Question

I’m trying to sort a list and to check my sorting algorithm is working as I would expect I would like to print out specific elements of the sorted list, something I would expect to be a simple task but is proving to be very difficult – I guess I’m going about it the wrong way.

data Candidate = Candidate Float Float Float String

…

getName :: Candidate -> String
getName (Candidate weight profit effic name) = name

…

main = do
    let items = [Candidate 0.20 4.17 (calculateEfficiency 0.20 4.17) "Weapon"]
    Candidate 3.11 4.53 (calculateEfficiency 3.11 4.53) "Tinned food":items
    Candidate 1.04 4.64 (calculateEfficiency 1.04 4.64) "Ammunition":items
    Candidate 2.70 1.19 (calculateEfficiency 2.70 1.19) "Water":items   

    let sortedItems = sortBy mySort items
    putStrLn (getName (sortedItems !! 0))

The error I get is:

Couldn't match expected type `[b0]' with actual type `IO ()'
In the return type of a call of `putStrLn'
In the expression: putStrLn (getName (sortedItems !! 0))
In the expression:
  do { let items = ...;
         Candidate 3.11 4.53 (calculateEfficiency 3.11 4.53) "Tinned food"
       : items;
         Candidate 1.04 4.64 (calculateEfficiency 1.04 4.64) "Ammunition"
       : items;
         Candidate 2.7 1.19 (calculateEfficiency 2.7 1.19) "Water" : items;
       .... }
Failed, modules loaded: none.

Thanks for any help.

Answer 1

The : operator is the prepend operator for lists, but it doesn’t actually modify a list. Imagine that instead of a list, you had a number; what you are doing is equivalent to this:

let items = 1
2 + items
7 + items
3 + items

That does’t make any sense.

You need to store the result of the prepend somewhere. You can do this by either creating new variables for each step:

let items = [Candidate 0.20 4.17 (calculateEfficiency 0.20 4.17) "Weapon"]
let items2 = Candidate 3.11 4.53 (calculateEfficiency 3.11 4.53) "Tinned food" : items

…or you can simply create a long list to begin with:

let items = [ Candidate 0.20 4.17 (calculateEfficiency 0.20 4.17) "Weapon"
            , Candidate 3.11 4.53 (calculateEfficiency 3.11 4.53) "Tinned food"
            , Candidate 1.04 4.64 (calculateEfficiency 1.04 4.64) "Ammunition"
            ]

This should yield the expected results, depending on if you also implemented mySort correctly.