Reputation: 1965
I'd like to pre-calculate some values that are then used when I need to do further lookups. I came up with the following:
import qualified Data.Vector.Unboxed as V
lcgen s =
lc
where
lc 0 b = lengths V.! b
lc a b = lengths V.! b - lengths V.! (a - 1) - 1
lengths = V.fromList $ scanl1 ((+) . (1 +)) $ map length $ words s
The function essentially returns the number of characters used in-between two words. I use it as follows:
let lc = lcgen "some sentence with a lot of words"
lc 0 0 -- == 4
lc 0 1 -- == 13
In this implementation, would the lengths
vector be memoized? Furthermore, how can I know and/or confirm this?
Upvotes: 2
Views: 178
Reputation: 52300
if you add a trace
to your lengths
like this:
lengths = trace "building list..." $ V.fromList ...
you will see the output every time the lengths
value is calculated
As it is it should only be evaluated/build once per lc = lcgen s
Upvotes: 1
Reputation: 43360
In this case lengths
clearly cannot be memoized, because it depends on the outer function's argument, which is why it is regenerated each time you call that function.
It could have been memoized if there was no such dependency. To check whether that happened you could use one of the methods suggested in the comments. To ensure that the memoization did happen, you could have used the standard approach of extracting the desired piece to the top level with the NOINLINE
pragma. E.g.,
{-# NOINLINE lengths #-}
lengths :: Vector Int
lengths =
error "define me"
Upvotes: 1