let fs = [(+2), (*2), (^2)]
let cs = concat $ map subsequences $ permutations fs
nub cs

<interactive>:31:1:
No instance for (Eq (Integer -> Integer))
  arising from a use of `nub'
Possible fix:
  add an instance declaration for (Eq (Integer -> Integer))
In the expression: nub cs
In an equation for `it': it = nub cs

> let fs = [AddTwo, Double, Square]
> let css = nub $ concat $ map subsequences $ permutations fs

> css

[[],[AddTwo],[Double],[AddTwo,Double],[Square],[AddTwo,Square],[Double,Square],[AddTwo,Double,Square],[Double,AddTwo],[Double,AddTwo,Square],[Square,Double],[Square,AddTwo],[Square,Double,AddTwo],[Double,Square,AddTwo],[Square,AddTwo,Double],[AddTwo,Square,Double]]

> map (cs-> call <$> cs <*> [3,4]) css

[[],[5,6],[6,8],[5,6,6,8],[9,16],[5,6,9,16],[6,8,9,16],[5,6,6,8,9,16],[6,8,5,6],[6,8,5,6,9,16],[9,16,6,8],[9,16,5,6],[9,16,6,8,5,6],[6,8,9,16,5,6],[9,16,5,6,6,8],[5,6,9,16,6,8]]

data Function = AddTwo | Double | Square deriving Eq

call AddTwo  =  (+2)
call Double  =  (*2)
call Square  =  (^2)

{-# LANGUAGE FlexibleInstances           #-}
{-# LANGUAGE NoMonomorphismRestriction   #-}

data NumResLog a = NRL { runNumRes :: a, runNumResLog :: String }
             deriving (Eq, Show)

instance (Num a) => Num (NumResLog a) where
  fromInteger n = NRL (fromInteger n) (show n)
  NRL a alog + NRL b blog
            = NRL (a+b) ( "("++alog++ ")+(" ++blog++")" )
  NRL a alog * NRL b blog
            = NRL (a*b) ( "("++alog++ ")*(" ++blog++")" )
  ...


instance (Num a) => Eq (NumResLog a -> NumResLog a) where
  f == g  = runNumResLog (f arg) == runNumResLog (g arg)
     where arg = NRL 0 "THE ARGUMENT"

unlogNumFn :: (NumResLog a -> NumResLog c) -> (a->c)
unlogNumFn f = runNumRes . f . (`NRL`"")

> let fs = [(+2), (*2), (^2)]
> let cs = concat $ map subsequences $ permutations fs
> let ncs = map (map unlogNumFn) $ nub cs
> map (map ($ 1)) ncs
[[],[3],[2],[3,2],[1],[3,1],[2,1],[3,2,1],[2,3],[2,3,1],[1,2],[1,3],[1,2,3],[2,1,3],[1,3,2],[3,1,2]]