How do I reverse a tuple in Haskell

Question

I am having trouble figuring out how to reverse a tuple, below is my code:

extractCode :: HTree -> HCodeMap 
extractCode t = extractCodeInner t []
  where extractCodeInner (Leaf _ c) bits = [(c, [])]
        extractCodeInner (Branch _ left right) bits = map (addBit Zero) (extractCode left ) ++ map (addBit One) (extractCode right)
        where addBit b = second $ ( b : )

Where HTree is defined as an Int (The frequency of a character occurring in a String) and Char from the String:

data HTree = Branch Int HTree HTree | Leaf Int Char 
             deriving (Show, Eq)

and HCodeMap is:

type HCodeMap = [(Char, [Bit])]

Where the Char is a character from the tree and [Bit] is:

data Bit = Zero | One deriving (Show, Eq)

Now when I pass the tree of the string "aaabbc"

I get:

[('c',[Zero, Zero]),('b',[Zero, One]),('a',[One])]

What I would like to do is reverse all of the items in the round bracket list (), such that it will look like this:

[('a',[One]),('b',[Zero, One]),('c',[Zero, Zero])]

I tried to using the sort . reverse I found in this article but I'm not sure how to https://codegolf.stackexchange.com/questions/3571/descending-sort-using-standard-ascending-sort-function

And I previously used a sortby( compare 'on' snd) function but trying to change this to reverse isn't that easy to work through in my head.

I am relatively new to Haskell, so I am sorry if this is a stupid question. Its just really easy to do on a normal list but I can't visualize how I would do it on a tuple.

Thank you in advance!

Bradley Bradley · Accepted Answer · 2015-01-05T22:18:56

As you have said yourself, you need to use the reverse function. Wherever you have your code to form the list of tuples, encapsulate that code in a reverse function using brackets (reverse(code)) or reverse $ <code> and the result will be the reverse.

Hope this helps!

How do I reverse a tuple in Haskell

1 Answers