Philip Armstrong wrote:
> genericLength :: (Num i) => [b] -> i
> genericLength [] = 0
> genericLength (_:l) = 1 + genericLength l
>
> So genericLength is lazily building up unevaluated (+) expressions and
> running out of stack space.
>
> Is there a good reason for genericLength to be lazy?
Yes, since addition in i may well be lazy.
data Nat = Zero | Succ Nat
instance Num Nat where
Zero + b = b
(Succ a) + b = Succ (a + b)
...
natLength :: [a] -> Nat
natLength = genericLength
Also, you can turn the foldr into a foldl' with a cleverly chosen data
type like
data Int' = I Int | Plus Int' Int'
eval :: Int' -> Int
eval = eval' 0
where
eval' (I i) k = k + i
eval' (Plus x y) k = eval' y $! eval' x k
instance Num Int' where
(+) = Plus
...
or in its bare essence
newtype DiffInt = DI { unDI :: Int -> Int }
instance Num DiffInt where
(+) f g k = DI $ unDI g $! unDI f k
evalDI :: DiffInt -> Int
evalDI f = unDI f 0
Regards,
apfelmus
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