[Haskell-cafe] How to check object's identity?

[Derek Elkins]

On Sun, 2009-01-04 at 16:19 +0800, Evan Laforge wrote:
> > Although equal? treats the two as the *same*, they're different lists
> > because if we modify one (e.g by set-car!) the other won't be affected.
> >
> > So here comes another question: when we say a function always give the
> > same output for the same input, what the *same* means here? ídentity
> > or equality?
> 
> If you don't have set-car!, then identity and equality are impossible
> to differentiate.  And haskell doesn't have set-car!.
> 
> However, as was noted, it does have something like mutable pointers
> via IORef, and sure enough, you can do pointer comparisons with them:
> 
> h <- newIORef 12
> h' <- newIORef 12
> print $ h == h --> True
> print $ h == h' --> False
> 
> Since they're pointers, to compare by value you have to explicitly
> derefence them:
> 
> print =<< liftM2 (==) (readIORef h) (readIORef h') --> True
> 
> 
> If you're wondering about the implementation of "(1:[], 1:[])", ghc
> might be smart enough to do CSE in this case and hence use the same
> memory for both lists, but in general CSE doesn't happen to avoid
> accidental recomputation.  There's some stuff in the ghc manual about
> lambda and let lifting that describes when CSE will and won't happen.
> I wouldn't count on it in general, but I don't read core well enough
> to tell.  Maybe someone who knows more about core can help me here:
> 
> a =
>   \ (eta_azv :: State# RealWorld) ->
>     case a24 stdout lvl7 eta_azv
>     of wild_aFu { (# new_s_aFw, a103_aFx #) ->
>     $wa13 stdout '\n' new_s_aFw
>     }
> 
> There are lots of apparently undefined variables, like the function
> 'a24'.  But it looks like the pair should come from 'lvl7', which is
> chained all the way down to 'lvl' like so:
> 
> lvl :: Integer
> lvl = S# 1
> 
> lvl1 :: [Integer]
> lvl1 =
>   : @ Integer lvl ([] @ Integer)
> 
> lvl2 :: ShowS
> lvl2 = showList lvl1
> 
> lvl3 :: [ShowS]
> lvl3 = : @ ShowS lvl2 ([] @ ShowS)
> 
> lvl4 :: [ShowS]
> lvl4 = : @ ShowS lvl2 lvl3
> 
> lvl5 :: [Char]
> lvl5 = : @ Char a2 ([] @ Char)
> 
> lvl6 :: String
> lvl6 =
>   foldr1
>     @ (String -> String) lvl16 lvl4 lvl5
> 
> lvl7 :: [Char]
> lvl7 = : @ Char a lvl6
> 
> So it *looks* like there's only one list created in 'lvl1', but I
> can't see where it's turning into a tuple, and I don't understand the
> ' = : ' stuff, 

You're reading it wrong. : is a name.  
It's lvl5 = (:) @ Char a2 ([] @ Char) where @ is type application
(instantiation).  Triming that, it's
lvl5 = (:) a2 [] or just lvl5 = a2:[]