Adding two functions together in Haskell

Question

Hi I am new in Haskell and I came across an interesting problem but I was not really sure on how I would go about solving it. I am about to show you only two parts of the qu

Answer 1

First, you need a way to break some large number into digits.

digits :: Integral x => x -> [x]
digits 0 = []
digits x = digits (x `div` 10) ++ [x `mod` 10]

Which gives you...

Prelude> digits 12345
[1,2,3,4,5]

You can then drop the last digit with init

Prelude> (init . digits) 12345
[1,2,3,4]

The a helper function to map over odd elements in a list.

mapOdd _ [] = []
mapOdd f (x:[]) = [f x]
mapOdd f (x:y:rest) = f x : y : mapOdd f rest

Giving you...

Prelude> mapOdd (+10) [1..10]
[11,2,13,4,15,6,17,8,19,10]

And a function to get back to a large number...

undigits = sum . zipWith (*) [10^n | n <- [0..]] . reverse

Resulting in...

Prelude> undigits [1, 2, 3, 4]
1234

And putting it all together

Prelude> undigits . mapOdd (*2) . init . digits $ 12345
2264

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In functional languages particularly, always try to solve a problem by composing solutions to smaller problems :)