break.cabal

name:                break
version:             0.1.0.0
build-type:          Simple
cabal-version:       >=1.10

library
  exposed-modules:     Lib
  build-depends:       base >=4.10 && <4.11
                     , transformers
  hs-source-dirs:      src
  default-language:    Haskell2010

executable test
  main-is:             Main.hs
  build-depends:       base
                     , break
  hs-source-dirs:      exe
  default-langauge:    Haskell2010

Lib.hs

module Lib where

import Control.Monad.Trans.State.Strict

eval :: Int -> State Int a -> a
eval p = fst . flip runState p

advance :: Int -> State Int ()
advance = modify' . (+)

loc :: State Int Int
loc = get

emit1 :: State Int ()
emit1 = advance 1

emitN :: Int -> State Int ()
-- adding in the 0 case, breaks with HEAD. 8.2.1 is fine with it.
-- emitN 0 = advance 0
emitN 0 = pure ()
emitN n = advance n

align8 :: State Int ()
align8 = do
  bits <- (`mod` 8) <$> loc
  emitN (8 - bits)

Main.hs

module Main where

import Lib
import System.Exit

main :: IO ()
main = do
  let p = eval 0 (emit1 >> align8 >> loc)
  putStrLn $ show p
  if p == 8
    then putStrLn "OK" >> exitSuccess
    else putStrLn "FAIL" >> exitFailure