Jonas Almström Duregård | 29 Jun 13:25 2012
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[GHC Users] Dictionary sharing

Hi,

Is there a way to ensure that functions in a class instance are
treated as top level definitions and not re-evaluated?

For instance if I have this:
>>>
class C a where
  list :: [a]

instance List a => List [a] where
  list = permutations list
<<<
How can I ensure that list :: [[X]] is evaluated at most once for any
type X (throughout my program)?

I assume this is potentially harmful, since list can never be garbage
collected and there may exist an unbounded number of X's.

I currently have a solution that uses Typeable to memoise the result
of the function based on its type. Is there an easier way?

Regards,
Jonas
Edward Z. Yang | 29 Jun 15:55 2012
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Re: [GHC Users] Dictionary sharing

Hello Jonas,

Like other top-level definitions, these instances are considered CAFs
(constant applicative forms), so these instances will in fact usually
be evaluated only once per type X.

    import System.IO.Unsafe
    class C a where
        dflt :: a
    instance C Int where
        dflt = unsafePerformIO (putStrLn "bang" >> return 2)
    main = do
        print (dflt :: Int)
        print (dflt :: Int)
        print (dflt :: Int)

ezyang <at> javelin:~/Dev/haskell$ ./caf
bang
2
2
2

Cheers,
Edward

Excerpts from Jonas Almström Duregård's message of Fri Jun 29 07:25:42 -0400 2012:
> Hi,
> 
> Is there a way to ensure that functions in a class instance are
> treated as top level definitions and not re-evaluated?
(Continue reading)

Jonas Almström Duregård | 29 Jun 17:21 2012
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Re: [GHC Users] Dictionary sharing

Thank you for your response Edward,

You write that it is usually only evaluated once, do you know the
circumstances under which it is evaluated more than once? I have some
examples of this but they are all very large.

The real issue I was having was actually not with a list but with a
memoised function i.e. something like:
>>>
class C a where
  memoised :: Int -> a
<<<

Perhaps functions are treated differently?

Regards,
Jonas

On 29 June 2012 15:55, Edward Z. Yang <ezyang <at> mit.edu> wrote:
> Hello Jonas,
>
> Like other top-level definitions, these instances are considered CAFs
> (constant applicative forms), so these instances will in fact usually
> be evaluated only once per type X.
>
>    import System.IO.Unsafe
>    class C a where
>        dflt :: a
>    instance C Int where
>        dflt = unsafePerformIO (putStrLn "bang" >> return 2)
(Continue reading)

Edward Z. Yang | 29 Jun 17:26 2012
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Re: [GHC Users] Dictionary sharing

I say "usually" because while I believe this to be true for the
current implementation of GHC, I don't think we necessary give
this operational guarantee.

But yes, your real problem is that there is a world of difference between
functions and non-functions.  You will need to use one of the usual tricks for
memoising functions, or forgo using a function altogether and lean on laziness.

Edward

Excerpts from Jonas Almström Duregård's message of Fri Jun 29 11:21:46 -0400 2012:
> Thank you for your response Edward,
> 
> You write that it is usually only evaluated once, do you know the
> circumstances under which it is evaluated more than once? I have some
> examples of this but they are all very large.
> 
> The real issue I was having was actually not with a list but with a
> memoised function i.e. something like:
> >>>
> class C a where
>   memoised :: Int -> a
> <<<
> 
> Perhaps functions are treated differently?
> 
> Regards,
> Jonas
> 
> On 29 June 2012 15:55, Edward Z. Yang <ezyang <at> mit.edu> wrote:
(Continue reading)

Bertram Felgenhauer | 2 Jul 20:11 2012

Re: [GHC Users] Dictionary sharing

Jonas Almström Duregård wrote:
> Thank you for your response Edward,
> 
> You write that it is usually only evaluated once, do you know the
> circumstances under which it is evaluated more than once? I have some
> examples of this but they are all very large.

Only the dictionaries for type class instances that do not depend on
other instances will be CAFs and evaluated at most once. When an
instance has such dependencies, as for example (from your initial
mail in this thread),

    instance List a => List [a] where
        list = permutations list

then dictionaries will be created on demand (causing re-evaluation of
'list' in this particular case). More precisely, when the compiler
finds that a function needs a List [a] instance where only a List a
instance is available, it will create a fresh dictionary for List [a]
using the above implementation.

I am not aware of GHC providing any caching or memoisation mechanism
for this, so I think that your solution of building your own using
Typeable is appropriate.

Best regards,

Bertram

-- Example program showing addresses of various Ord dictionaries.
(Continue reading)


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