Cannot cons to a list in Haskell data structure

Question

The code below has a data structure Bot which has a parameter values of type [Int]. The function giveValue retrieves the bot of a given index from a hashmap, adds a value to that bot's values parameter and returns the updated hashmap. However, it currently replaces the existing Int in values, rather than append to the array. Could someone explain to me why this is happening?

import Data.List.Split
import Data.List
import Data.Map (Map)
import qualified Data.Map.Strict as Map

data Bot = Bot {values :: [Int], low :: Int, high :: Int}
instance Show (Bot) where
    show b = show (values b) ++ " " ++ show (low b) ++ " " ++ show (high b)


getBot :: Int -> Map Int (Bot)-> Bot
getBot i m
    | (length bots == 1) = bots !! 0
    | otherwise = Bot [] falseVal falseVal
    where
        falseVal = -1
        bots = [b | (i2, b) <- Map.toList m, i2==i]


giveValue :: Int -> Int -> Map Int (Bot) -> Map Int (Bot)
giveValue botInd val oldM = newM
    where
        bot = getBot val oldM
        newM = Map.insert botInd (Bot (val : values bot) (low bot) (high bot)) oldM

main = do
    let bot = Bot [17] 3 4
    let bots = Map.insert 0 bot (Map.empty)

    print $ giveValue 0 61 bots  
    -- Prints fromList [(0, [12] -1 -1)]

Answer 1

I think you mean bot = getBot botInd oldM not bot = getBot val oldM .

The compiler could have caught this, except you're using Int as the type of both the value you want to put in and how you're looking up the bot.

Maybe if you pass in the whole Bot to giveValue instead, then you could be sure that you're giving the right value to the right bot?

You could also wrap the "bot id" in a newtype, like newtype BotId = BotId { unBotId :: Int } . getBot would take a BotId , and then ghc would've complained that you were passing in an integer instead of an id!

Also, consider using Maybe Int instead of Int for low and high . The fact that you're using a falseVal is a big red flag that you should be using Maybe instead.