[Haskell-cafe] StableNames and monadic functions
Lorenzo Bolla
lbolla at gmail.com
Tue Jun 26 16:57:28 CEST 2012
In other words there is a difference between Identity and Equivalence. What
you have implemented with StableName is an "Identity" (sometimes called
"reference equality"), as opposed to an "Equivalence" (aka "value
equality").
In Python, for example:
>>> x = {1:2}
>>> y = {1:2}
>>> x == y
True
>>> x is y
False
L.
On Tue, Jun 26, 2012 at 3:42 PM, Lorenzo Bolla <lbolla at gmail.com> wrote:
> I think about StableName like the "&" operator in C, that returns you the
> memory address of a variable. It's not the same for many reasons, but by
> analogy, if &x == &y then x == y, but &x != &y does not imply x != y.
>
> So, values that are semantically equal, may be stored in different memory
> locations and have different StableNames.
>
> The fact that changing the order of the lines also changes the result of
> the computation is obviously stated in the type signature of
> makeStableName, which lives in the IO monad. On the other hand
> hashStableNAme is a pure function.
>
> L.
>
>
>
> On Tue, Jun 26, 2012 at 3:26 PM, Ismael Figueroa Palet <
> ifigueroap at gmail.com> wrote:
>
>>
>>
>> 2012/6/26 Lorenzo Bolla <lbolla at gmail.com>
>>
>>> The point I was making is that StableName might be what you want. You
>>> are using it to check if two functions are the same by comparing their
>>> "stablehash". But from StableName documentation:
>>>
>>> The reverse is not necessarily true: if two stable names are not equal,
>>>> then the objects they name may still be equal.
>>>
>>>
>>> The `eq` you implemented means this, I reckon: if `eq` returns True then
>>> the 2 functions are equal, if `eq` returns False then you can't tell!
>>>
>>> Does it make sense?
>>> L.
>>>
>>
>> Yes it does make sense, and I'm wondering why the hash are equal in one
>> case but are not equal on the other case (i.e. using let/where vs not using
>> it) because I'd like it to behave the same in both situations
>>
>> Thanks again
>>
>>
>>>
>>>
>>> On Tue, Jun 26, 2012 at 1:54 PM, Ismael Figueroa Palet <
>>> ifigueroap at gmail.com> wrote:
>>>
>>>> Thanks Lorenzo, I'm cc'ing the list with your response also:
>>>>
>>>> As you point out, when you do some kind of "let-binding", using the
>>>> where clause, or explicit let as in:
>>>>
>>>> main :: IO ()
>>>> main = do
>>>> let f1 = (successor :: Int -> State Int Int)
>>>> let f2 = (successor :: Int -> Maybe Int)
>>>> b2 <- eq f2 f2
>>>> b1 <- eq f1 f1
>>>> print (show b1 ++ " " ++ show b2)
>>>>
>>>> The behavior is as expected. I guess the binding triggers some internal
>>>> optimization or gives more information to the type checker; but I'm still
>>>> not clear why it is required to be done this way -- having to let-bind
>>>> every function is kind of awkward.
>>>>
>>>> I know the details of StableNames are probably
>>>> implementation-dependent, but I'm still wondering about how to detect /
>>>> restrict this situation.
>>>>
>>>> Thanks
>>>>
>>>>
>>>> 2012/6/26 Lorenzo Bolla <lbolla at gmail.com>
>>>>
>>>>> From StableName docs:
>>>>>
>>>>>> The reverse is not necessarily true: if two stable names are not
>>>>>> equal, then the objects they name may still be equal.
