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:

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class C a where memoised :: Int -> a <<<

Perhaps functions are treated differently? Regards, Jonas On 29 June 2012 15:55, Edward Z. Yang 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)    main = do        print (dflt :: Int)        print (dflt :: Int)        print (dflt :: Int)

ezyang@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:

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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:

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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

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. -- Contents may be hazardous if swallowed! Keep away from children! {-# LANGUAGE MagicHash, Rank2Types #-} module Main where import GHC.Exts import GHC.Int newtype GetDict = GetDict { unGetDict :: forall a . Ord a => a -> Int } -- Evil hack for extracting the address of a dictionary from a function -- call. Note that these addresses may change during GC! getDict :: Ord a => a -> Int getDict = unGetDict (unsafeCoerce# getDict') where getDict' :: Addr# -> Addr# -> Int getDict' d _ = I# (addr2Int# d) {-# NOINLINE bar #-} -- newListDict is designed to force the creation of a new Ord [a] -- dictionary given an Ord a dictionary, and return the new dictionary's -- address. getListDict :: Ord a => a -> Int getListDict x = unGetDict (GetDict (\x -> getDict [x])) x main = do print $ getDict (1 :: Int) -- using a CAF dictionary print $ getDict (2 :: Int) -- same as previous print $ getDict (2 :: Word) -- a different CAF dictionary print $ getDict ([1] :: [Int]) -- also a CAF! print $ getDict ([2] :: [Int]) -- same as previous print $ getListDict (1 :: Int) -- a dynamically created dictionary print $ getListDict (2 :: Int) -- different from previous