Jan-Willem Maessen <[EMAIL PROTECTED]> writes:
> Tomasz Zielonka <[EMAIL PROTECTED]> wrote:
> [...]
> > data Stat i o = -- aggregate function taking i's on input and producing o
> > forall s. Stat
> > s -- init
> > (s -> i -> s) -- update
> > (s -> o) -- result
> [...]
> * But it bugs me that an awful lot of examples of existential typing
> could be obtained simply by currying / lazy evaluation. In this
> case, however, the "update" function lets us absorb additional input
> as in the subsequent message (which I've now accidentally deleted):
I'm not convinced that existentials are needed here.
mike
import Prelude hiding ( sum )
data Stat i o = Stat { update :: i -> Stat i o
, result :: o }
runStat :: Stat i o -> [i] -> o
runStat stat = result . foldl update stat
stateStat :: (s -> i -> s) -> (s -> o) -> s -> Stat i o
stateStat updateF resultF initState = Stat
{ update = \i -> stateStat updateF resultF (updateF initState i)
, result = resultF initState }
instance Functor (Stat a) where
fmap f st = Stat { update = fmap f . update st, result = f (result st) }
avg :: Fractional n => Stat n n
avg = fmap (\(s,c) -> if c /= 0 then s/c else 0) (pair sum count)
fold :: (a -> b -> a) -> a -> Stat b a
fold f = stateStat f id
count :: Num n => Stat a n
count = fold (\s _ -> s+1) 0
sum :: Num n => Stat n n
sum = fold (+) 0
pair :: Stat a b -> Stat a c -> Stat a (b,c)
pair (Stat upd1 res1) (Stat upd2 res2) = Stat (\i -> pair (upd1 i) (upd2 i)) (res1,
res2)
main = error "no main"
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