Question

I know it is verbose but that is how I am learning the syntax. With this line at

(abCombo’ a 2 lst) … I want to return the ‘list’ and/or print the list but I am having trouble extracting the list with this return type, ‘ Writer [String] [Int]’.

-- Loop through several integer values
-- and calculate the power of a^b, append to list
abCombo' :: Int -> Int -> [Int] -> Writer [String] [Int]
abCombo' a b lst
    | b == maxB = do
        tell [ "  ... x-Done(1): a^b = " ++ show (a^b) ++ " // " ++ show lst  ]
        return ((a^b):lst)
    | otherwise = do
        tell [ "  ... x-Processing: a^b = " ++ show (a^b) ++ " // " ++ show lst ]
        abCombo' a (b+1) ((a^b):lst)

-- Loop through several integer values
-- and calculate the power of a^b, append to list
abCombo :: Int -> [Int] -> Writer [String] [Int]
abCombo a lst
    | a == maxA = do              
        tell [ "- Done(2): a=" ++ show a ]
        abCombo' a 2 lst
    | otherwise = do
        (abCombo' a 2 lst) <<<<<<<<<<<<<<<<<<<<<< line of interest, here
        tell ["- Processing: a=" ++ show a]
        abCombo (a + 1) lst 

…

That is the current code above, I want to change it to:

abCombo :: Int -> [Int] -> Writer [String] [Int]
abCombo a lst
    | a == maxA = do              
        tell [ "- Done(2): a=" ++ show a ]
        abCombo' a 2 lst
    | otherwise = do
        let res = (abCombo' a 2 lst) <<<<<<<<<<<<<<<<<<<<<< line of interest, here            
        tell ["- Processing: a=" ++ show a]
        abCombo (a + 1) (flatten snd res)

Answer 1

To bind the result of an action in a do-block you need to use <- instead of let.

res <- abCombo' a 2 lst      -- res :: [Int]

This is because with let, you’re just putting a name on the action itself.

let res = abCombo' a 2 lst   -- res :: Writer [String] [Int]