Solution for code dojo 30/08/2016 with @miguel-vila

sherlock_and_the_beast.hs

-- Slow solution that fails for n>19 due to integer overflow
import Data.List
import Data.Ord

maxPair n = sortBy (comparing snd) [(x,y) | x <- [0,3..n], y <- [0,5..(n-x)], x+y==n]
gen [] = -1
gen ((x,y):_) = let
    noOf5s = (replicate x '5') 
    noOf3s = (replicate y '3') 
    in read (noOf5s ++ noOf3s) :: Int

largest n = gen $ maxPair n

-- Faster solution that passes all test cases:
import Data.Maybe
maxPair' n = listToMaybe [(x,y) | x <- [0,5..n], y <- [0,3..(n-x)], x+y==n]

gen' Nothing = "-1"
gen' (Just (x,y)) = let
    noOf5s = (replicate y '5') 
    noOf3s = (replicate x '3') 
    in (noOf5s ++ noOf3s)
largest' n = gen' $ maxPair' n