>>>>>
>>>>>
>>>>> This version works as expected:
>>>>>
>>>>> import System.Mem.StableName
>>>>> import Control.Monad.State
>>>>>
>>>>> eq :: a -> b -> IO Bool
>>>>> eq a b = do
>>>>> pa <- makeStableName a
>>>>> pb <- makeStableName b
>>>>> return (hashStableName pa == hashStableName pb)
>>>>>
>>>>> successor :: (Num a, Monad m) => a -> m a
>>>>> successor n = return (n+1)
>>>>>
>>>>> -- main :: IO ()
>>>>> -- main = do
>>>>> -- b2 <- eq (successor :: Int -> State Int Int) (successor ::
>>>>> Int -> State Int Int)
>>>>> -- b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int
>>>>> -> Maybe Int)
>>>>> -- print (show b1 ++ " " ++ show b2)
>>>>>
>>>>> main :: IO ()
>>>>> main = do
>>>>> b2 <- eq f2 f2
>>>>> b1 <- eq f1 f1
>>>>> print (show b1 ++ " " ++ show b2)
>>>>> where f1 = (successor :: Int -> Maybe Int)
>>>>> f2 = (successor :: Int -> State Int Int)
>>>>>
>>>>>
>>>>>
>>>>> hth,
>>>>> L.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Tue, Jun 26, 2012 at 1:15 PM, Ismael Figueroa Palet <
>>>>> ifigueroap at gmail.com> wrote:
>>>>>
>>>>>> I'm using StableNames to have a notion of function equality, and I'm
>>>>>> running into problems when using monadic functions.
>>>>>>
>>>>>> Consider the code below, file Test.hs
>>>>>>
>>>>>> import System.Mem.StableName
>>>>>> import Control.Monad.State
>>>>>>
>>>>>> eq :: a -> b -> IO Bool
>>>>>> eq a b = do
>>>>>> pa <- makeStableName a
>>>>>> pb <- makeStableName b
>>>>>> return (hashStableName pa == hashStableName pb)
>>>>>>
>>>>>> successor :: (Num a, Monad m) => a -> m a
>>>>>> successor n = return (n+1)
>>>>>>
>>>>>> main :: IO ()
>>>>>> main = do
>>>>>> b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int ->
>>>>>> Maybe Int)
>>>>>> b2 <- eq (successor :: Int -> State Int Int) (successor :: Int
>>>>>> -> State Int Int)
>>>>>> print (show b1 ++ " " ++ show b2)
>>>>>>
>>>>>> Running the code into ghci the result is "False False". There is some
>>>>>> old post saying that this is due to the dictionary-passing style for
>>>>>> typeclasses, and compiling with optimizations improves the situation.
>>>>>>
>>>>>> Compiling with ghc --make -O Tests.hs and running the program, the
>>>>>> result is "True True", which is what I expect.
>>>>>> However, if I change main to be like the following:
>>>>>>
>>>>>> main :: IO ()
>>>>>> main = do
>>>>>> b2 <- eq (successor :: Int -> State Int Int) (successor :: Int
>>>>>> -> State Int Int)
>>>>>> b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int ->
>>>>>> Maybe Int)
>>>>>> print (show b1 ++ " " ++ show b2)
>>>>>>
>>>>>> i.e. just changing the sequential order, and then compiling again
>>>>>> with the same command, I get "True False", which is very confusing for me.
>>>>>> Similar situations happens when using the state monad transformer,
>>>>>> and manually built variations of it.
>>>>>>
>>>>>> It sounds the problem is with hidden closures created somewhere that
>>>>>> do not point to the same memory locations, so StableNames yields false for
>>>>>> that cases, but it is not clear to me under what circumstances this
>>>>>> situation happens. Is there other way to get some approximation of function
>>>>>> equality? or a way to "configure" the behavior of StableNames in presence
>>>>>> of class constraints?
>>>>>>
>>>>>> I'm using the latests Haskell Platform on OS X Lion, btw.
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> --
>>>>>> Ismael
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Haskell-Cafe mailing list
>>>>>> Haskell-Cafe at haskell.org
>>>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>>>>
>>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Ismael
>>>>
>>>>
>>>
>>
>>
>> --
>> Ismael
>>
>>
>
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