How to merge two hashtables?

[N0lim]

No description provided.

[erikd]

A naive solution would be to convert the two Hashtables to lists, concatenate the two lists and then convert the resulting list back to a Hashtable.

Warning, this code has never been compiled.

naiveMerge :: HashTable k v -> HashTable k v -> ST (HashTable k v)
naiveMerge a b = do
     as <-toList a
     bs <- toList b
     fromList $ as + bs

And no thought whatever has gone into how to handle key/value pairs that exist in both lists.

[N0lim]

I wrote this mutable version and a test for it, I'm sure a more flexible version can be made using mutate or maybe an immutable version will turn out

import Data.HashTable.Class
import qualified Data.HashTable.ST.Cuckoo as C
import Control.Monad.ST

--merge :: (HashTable h1, HashTable h2, Hashable a) => h1 s a b -> h2 s a b -> ST s ()
merge tbl1 tbl2 = do
  l1 <- toList tbl1
  mapM (uncurry (insert tbl2)) l1 
  return ()

testST = do
  tbl1 <- C.new
  insert tbl1 "a" True
  insert tbl1 "b" True
  tbl2 <- C.new
  insert tbl2 "b" False
  insert tbl2 "c" False
  merge tbl1 tbl2
  a <- Data.HashTable.Class.lookup tbl2 "a"
  b <- Data.HashTable.Class.lookup tbl2 "b"
  c <- Data.HashTable.Class.lookup tbl2 "c"
  return (a, b, c)

test = runST testST