a solver for https://twitter.com/miharasan/status/1348567822377521154

main.hs

import Control.Monad (guard, replicateM)
import Data.List (foldl')

main :: IO ()
main = do
    print $ head $ do
        [a, b, c, d, e, g, h, j] <- replicateM 8 [0 .. 9]
        [f, i] <- replicateM 2 [1 .. 9]
        guard $ h * h `mod` 10 == h
        guard $ dec [f, d, c, a, j, h] * dec [i, b, c, f, e, h] == dec [f, b, a, e, c, i, i, j, e, g, i, h]
        pure (a, b, c, d, e, f, g, h, i, j)

dec :: [Int] -> Int
dec = foldl' (\acc n -> 10 * acc + n) 0