- Implement selectNApart using drop
selectNApart :: Int -> [e] -> [e]
selectNApart n [] = []
selectNApart n xs = head part ++ selectNApart n rest
  where
    (part, rest) = splitAt (min n (length xs)) xs

-- Implement evalIntExpr using visitor pattern
evalIntExpr :: IntExpr -> Int
evalIntExpr (Leaf i) = i
evalIntExpr (Add e1 e2) = evalIntExpr e1 + evalIntExpr e2
evalIntExpr (Sub e1 e2) = evalIntExpr e1 - evalIntExpr e2
evalIntExpr (Mul e1 e2) = evalIntExpr e1 * evalIntExpr e2
evalIntExpr (Uminus e) = negate (evalIntExpr e)

-- Implement evalIdExpr using visitor pattern
evalIdExpr :: IdExpr -> Assoc Int -> Int
evalIdExpr (Leaf i) _ = i
evalIdExpr (Id idStr) env = maybe 0 idVal envLookup
  where
    idVal = fromIntegral
    envLookup = findWithDefault 0 idStr env

findWithDefault :: a -> k -> [(k, a)] -> a
findWithDefault dflt key = fromJust . lookup key

-- Implement evalMaybeExpr using visitor pattern
evalMaybeExpr :: MaybeExpr -> Assoc Int -> Maybe Int
evalMaybeExpr (Leaf i) _ = Just i
evalMaybeExpr (Id idStr) env = findWithDefault Nothing idStr env
evalMaybeExpr (Add m1 m2) env = liftA2 addMaybes (evalMaybeExpr m1 env) (evalMaybeExpr m2 env)
evalMaybeExpr (Sub m1 m2) env = liftA2 subtractMaybes (evalMaybeExpr m1 env) (evalMaybeExpr m2 env)
evalMaybeExpr (Mul m1 m2) env = liftA2 multiplyMaybes (evalMaybeExpr m1 env) (evalMaybeExpr m2 env)
evalMaybeExpr (Uminus m) env = fmap negate (evalMaybeExpr m env)

addMaybes :: Maybe Int -> Maybe Int -> Maybe Int
addMaybes Nothing (Just i) = Just i
addMaybes (Just i) Nothing = Just i
addMaybes (Just i) (Just j) = Just (i+j)

subtractMaybes :: Maybe Int -> Maybe Int -> Maybe Int
subtractMaybes Nothing (Just i) = Just (-i)
subtractMaybes (Just i) Nothing = Just i
subtractMaybes (Just i) (Just j) = Just (i-j)

multiplyMaybes :: Maybe Int -> Maybe Int -> Maybe Int
multiplyMaybes Nothing _ = Nothing
multiplyMaybes _ Nothing = Nothing
multiplyMaybes (Just i) (Just j) = Just (i*j)

-- Implement postfixExpr parser
parseToken :: String -> Either ParseError Token
parseToken str = case parseString str of
  Left errMsg -> Left (ParseError ("Invalid character in token: " ++ errMsg))
  Right tkn -> Right tkn

data Token = Num Int | Op Operator
  deriving (Show)

data Operator = Plus | Minus | Multiply | UnaryMinus
  deriving (Show)

data ParseError = ParseError String
  deriving (Show)

parseString :: String -> Either ParseError Token
parseString ('+' : rest) = Right (Op Plus)
parseString ('-' : rest) = Right (Op Minus)
parseString ('*' : rest) = Right (Op Multiply)
parseString (('-':rest') & notFollowedBy ('-')) = Right (Op UnaryMinus)
parseString (numChars : rest) =
  case reads numChars of
    [(value, "")
``` complete this program