SSEM simulator in Haskell

A.hs

{-# LANGUAGE BangPatterns #-}
import Data.Array.IO
import Data.Array.Base
import Data.Bits
import Data.Int (Int32)

main = do
  !inits <- return . map (fromIntegral . bin2signed) . lines =<< getContents
  program <- newListArray (0,31) inits :: IO (IOUArray Int Int)
  v <- ssem program
  print v
 where
  bin2signed :: String -> Int32
  bin2signed = foldl (\n c -> 2*n + if c=='1' then 1 else 0) 0

ssem stack = go stack 0 0
 where
  go stack !acc !pc0 = do
    let pc = pc0 + 1
    instr <- unsafeRead stack pc
    let opcode = (instr `shiftR` 29) .&. 7
        operand = instr .&. 31
    v <- unsafeRead stack operand
    case opcode of
      0 -> go stack acc v
      1 -> go stack acc (pc + v)
      2 -> go stack (-v) pc
      3 -> unsafeWrite stack operand acc >> go stack acc pc
      4 -> go stack (acc - v) pc
      6 -> go stack acc (pc + if acc < 0 then 1 else 0)
      7 -> unsafeRead stack 27 >>= return
      _ -> fail $ show opcode