haskell script to asciify image, scaling is next step

asciify.hs

#!/usr/bin/env stack
{- stack
    --resolver lts-14.5
    --install-ghc
    exec ghci
    --package JuicyPixels
    --package lens
    --package split
-}
{-# LANGUAGE ScopedTypeVariables #-}
import Codec.Picture
import Codec.Picture.Metadata
import Codec.Picture.Types
import Control.Lens
import Control.Lens.Prism
import Control.Monad
import Data.Monoid
import Data.List.Split(chunksOf)

main = readImage "poggers.png" >>= asciify

          where
            asciify (Left errorMsg) = putStrLn $ "error: " <> errorMsg
            asciify (Right dynamicImage) =
               let rgb8 = pixelMap gray $ convertRGB8 dynamicImage in
               forM_ (chunksOf (imageWidth rgb8) (rgb8 ^.. imagePixels)) (putStrLn . concatMap grayRamp)
            gray = computeLuma :: PixelRGB8 -> Pixel8
            grayRamp (i :: Pixel8) = 
                let offset = round (fromIntegral i / 256 * 69) :: Int
                in "$@B%8&WM#*oahkbdpqwmZO0QLCJUYXzcvunxrjft/\\|()1{}[]?-_+~<>i!lI;:,\"^`'." ^.. traversed.index offset