Haskell -> Printing sortBy list - error: parse error on input ‘print’-CodePudding

I was trying to run this program on ghci, where it reorders the names in the ascending order of their last names. However, when I run it, I get this error " error: parse error on input ‘print’ ".
I would truly appreciate it if you can help me out with this. Thank you![enter image description here][1]

import Data.List

main :: IO ()

names = [("Tatsunori", "Ono"), ("Kishore", "Palanisamy"), ("Calder", "Hosgood"), ("Yiling", "Zhuang")]

main = do
 let compareLastNames name1 name2 = if lastName1 > lastName2
                                      then GT
                                      else if lastName1 < lastName2
                                            then LT
                                            else EQ
   where lastName1 = snd name1
         lastName2 = snd name2
   
   print (sortBy compareLastNames names)

CodePudding user response：

The reason is the where that is indented at the same level of the let, and not more indented than the compareLastNames function.

You can indent this with:

{-# LANGUAGE BlockArguments #-}

main = do
    let compareLastNames name1 name2 = if lastName1 > lastName2 then GT else if lastName1 < lastName2 then LT else EQ
            where lastName1 = snd name1
                  lastName2 = snd name2
    print (sortBy compareLastNames names)

But you make this too complicated, you can work with on :: (b -> b -> c) -> (a -> b) -> a -> a -> c:

import Data.Function(on)

main = print (sortBy (compare `on` snd) names)

or use sortOn :: Ord b => (a -> b) -> [a] -> [a]:

import Data.List(sortOn)

main = print (sortOn snd names)