Snippets of Int vs String IORef and basic CAS test. Spot the difference. These are both from .dump-simpl files.

Int_Version.hs

    case newMutVar# @  Int   @ RealWorld main4 eta_Xg
    of _ [Occ=Dead] { (# ipv_a7jX, ipv1_a7jY #) ->
    case ({__pkg_ccall_GC atomic-primops-0.6.0.4 stg_readMutVar2zh MutVar#
                                                           RealWorld ()
                                                         -> State# RealWorld
                                                         -> (# State# RealWorld, Any () #)}_a7mV
            (ipv1_a7jY
             `cast` ((MutVar# <RealWorld>_N (UnivCo representational  Int   ()))_R
                     :: MutVar# RealWorld  Int   ~# MutVar# RealWorld ()))
            ipv_a7jX)
         `cast` ((# <State# RealWorld>_R,
                    (Any (UnivCo nominal ()  Int  ))_R #)_R
                 :: (# State# RealWorld, Any () #)
                      ~#
                    (# State# RealWorld, Any  Int   #))
    of _ [Occ=Dead] { (# ipv2_X4yl, ipv3_X4yn #) ->
    case ipv3_X4yn of tick1_ { __DEFAULT ->

    case ({__pkg_ccall_GC atomic-primops-0.6.0.4 stg_casMutVar2zh MutVar#
                                                          RealWorld ()
                                                        -> Any ()
                                                        -> Any ()
                                                        -> State# RealWorld
                                                        -> (# State# RealWorld, Int#, Any () #)}_a7lX
            (ipv1_a7jY
             `cast` ((MutVar# <RealWorld>_N (UnivCo representational  Int   ()))_R
                     :: MutVar# RealWorld  Int   ~# MutVar# RealWorld ()))
            (tick1_
             `cast` ((Any (UnivCo nominal  Int   ()))_R :: Any  Int   ~# Any ()))
            (main3
             `cast` (UnivCo representational Int (Ticket Int)
                     ; (Any (UnivCo nominal  Int   ()))_R
                     ::  Int   ~# Any ()))
            ipv2_X4yl)
         `cast` ((# <State# RealWorld>_R,
                    <Int#>_R,
                    (Any (UnivCo nominal ()  Int  ))_R #)_R
                 :: (# State# RealWorld, Int#, Any () #)
                      ~#
                    (# State# RealWorld, Int#, Any  Int   #))
    of _ [Occ=Dead] { (# ipv4_a7j8, ipv5_a7j9, ipv6_a7ja #) ->
    case ==# ipv5_a7j9 0 of _ [Occ=Dead] {
      __DEFAULT -> (# ipv4_a7j8, () #);
      0 ->
        ((failIO @ () main2)
         `cast` (NTCo:IO[0] <()>_R
                 :: IO () ~# (State# RealWorld -> (# State# RealWorld, () #))))
          ipv4_a7j8
    }
    }
    }
    }
    }

String_Version.hs

    case newMutVar# @ [Char] @ RealWorld old eta_Xg
    of _ [Occ=Dead] { (# ipv_a7jX, ipv1_a7jY #) ->
    case ({__pkg_ccall_GC atomic-primops-0.6.0.4 stg_readMutVar2zh MutVar#
                                                           RealWorld ()
                                                         -> State# RealWorld
                                                         -> (# State# RealWorld, Any () #)}_a7k3
            (ipv1_a7jY
             `cast` ((MutVar# <RealWorld>_N (UnivCo representational [Char] ()))_R
                     :: MutVar# RealWorld [Char] ~# MutVar# RealWorld ()))
            ipv_a7jX)
         `cast` ((# <State# RealWorld>_R,
                    (Any (UnivCo nominal () [Char]))_R #)_R
                 :: (# State# RealWorld, Any () #)
                      ~#
                    (# State# RealWorld, Any [Char] #))
    of _ [Occ=Dead] { (# ipv2_X4yo, ipv3_X4yq #) ->
    case ipv3_X4yq of tick1_ { __DEFAULT ->
    case unpackCString# "bye"# of new1_a7j2 { __DEFAULT ->
    case ({__pkg_ccall_GC atomic-primops-0.6.0.4 stg_casMutVar2zh MutVar#
                                                          RealWorld ()
                                                        -> Any ()
                                                        -> Any ()
                                                        -> State# RealWorld
                                                        -> (# State# RealWorld, Int#, Any () #)}_a7j5
            (ipv1_a7jY
             `cast` ((MutVar# <RealWorld>_N (UnivCo representational [Char] ()))_R
                     :: MutVar# RealWorld [Char] ~# MutVar# RealWorld ()))
            (tick1_
             `cast` ((Any (UnivCo nominal [Char] ()))_R :: Any [Char] ~# Any ()))
            (new1_a7j2
             `cast` (UnivCo representational [] Any 
                     (UnivCo nominal Char [Char]; UnivCo nominal [Char] ())
                     :: [Char] ~# Any ()))
            ipv2_X4yo)
         `cast` ((# <State# RealWorld>_R,
                    <Int#>_R,
                    (Any (UnivCo nominal () [Char]))_R #)_R
                 :: (# State# RealWorld, Int#, Any () #)
                      ~#
                    (# State# RealWorld, Int#, Any [Char] #))
    of _ [Occ=Dead] { (# ipv4_a7j8, ipv5_a7j9, ipv6_a7ja #) ->
    case ==# ipv5_a7j9 0 of _ [Occ=Dead] {
      __DEFAULT -> (# ipv4_a7j8, () #);
      0 ->
        ((failIO @ () main2)
         `cast` (NTCo:IO[0] <()>_R
                 :: IO () ~# (State# RealWorld -> (# State# RealWorld, () #))))
          ipv4_a7j8
    }
    }
    }
    }
    }
    }