[Haskell-cafe] Trying to reduce memory costs of String duplicates

wren ng thornton wren at freegeek.org
Sat Sep 5 20:31:28 EDT 2009

Günther Schmidt wrote:
> Hi all,
> I'm reading in a data of 216k records into a map of Key, Values Pairs, 
> the values being strings.
> As it happens out of 216k String values there really are only about 6.6k 
> distinct string values, so I could save a lot of RAM if I was able to 
> "insert" only actually *new* string values into the map and use 
> references to (string) values that already are in memory instead.
> Is there a container that would, if I wanted to insert an element, 
> return a pair of either the previously inserted, equal value and the 
> container unchanged, or the new, previously unknown value and the new 
> container amended by that element?

If by "strings" you allow ByteStrings, then you could use the 
bytestring-trie package[1]. This will be more worthwhile than other 
approaches if your 6.6k strings have a lot of repeated prefixes, since 
repeated prefixes will be shared among the unique strings.

Something like the following should work:

     intern    :: ByteString -> Trie ByteString
               -> (ByteString, Trie ByteString)

     intern s t = (fromJust (lookup s t'), t')
         t' = alterBy (\_ _ -> maybe (Just s) Just) s s t


Live well,

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