Writing a faster mutable trie

I am implementing a mutable trie. I tested and it produces correct results. However, it is very slow. I benchmark it against plain old Data.Map, which is more than twice as fast.

Full source

So what am I doing wrong? I am expecting a fundamental flaw in the way I write mutable code here and not some micro performance tricks. I assume STVector is fast enough.

module Trie(Trie, empty, insert, member) where

import Data.Char (ord, toUpper)
import Data.Maybe (isJust)
import Control.Monad.ST
import qualified Data.Vector.Mutable as V 

-- In a real world scenario, we would probably want to our mutable trie to base
-- on PrimMonad and PrimState so that it can work within both IO and ST. Also,
-- our trie only accepts String keys composed of capitalized English alphabets
-- ([A-Z]+). Values, though, can be of any type. Finally, there should have
-- been a function to retrieve the value given a key, but we omitted it because
-- of laziness (pun intended).
data Trie s a = Trie {
    trieValue :: Maybe a ,
    trieChildren :: V.STVector s (Maybe (Trie s a))
}

toIndex :: Char -> Int
toIndex c = (ord (toUpper c) - ord 'A') `mod` 26

empty :: ST s (Trie s a)
empty = emptyWith Nothing

newChildren :: ST s (V.STVector s (Maybe (Trie s a)))
newChildren = V.replicate 26 Nothing

emptyWith :: Maybe a -> ST s (Trie s a)
emptyWith x = newChildren >>= return . Trie x

insert :: Trie s a -> String -> a -> ST s (Trie s a)
insert = insert' . Just

insert' :: Maybe (Trie s a) -> String -> a -> ST s (Trie s a)
insert' Nothing cs z = do
    node >= V.write ys i . Just
    return root

member :: Trie s a -> String -> ST s Bool
member = member' . Just

member' :: Maybe (Trie s a) -> String -> ST s Bool
member' Nothing _ = return False
member' (Just (Trie x _)) [] = return (isJust x)
member' (Just (Trie _ ys)) (c:cs) = 
    V.read ys (toIndex c) >>= flip member' cs

The big thing I can see is that you don't seem to know why to use a mutable structure in Haskell. They're not automatically faster. In fact, they have some GC-related overhead that exceeds that of immutable structures in GHC. Mutable structures in GHC are only a win if they can actually reduce the amount of allocation, and this implementation looks like it allocates a lot.

When you consider using a mutable structure, you need to examine how mutability will reduce total allocation. If it doesn't reduce allocation (and usually by an asymptotic factor) it's unlikely that mutability alone is going to help performance.

StackExchange Code Review Q#61144, answer score: 3