extensible_list.hs

main.hs

{-# LANGUAGE OverloadedStrings, FlexibleInstances #-}

import Data.String (IsString(..))
import Data.Char (ord, chr)
import Math.OEIS (extendSequence)

main :: IO ()
main = do
    print ("ASDF" + 2)
    print (1 + "KLMN")

data ExtensibleList a = List [a] | Amount Integer

default (ExtensibleList Char)

instance Functor ExtensibleList where
    fmap f (List xs) = List (fmap f xs)
    fmap f (Amount n) = Amount n

instance Num (ExtensibleList Char) where
    fromInteger = Amount
    x + y = toChars (toInts x + toInts y)
        where
        toInts = fmap (toInteger . ord)
        toChars = fmap (chr . fromInteger)

instance Num (ExtensibleList Integer) where
    fromInteger = Amount
    (List x) + (List y) = List (x ++ y)
    (List x) + (Amount n) = List (take (length x + fromInteger n) (extendSequence x))
    (Amount n) + (List x) = List (reverse (take (length x + fromInteger n) (extendSequence (reverse x))))
    (Amount n) + (Amount n') = Amount (n + n')
    -- implement the rest yourself

instance IsString (ExtensibleList Char) where
    fromString = List

instance Show (ExtensibleList Char) where
    show (Amount n) = show n
    show (List x) = x

instance Show (ExtensibleList Integer) where
    show (Amount n) = show n
    show (List x) = show x