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:
> 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
>
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